compare compiler performance with `hashbrown`

[nikomatsakis]

This https://github.com/Amanieu/hashbrown crate looks nice!

We should do a performance comparison.

cc @Amanieu

[Mark-Simulacrum]

Should be fairly easy -- well, except I think we now don't have the FxHash implementations in-tree so that might be a bit harder...

I think this is a great issue for someone to take on though, if they want to swap out the FxHash implementation and post a PR we can run perf on it!

[Centril]

Better yet... if it is a fully drop-in replacement for HashMap, could we just replace the standard library's implementation with the one in hashbrown?

[shepmaster]

a fully drop-in replacement

depends on if "drop in" means "adheres to every expected requirement of HashMap" or just "API compatible".

[shepmaster]

It also lists

Uses SIMD to speed up lookups

It'd be very good to performance test it on all the architectures we care about people running the compiler on. If we replace the standard library implementation , then expand that to every tier 1 platform.

[Amanieu]

@shepmaster "drop-in" means that I literally copied the HashMap API from libstd and ripped out the internals. The only significant change API-wise is that the default hasher is changed.

[Centril]

@Amanieu are there any guarantees not covered in the signatures that don't fit other than the default hasher? (we would retain the standard library's default hasher...)

[Amanieu]

There is one small detail due to the way re-hashing works: if the Hash impl panics during rehashing then any elements that haven't been rehashed yet will be dropped from the table. There is unfortunately no way around this since the full hashes are not stored in the table.

[Centril]

@Amanieu ah; that seems like sane behavior tho (or at least I don't think it's weird behavior...) and very subtle / unlikely to happen. I'm having a hard time imagining a situation where the the Hash impl would not panic on insertion but then panic on rehash... maybe for some type with interior mutability perhaps (where the Hash impl alters the value of the type / using some sort of global side effect, e.g. reading from file... but that's probably going to be an incorrect implementation...)? Perhaps I'm not creative enough... =P

[scottmcm]

Is there anything that needs to be accounted for in #54043 to ensure that such a replacement would be possible in the future?

[Amanieu]

@scottmcm There is no issue there, the raw entry API can easily be ported to hashbrown.

[Zoxc]

We'd also want a way to avoid double lookups when interning like my PR did. The raw_entry API is supposed to enable that for the libstd's HashMap.

It would be a good idea to pick up the refactoring to use a intern method in my PR. It could just be an utility function in rustc instead of being a method on the hash map.

[Amanieu]

I'm having some trouble replacing rustc_data_structures::FxHashMap: hashbrown uses core::arch for SSE2 support, but core::arch is missing in stage0.

[Zoxc]

You can replace it only for stage1+, using #[cfg(not(stage0))] and #[cfg(stage0)]

[Amanieu]

Is there a way to specify a dependency only for stage1+ in Cargo.toml? I already have the #[cfg], but Cargo is still trying to compile the crate since it is listed as a dependency of rustc_data_structures.

[Zoxc]

I don't know of a way to do that.

@gnzlbg Do you know of a workaround here? Or why arch is not in stage0?

[gnzlbg]

Or why arch is not in stage0?

arch depends on compiler and LLVM internals, so it is not in stage0 because it often cannot be built with beta. Does that make sense?

Do you know of a workaround here?

@Amanieu do you have a way to manually enable the usage of arch in your library, e.g., via a cargo feature ? If so you could maybe try disabling the usage of arch for stage0, and enabling it in stage1. The alternative would be to try to do what you mention: use the std Hash{Map,Set} in stage0, and only use hashbrown in stage1, but I don't know how to do that =/

Is there a way to use the arch module from beta while building stage0 instead of the arch module from the std lib being built? That might be another option.

---

Looking at hashbrown, that looks a lot like the new swiss tables data-structures in abseil (https://abseil.io/blog/20180927-swisstables), looks like a really cool project!

[shepmaster]

that looks a lot like the new swiss tables data-structures

Yes, the project description suggests that's not an accident:

[Amanieu]

I ended up just making a separate branch for it: https://github.com/Amanieu/hashbrown/tree/stage0

[shepmaster]

@Amanieu If your library cannot compile without arch, does that mean that mean it doesn't support non-x86 platforms?

[Amanieu]

No, it's just that it tries to use arch if the sse2 feature is available on x86.

[eddyb]

cc @Gankro

[nikomatsakis]

@Amanieu are you pursuing this integration further? I think if you can get a branch that works, you just need to open a PR and we can run the initial perf measurements.

[michaelwoerister]

cc @rust-lang/wg-compiler-performance

[nagisa]

Discussed during the compiler meeting. It would be nice to have a faster hashmap for the compiler, but it would also be interesting to see what are the options for including the algorithm into the standard library in the first place (the compiler would automatically reap the benefits without any extra effort necessary).

There were questions about what exactly makes these hashmaps faster/better than those in libstd. What are the pros/cons etc.

[Amanieu]

I started work on a branch which replaces FxHashMap in rustc_data_structures: Amanieu@2642619

It doesn't compile yet, and I am currently busy with other things so I can't work on this.

Simply replacing the standard hash map in libstd as @nagisa suggests might be an option, however there are several factors that we need to take into account:

• Table resizing/rehashing is going to be slower since it will need to re-invoke the hasher for every element. This is going to particularly slow for large keys and if we are keeping SipHash as the default hasher.
• SwissTable uses tombstones when an element is deleted, which means that rehashes can occurs if you insert/delete elements even if the maximum number of elements at any point never grows.
• The current HashMap has "adaptive early resizing" which apparently helps with HashDoS.
• As mentioned in my comment above, if a Hash implementation panics during a rehash then it may lead to the table losing some elements (these elements are still properly dropped, so it will not leak memory).

[Gankro]

I definitely wouldn't humor replacing the implementation in libstd without way more benchmarking, but rustc is basically a giant hashmap benchmark if I recall correctly, so doing a swap in should be pretty useful info?

I don't understand why this crate is supposed to be API compatible but isn't trivial to swap in and test? whoops found it; yuck

[Gankro]

Also if I recall correctly, I explicitly made sure that requiring rehashing on insert was API compatible with the new raw_entry API (it requires K: Hash even if you're providing a custom hash function to get the entry). Someone should make sure that wasn't dropped in the new PR which I haven't been following.

[jonhoo]

@Gankro pinged @fintelia about it!

[fintelia]

Yeah all the new functions also have the K: Hash bound place, so the new raw_entry API should allow rehashing at any point. In fact, since HashMap::new() and similar all require that same bound, you can't even create a HashMap without hashable keys!

[nnethercote]

rustc is basically a giant hashmap benchmark if I recall correctly

Two years ago I described rustc's execution as "mostly malloc/free and hash tables". That is less true than it used to be, but hash tables are still used a lot.

If hashbrown doesn't store key hashes it should use less memory, right?

[eddyb]

It might be interesting to see if we could start caching hashes e.g. in Ty, to avoid the cost of hashing a pointer, and maybe even allow the hashes to be persisted.

[Amanieu]

I found a potential breaking change while porting hashbrown to std. The Entry<'a, K, V> type needs to be changed to Entry<'a, K, V, S>. This is needed because VacantEntry::insert needs to be able to re-hash the table contents when it needs to grow the table.

[Amanieu]

cc #56241

[tkaitchuck]

I notice that the benchmarks listed are with maps using integers as keys. How does it do with medium length strings? (5-20 character) This is a very common for hashmap keys, and hashing time would likely dominate, making re-hashing potentially expensive.

[Voultapher]

Some time ago I started working on a hashmap benchmark and comparison project https://github.com/Voultapher/hashmap-compare. It's still very WIP. Among other things its missing result analysis and visualization.

Looking at the raw output of std::collections::HashMap with fx_hasher and hashbrown::HashMap shows hashbrown only being faster under certain circumstances, and slower in others, especially for small hash map sizes.

https://gist.github.com/Voultapher/126dda90923c4c5b24135a9ecff403f3

[stjepang]

@Voultapher Can we perhaps get the difference between hash tables printed in the cargo benchcmp format? It'd help to see the difference between fnv and hashbrown side-by-side.

Unless I'm missing something, it looks like hashbrown is worryingly slower only in the tiny case with integer keys and values?

@Amanieu Can we do anything to speed up small hash tables (~10 entries)?

[Amanieu]

What is happening here is that the benchmarks include the time taken to grow the table. Unlike the std hashmap, hashbrown starts at a size of 1 element and double in size as needed. On the other hand the standard hashmap immediately starts with 32 elements.

Of course, this is something that can be tweaked. The intention was to avoid excessive memory usage when having many tables with few elements.

[stjepang]

Of course, this is something that can be tweaked. The intention was to avoid excessive memory usage when having many tables with few elements.

Good to hear this will be easy to fix then! We'll probably want to have faster growth for small hash tables without compromising too much on memory usage. Maybe something like 0 -> 4 -> 16 -> 32 -> 64 ...

[Amanieu]

@Voultapher If you want, try changing HashMap::new() to HashMap::with_capacity(32). This should give a better performance comparison.

[SimonSapin]

Unlike the std hashmap, hashbrown starts at a size of 1 element and double in size as needed. On the other hand the standard hashmap immediately starts with 32 elements.

Perhaps this behavior could be adaptive based on size_of::<(K, V)>()?

[Voultapher]

@Voultapher If you want, try changing HashMap::new() to HashMap::with_capacity(32). This should give a better performance comparison.

CONFIGURATION: hashbrown_map,reserve_hm is essentially that.

[shepmaster]

hashbrown starts at a size of 1 element

Do I understand correctly that this means that HashMap::new would now allocate memory? As I understand it, that's likely to cause performance degradation in the overall system. It would also cause a break with the current documentation of HashMap:

it will not allocate until it is first inserted into

[Amanieu]

@shepmaster No, both the old and new HashMap do not allocate any memory when created. However once an element is inserted, hashbrown will gradually grow the capacity (2,4,8,16, ...) whereas the current HashMap will immediately start at 32 buckets.

[gnzlbg]

FWIW I prefer @Amanieu 's approach of not starting at 32 elements. If a user finds this to be a performance issue, the solution is easy: use ::with_capacity(32).

However, if a user needs hash maps that are smaller than 32 elements (e.g. due to memory consumption concerns), and the hash map always start at 32 elements, the solution isn't really that easy anymore.

This might worsen the performance of some applications for which a 32 element start was paying off, but AFAICT these applications might perform even better by reserving some other number of elements, and relying on a specific growth policy of ::new() + insert sounds like a performance bug to me.

[Gankro]

I agree to a point. But if this is super common we're just creating a performance trap for most of our users to make the system more flexible for the minority. Also we could just as easily write the map with the following logic (in fact I would expect it to have this form):

if is_empty() { alloc(32) } else { realloc(cap * 2) }

And users who want slow growing can with_capacity(1).

That said I think it's fine to land with the either growing logic and refine it in followups with realworld data.

[arthurprs]

I notice that the benchmarks listed are with maps using integers as keys. How does it do with medium length strings? (5-20 character) This is a very common for hashmap keys, and hashing time would likely dominate, making re-hashing potentially expensive.

We probably want to take a look at this sooner rather than later, specially considering that Rust encourages/defaults to Sip13.

Most (All?) benchmarks so far are focusing on very fast hashers and integer keys.

[stjepang]

Here are @Voultapher's results in cargo benchcmp format (fx vs hashbrown):

cmp

 name                              fx_hash ns/iter  hb_map ns/iter  diff ns/iter   diff %  speedup 
 tests::big::clone                 6,507,451        4,412,083         -2,095,368  -32.20%   x 1.47 
 tests::big::copy_element_wise     9,405,607        8,101,581         -1,304,026  -13.86%   x 1.16 
 tests::big::fill_only             5,362,458        3,664,157         -1,698,301  -31.67%   x 1.46 
 tests::big::insert_random         7,773,228        6,236,963         -1,536,265  -19.76%   x 1.25 
 tests::big::lookup_all            5,816,665        4,389,039         -1,427,626  -24.54%   x 1.33 
 tests::big::lookup_missing        7,505,441        4,685,889         -2,819,552  -37.57%   x 1.60 
 tests::big::lookup_one            5,367,760        3,664,826         -1,702,934  -31.73%   x 1.46 
 tests::big::lookup_random         6,578,814        5,066,531         -1,512,283  -22.99%   x 1.30 
 tests::big::random_gen            760,432          746,856              -13,576   -1.79%   x 1.02 
 tests::big::traversal             5,466,632        3,768,946         -1,697,686  -31.06%   x 1.45 
 tests::large::clone               366,667          285,431              -81,236  -22.16%   x 1.28 
 tests::large::copy_element_wise   1,011,849        675,146             -336,703  -33.28%   x 1.50 
 tests::large::fill_only           331,824          250,706              -81,118  -24.45%   x 1.32 
 tests::large::insert_random       641,432          559,385              -82,047  -12.79%   x 1.15 
 tests::large::lookup_all          361,762          286,708              -75,054  -20.75%   x 1.26 
 tests::large::lookup_missing      506,489          408,327              -98,162  -19.38%   x 1.24 
 tests::large::lookup_one          333,107          251,079              -82,028  -24.63%   x 1.33 
 tests::large::lookup_random       475,511          401,555              -73,956  -15.55%   x 1.18 
 tests::large::random_gen          114,740          112,922               -1,818   -1.58%   x 1.02 
 tests::large::traversal           344,705          263,406              -81,299  -23.59%   x 1.31 
 tests::medium::clone              33,535           30,858                -2,677   -7.98%   x 1.09 
 tests::medium::copy_element_wise  62,100           60,295                -1,805   -2.91%   x 1.03 
 tests::medium::fill_only          30,040           27,596                -2,444   -8.14%   x 1.09 
 tests::medium::insert_random      45,116           41,869                -3,247   -7.20%   x 1.08 
 tests::medium::lookup_all         32,223           31,341                  -882   -2.74%   x 1.03 
 tests::medium::lookup_missing     35,241           34,807                  -434   -1.23%   x 1.01 
 tests::medium::lookup_one         30,081           27,993                -2,088   -6.94%   x 1.07 
 tests::medium::lookup_random      37,347           35,581                -1,766   -4.73%   x 1.05 
 tests::medium::random_gen         4,144            4,030                   -114   -2.75%   x 1.03 
 tests::medium::traversal          31,348           28,976                -2,372   -7.57%   x 1.08 
 tests::small::clone               2,328            2,717                    389   16.71%   x 0.86 
 tests::small::copy_element_wise   4,582            4,992                    410    8.95%   x 0.92 
 tests::small::fill_only           2,171            2,370                    199    9.17%   x 0.92 
 tests::small::insert_random       4,397            4,598                    201    4.57%   x 0.96 
 tests::small::lookup_all          2,387            2,701                    314   13.15%   x 0.88 
 tests::small::lookup_missing      3,825            3,441                   -384  -10.04%   x 1.11 
 tests::small::lookup_one          2,169            2,372                    203    9.36%   x 0.91 
 tests::small::lookup_random       3,235            3,524                    289    8.93%   x 0.92 
 tests::small::random_gen          744              733                      -11   -1.48%   x 1.02 
 tests::small::traversal           2,245            2,448                    203    9.04%   x 0.92 
 tests::tiny::clone                247              429                      182   73.68%   x 0.58 
 tests::tiny::copy_element_wise    428              753                      325   75.93%   x 0.57 
 tests::tiny::fill_only            201              376                      175   87.06%   x 0.53 
 tests::tiny::insert_random        503              691                      188   37.38%   x 0.73 
 tests::tiny::lookup_all           217              405                      188   86.64%   x 0.54 
 tests::tiny::lookup_missing       322              534                      212   65.84%   x 0.60 
 tests::tiny::lookup_one           204              384                      180   88.24%   x 0.53 
 tests::tiny::lookup_random        355              542                      187   52.68%   x 0.65 
 tests::tiny::random_gen           128              128                        0    0.00%   x 1.00 
 tests::tiny::traversal            222              377                      155   69.82%   x 0.59 

cmp_reserve

 name                              fx_hash_reserve ns/iter  hb_map_reserve ns/iter  diff ns/iter   diff %  speedup 
 tests::big::clone                 4,754,128                2,956,167                 -1,797,961  -37.82%   x 1.61 
 tests::big::copy_element_wise     6,386,798                3,952,174                 -2,434,624  -38.12%   x 1.62 
 tests::big::fill_only             3,160,160                1,962,929                 -1,197,231  -37.89%   x 1.61 
 tests::big::insert_random         5,534,253                4,527,364                 -1,006,889  -18.19%   x 1.22 
 tests::big::lookup_all            3,527,120                2,684,407                   -842,713  -23.89%   x 1.31 
 tests::big::lookup_missing        5,306,770                2,979,452                 -2,327,318  -43.86%   x 1.78 
 tests::big::lookup_one            3,170,956                1,961,437                 -1,209,519  -38.14%   x 1.62 
 tests::big::lookup_random         4,313,456                3,359,152                   -954,304  -22.12%   x 1.28 
 tests::big::random_gen            757,628                  749,122                       -8,506   -1.12%   x 1.01 
 tests::big::traversal             3,256,776                2,066,055                 -1,190,721  -36.56%   x 1.58 
 tests::large::clone               259,178                  145,420                     -113,758  -43.89%   x 1.78 
 tests::large::copy_element_wise   408,293                  240,134                     -168,159  -41.19%   x 1.70 
 tests::large::fill_only           222,180                  109,206                     -112,974  -50.85%   x 2.03 
 tests::large::insert_random       536,214                  420,179                     -116,035  -21.64%   x 1.28 
 tests::large::lookup_all          253,566                  145,440                     -108,126  -42.64%   x 1.74 
 tests::large::lookup_missing      401,967                  263,920                     -138,047  -34.34%   x 1.52 
 tests::large::lookup_one          222,645                  109,125                     -113,520  -50.99%   x 2.04 
 tests::large::lookup_random       372,254                  259,761                     -112,493  -30.22%   x 1.43 
 tests::large::random_gen          114,193                  114,292                           99    0.09%   x 1.00 
 tests::large::traversal           235,224                  120,938                     -114,286  -48.59%   x 1.94 
 tests::medium::clone              20,261                   13,350                        -6,911  -34.11%   x 1.52 
 tests::medium::copy_element_wise  35,255                   23,405                       -11,850  -33.61%   x 1.51 
 tests::medium::fill_only          16,409                   10,165                        -6,244  -38.05%   x 1.61 
 tests::medium::insert_random      30,916                   24,358                        -6,558  -21.21%   x 1.27 
 tests::medium::lookup_all         18,645                   14,055                        -4,590  -24.62%   x 1.33 
 tests::medium::lookup_missing     22,384                   17,169                        -5,215  -23.30%   x 1.30 
 tests::medium::lookup_one         16,279                   8,361                         -7,918  -48.64%   x 1.95 
 tests::medium::lookup_random      23,889                   15,973                        -7,916  -33.14%   x 1.50 
 tests::medium::random_gen         4,155                    4,028                           -127   -3.06%   x 1.03 
 tests::medium::traversal          17,858                   9,831                         -8,027  -44.95%   x 1.82 
 tests::small::clone               1,845                    1,280                           -565  -30.62%   x 1.44 
 tests::small::copy_element_wise   3,478                    2,399                         -1,079  -31.02%   x 1.45 
 tests::small::fill_only           1,689                    935                             -754  -44.64%   x 1.81 
 tests::small::insert_random       3,905                    3,135                           -770  -19.72%   x 1.25 
 tests::small::lookup_all          1,940                    1,279                           -661  -34.07%   x 1.52 
 tests::small::lookup_missing      3,360                    1,946                         -1,414  -42.08%   x 1.73 
 tests::small::lookup_one          1,699                    935                             -764  -44.97%   x 1.82 
 tests::small::lookup_random       2,772                    2,021                           -751  -27.09%   x 1.37 
 tests::small::random_gen          740                      734                               -6   -0.81%   x 1.01 
 tests::small::traversal           1,797                    1,001                           -796  -44.30%   x 1.80 
 tests::tiny::clone                247                      215                              -32  -12.96%   x 1.15 
 tests::tiny::copy_element_wise    460                      338                             -122  -26.52%   x 1.36 
 tests::tiny::fill_only            203                      170                              -33  -16.26%   x 1.19 
 tests::tiny::insert_random        517                      482                              -35   -6.77%   x 1.07 
 tests::tiny::lookup_all           234                      206                              -28  -11.97%   x 1.14 
 tests::tiny::lookup_missing       340                      325                              -15   -4.41%   x 1.05 
 tests::tiny::lookup_one           209                      175                              -34  -16.27%   x 1.19 
 tests::tiny::lookup_random        360                      332                              -28   -7.78%   x 1.08 
 tests::tiny::random_gen           127                      129                                2    1.57%   x 0.98 
 tests::tiny::traversal            232                      177                              -55  -23.71%   x 1.31 

cmp_string_key

 name                              fx_hash_string_key ns/iter  hb_map_string_key ns/iter  diff ns/iter   diff %  speedup 
 tests::big::clone                 41,640,749                  37,231,056                   -4,409,693  -10.59%   x 1.12 
 tests::big::copy_element_wise     50,113,165                  44,930,224                   -5,182,941  -10.34%   x 1.12 
 tests::big::fill_only             31,153,347                  22,566,193                   -8,587,154  -27.56%   x 1.38 
 tests::big::insert_random         54,839,289                  41,885,814                  -12,953,475  -23.62%   x 1.31 
 tests::big::lookup_all            44,268,429                  34,459,033                   -9,809,396  -22.16%   x 1.28 
 tests::big::lookup_missing        42,657,665                  32,314,927                  -10,342,738  -24.25%   x 1.32 
 tests::big::lookup_one            31,245,774                  22,369,691                   -8,876,083  -28.41%   x 1.40 
 tests::big::lookup_random         52,480,971                  41,227,666                  -11,253,305  -21.44%   x 1.27 
 tests::big::random_gen            747,276                     752,287                           5,011    0.67%   x 0.99 
 tests::big::traversal             31,487,239                  22,135,437                   -9,351,802  -29.70%   x 1.42 
 tests::large::clone               3,539,273                   2,176,247                    -1,363,026  -38.51%   x 1.63 
 tests::large::copy_element_wise   4,053,978                   2,606,888                    -1,447,090  -35.70%   x 1.56 
 tests::large::fill_only           2,931,351                   1,650,922                    -1,280,429  -43.68%   x 1.78 
 tests::large::insert_random       4,456,544                   2,809,220                    -1,647,324  -36.96%   x 1.59 
 tests::large::lookup_all          3,938,244                   2,454,814                    -1,483,430  -37.67%   x 1.60 
 tests::large::lookup_missing      4,069,325                   2,541,896                    -1,527,429  -37.54%   x 1.60 
 tests::large::lookup_one          2,960,518                   1,657,446                    -1,303,072  -44.02%   x 1.79 
 tests::large::lookup_random       4,210,819                   2,715,891                    -1,494,928  -35.50%   x 1.55 
 tests::large::random_gen          114,645                     114,111                            -534   -0.47%   x 1.00 
 tests::large::traversal           2,971,583                   1,662,944                    -1,308,639  -44.04%   x 1.79 
 tests::medium::clone              275,756                     201,411                         -74,345  -26.96%   x 1.37 
 tests::medium::copy_element_wise  336,514                     218,863                        -117,651  -34.96%   x 1.54 
 tests::medium::fill_only          209,900                     152,374                         -57,526  -27.41%   x 1.38 
 tests::medium::insert_random      315,836                     241,154                         -74,682  -23.65%   x 1.31 
 tests::medium::lookup_all         287,676                     227,982                         -59,694  -20.75%   x 1.26 
 tests::medium::lookup_missing     287,370                     224,582                         -62,788  -21.85%   x 1.28 
 tests::medium::lookup_one         211,345                     154,994                         -56,351  -26.66%   x 1.36 
 tests::medium::lookup_random      296,956                     236,349                         -60,607  -20.41%   x 1.26 
 tests::medium::random_gen         4,022                       4,100                                78    1.94%   x 0.98 
 tests::medium::traversal          210,894                     156,688                         -54,206  -25.70%   x 1.35 
 tests::small::clone               21,570                      18,787                           -2,783  -12.90%   x 1.15 
 tests::small::copy_element_wise   25,194                      18,151                           -7,043  -27.96%   x 1.39 
 tests::small::fill_only           16,074                      13,989                           -2,085  -12.97%   x 1.15 
 tests::small::insert_random       27,688                      22,734                           -4,954  -17.89%   x 1.22 
 tests::small::lookup_all          25,408                      20,805                           -4,603  -18.12%   x 1.22 
 tests::small::lookup_missing      25,629                      21,534                           -4,095  -15.98%   x 1.19 
 tests::small::lookup_one          16,161                      14,087                           -2,074  -12.83%   x 1.15 
 tests::small::lookup_random       26,615                      21,665                           -4,950  -18.60%   x 1.23 
 tests::small::random_gen          732                         737                                   5    0.68%   x 0.99 
 tests::small::traversal           16,949                      13,982                           -2,967  -17.51%   x 1.21 
 tests::tiny::clone                1,794                       1,653                              -141   -7.86%   x 1.09 
 tests::tiny::copy_element_wise    1,686                       1,939                               253   15.01%   x 0.87 
 tests::tiny::fill_only            1,172                       1,322                               150   12.80%   x 0.89 
 tests::tiny::insert_random        2,304                       2,185                              -119   -5.16%   x 1.05 
 tests::tiny::lookup_all           1,842                       1,914                                72    3.91%   x 0.96 
 tests::tiny::lookup_missing       2,045                       2,168                               123    6.01%   x 0.94 
 tests::tiny::lookup_one           1,236                       1,320                                84    6.80%   x 0.94 
 tests::tiny::lookup_random        2,023                       2,114                                91    4.50%   x 0.96 
 tests::tiny::random_gen           128                         128                                   0    0.00%   x 1.00 
 tests::tiny::traversal            1,162                       1,332                               170   14.63%   x 0.87 

cmp_string_value

 name                              fx_hash_string_value ns/iter  hb_map_string_value ns/iter  diff ns/iter   diff %  speedup 
 tests::big::clone                 28,788,708                    28,156,111                       -632,597   -2.20%   x 1.02 
 tests::big::copy_element_wise     39,861,523                    25,823,906                    -14,037,617  -35.22%   x 1.54 
 tests::big::fill_only             20,341,366                    15,939,241                     -4,402,125  -21.64%   x 1.28 
 tests::big::insert_random         38,308,105                    34,449,390                     -3,858,715  -10.07%   x 1.11 
 tests::big::lookup_all            21,341,971                    17,606,206                     -3,735,765  -17.50%   x 1.21 
 tests::big::lookup_missing        22,491,193                    16,692,648                     -5,798,545  -25.78%   x 1.35 
 tests::big::lookup_one            19,936,818                    15,639,018                     -4,297,800  -21.56%   x 1.27 
 tests::big::lookup_random         22,219,383                    18,204,456                     -4,014,927  -18.07%   x 1.22 
 tests::big::random_gen            754,522                       757,921                             3,399    0.45%   x 1.00 
 tests::big::traversal             19,329,821                    16,025,152                     -3,304,669  -17.10%   x 1.21 
 tests::large::clone               2,296,144                     1,914,849                        -381,295  -16.61%   x 1.20 
 tests::large::copy_element_wise   6,338,369                     1,806,096                      -4,532,273  -71.51%   x 3.51 
 tests::large::fill_only           1,669,212                     1,324,006                        -345,206  -20.68%   x 1.26 
 tests::large::insert_random       2,672,227                     2,383,659                        -288,568  -10.80%   x 1.12 
 tests::large::lookup_all          1,702,581                     1,377,023                        -325,558  -19.12%   x 1.24 
 tests::large::lookup_missing      1,850,643                     1,474,750                        -375,893  -20.31%   x 1.25 
 tests::large::lookup_one          1,668,583                     1,319,367                        -349,216  -20.93%   x 1.26 
 tests::large::lookup_random       1,819,394                     1,490,359                        -329,035  -18.08%   x 1.22 
 tests::large::random_gen          113,680                       113,100                              -580   -0.51%   x 1.01 
 tests::large::traversal           1,665,136                     1,760,164                          95,028    5.71%   x 0.95 
 tests::medium::clone              216,942                       169,949                           -46,993  -21.66%   x 1.28 
 tests::medium::copy_element_wise  265,627                       158,921                          -106,706  -40.17%   x 1.67 
 tests::medium::fill_only          160,125                       119,402                           -40,723  -25.43%   x 1.34 
 tests::medium::insert_random      247,924                       201,818                           -46,106  -18.60%   x 1.23 
 tests::medium::lookup_all         168,105                       122,051                           -46,054  -27.40%   x 1.38 
 tests::medium::lookup_missing     174,783                       126,260                           -48,523  -27.76%   x 1.38 
 tests::medium::lookup_one         165,979                       119,581                           -46,398  -27.95%   x 1.39 
 tests::medium::lookup_random      172,994                       129,014                           -43,980  -25.42%   x 1.34 
 tests::medium::random_gen         4,050                         4,106                                  56    1.38%   x 0.99 
 tests::medium::traversal          165,668                       120,305                           -45,363  -27.38%   x 1.38 
 tests::small::clone               18,834                        17,412                             -1,422   -7.55%   x 1.08 
 tests::small::copy_element_wise   21,553                        14,849                             -6,704  -31.10%   x 1.45 
 tests::small::fill_only           13,707                        11,626                             -2,081  -15.18%   x 1.18 
 tests::small::insert_random       22,988                        20,431                             -2,557  -11.12%   x 1.13 
 tests::small::lookup_all          13,917                        11,934                             -1,983  -14.25%   x 1.17 
 tests::small::lookup_missing      15,470                        12,674                             -2,796  -18.07%   x 1.22 
 tests::small::lookup_one          13,677                        11,738                             -1,939  -14.18%   x 1.17 
 tests::small::lookup_random       14,795                        13,108                             -1,687  -11.40%   x 1.13 
 tests::small::random_gen          742                           735                                    -7   -0.94%   x 1.01 
 tests::small::traversal           13,865                        11,920                             -1,945  -14.03%   x 1.16 
 tests::tiny::clone                1,772                         1,379                                -393  -22.18%   x 1.28 
 tests::tiny::copy_element_wise    1,611                         1,382                                -229  -14.21%   x 1.17 
 tests::tiny::fill_only            1,172                         1,019                                -153  -13.05%   x 1.15 
 tests::tiny::insert_random        2,291                         1,894                                -397  -17.33%   x 1.21 
 tests::tiny::lookup_all           1,150                         1,033                                -117  -10.17%   x 1.11 
 tests::tiny::lookup_missing       1,325                         1,170                                -155  -11.70%   x 1.13 
 tests::tiny::lookup_one           1,213                         1,055                                -158  -13.03%   x 1.15 
 tests::tiny::lookup_random        1,357                         1,207                                -150  -11.05%   x 1.12 
 tests::tiny::random_gen           130                           130                                     0    0.00%   x 1.00 
 tests::tiny::traversal            1,222                         1,006                                -216  -17.68%   x 1.21 

cmp_string_key_string_value

 name                              fx_hash_string_key_string_value ns/iter  hb_map_string_key_string_value ns/iter  diff ns/iter   diff %  speedup 
 tests::big::clone                 74,760,564                               72,572,547                                -2,188,017   -2.93%   x 1.03 
 tests::big::copy_element_wise     74,554,969                               78,397,734                                 3,842,765    5.15%   x 0.95 
 tests::big::fill_only             50,268,783                               43,268,444                                -7,000,339  -13.93%   x 1.16 
 tests::big::insert_random         89,918,488                               80,498,378                                -9,420,110  -10.48%   x 1.12 
 tests::big::lookup_all            64,704,076                               58,354,002                                -6,350,074   -9.81%   x 1.11 
 tests::big::lookup_missing        61,851,802                               55,440,549                                -6,411,253  -10.37%   x 1.12 
 tests::big::lookup_one            50,035,424                               43,687,211                                -6,348,213  -12.69%   x 1.15 
 tests::big::lookup_random         74,474,298                               69,774,102                                -4,700,196   -6.31%   x 1.07 
 tests::big::random_gen            748,833                                  749,853                                        1,020    0.14%   x 1.00 
 tests::big::traversal             52,973,648                               44,830,385                                -8,143,263  -15.37%   x 1.18 
 tests::large::clone               4,975,082                                3,619,651                                 -1,355,431  -27.24%   x 1.37 
 tests::large::copy_element_wise   5,270,695                                3,591,497                                 -1,679,198  -31.86%   x 1.47 
 tests::large::fill_only           3,833,030                                2,536,580                                 -1,296,450  -33.82%   x 1.51 
 tests::large::insert_random       6,218,582                                4,559,051                                 -1,659,531  -26.69%   x 1.36 
 tests::large::lookup_all          4,842,136                                3,382,923                                 -1,459,213  -30.14%   x 1.43 
 tests::large::lookup_missing      4,878,035                                3,444,655                                 -1,433,380  -29.38%   x 1.42 
 tests::large::lookup_one          3,848,832                                2,552,879                                 -1,295,953  -33.67%   x 1.51 
 tests::large::lookup_random       5,138,614                                3,686,649                                 -1,451,965  -28.26%   x 1.39 
 tests::large::random_gen          114,083                                  113,766                                         -317   -0.28%   x 1.00 
 tests::large::traversal           4,070,807                                2,635,759                                 -1,435,048  -35.25%   x 1.54 
 tests::medium::clone              437,972                                  335,931                                     -102,041  -23.30%   x 1.30 
 tests::medium::copy_element_wise  487,463                                  308,724                                     -178,739  -36.67%   x 1.58 
 tests::medium::fill_only          321,994                                  235,168                                      -86,826  -26.97%   x 1.37 
 tests::medium::insert_random      500,262                                  392,281                                     -107,981  -21.58%   x 1.28 
 tests::medium::lookup_all         402,767                                  312,075                                      -90,692  -22.52%   x 1.29 
 tests::medium::lookup_missing     403,073                                  306,051                                      -97,022  -24.07%   x 1.32 
 tests::medium::lookup_one         322,310                                  236,379                                      -85,931  -26.66%   x 1.36 
 tests::medium::lookup_random      413,442                                  323,796                                      -89,646  -21.68%   x 1.28 
 tests::medium::random_gen         4,183                                    4,067                                           -116   -2.77%   x 1.03 
 tests::medium::traversal          341,593                                  243,975                                      -97,618  -28.58%   x 1.40 
 tests::small::clone               36,063                                   31,253                                        -4,810  -13.34%   x 1.15 
 tests::small::copy_element_wise   36,840                                   26,096                                       -10,744  -29.16%   x 1.41 
 tests::small::fill_only           25,279                                   21,553                                        -3,726  -14.74%   x 1.17 
 tests::small::insert_random       44,857                                   36,801                                        -8,056  -17.96%   x 1.22 
 tests::small::lookup_all          34,446                                   28,472                                        -5,974  -17.34%   x 1.21 
 tests::small::lookup_missing      36,286                                   29,268                                        -7,018  -19.34%   x 1.24 
 tests::small::lookup_one          25,894                                   21,539                                        -4,355  -16.82%   x 1.20 
 tests::small::lookup_random       36,469                                   29,525                                        -6,944  -19.04%   x 1.24 
 tests::small::random_gen          734                                      734                                                0    0.00%   x 1.00 
 tests::small::traversal           28,355                                   22,041                                        -6,314  -22.27%   x 1.29 
 tests::tiny::clone                3,051                                    2,420                                           -631  -20.68%   x 1.26 
 tests::tiny::copy_element_wise    2,699                                    2,270                                           -429  -15.89%   x 1.19 
 tests::tiny::fill_only            1,989                                    1,730                                           -259  -13.02%   x 1.15 
 tests::tiny::insert_random        3,781                                    3,245                                           -536  -14.18%   x 1.17 
 tests::tiny::lookup_all           2,741                                    2,424                                           -317  -11.57%   x 1.13 
 tests::tiny::lookup_missing       2,890                                    2,591                                           -299  -10.35%   x 1.12 
 tests::tiny::lookup_one           2,206                                    1,839                                           -367  -16.64%   x 1.20 
 tests::tiny::lookup_random        2,927                                    2,565                                           -362  -12.37%   x 1.14 
 tests::tiny::random_gen           127                                      130                                                3    2.36%   x 0.98 
 tests::tiny::traversal            2,211                                    1,753                                           -458  -20.71%   x 1.26 

cmp_string_pad_string_key

 name                              fx_hash_string_pad_string_key ns/iter  hb_map_string_pad_string_key ns/iter  diff ns/iter   diff %  speedup 
 tests::big::clone                 41,272,108                             39,915,804                              -1,356,304   -3.29%   x 1.03 
 tests::big::copy_element_wise     47,931,060                             47,404,488                                -526,572   -1.10%   x 1.01 
 tests::big::fill_only             33,041,000                             25,065,857                              -7,975,143  -24.14%   x 1.32 
 tests::big::insert_random         59,294,617                             48,454,355                             -10,840,262  -18.28%   x 1.22 
 tests::big::lookup_all            50,607,791                             42,484,663                              -8,123,128  -16.05%   x 1.19 
 tests::big::lookup_missing        46,996,493                             37,527,247                              -9,469,246  -20.15%   x 1.25 
 tests::big::lookup_one            33,313,558                             24,501,042                              -8,812,516  -26.45%   x 1.36 
 tests::big::lookup_random         56,667,651                             48,438,774                              -8,228,877  -14.52%   x 1.17 
 tests::big::random_gen            747,711                                752,593                                      4,882    0.65%   x 0.99 
 tests::big::traversal             33,510,878                             24,669,835                              -8,841,043  -26.38%   x 1.36 
 tests::large::clone               3,127,811                              2,494,388                                 -633,423  -20.25%   x 1.25 
 tests::large::copy_element_wise   4,233,900                              3,043,392                               -1,190,508  -28.12%   x 1.39 
 tests::large::fill_only           2,446,574                              1,938,230                                 -508,344  -20.78%   x 1.26 
 tests::large::insert_random       4,096,627                              3,428,168                                 -668,459  -16.32%   x 1.19 
 tests::large::lookup_all          3,681,006                              3,091,176                                 -589,830  -16.02%   x 1.19 
 tests::large::lookup_missing      3,744,526                              3,071,961                                 -672,565  -17.96%   x 1.22 
 tests::large::lookup_one          2,455,870                              1,926,694                                 -529,176  -21.55%   x 1.27 
 tests::large::lookup_random       3,865,338                              3,343,224                                 -522,114  -13.51%   x 1.16 
 tests::large::random_gen          113,971                                113,770                                       -201   -0.18%   x 1.00 
 tests::large::traversal           2,464,109                              1,930,872                                 -533,237  -21.64%   x 1.28 
 tests::medium::clone              290,379                                233,367                                    -57,012  -19.63%   x 1.24 
 tests::medium::copy_element_wise  338,738                                278,766                                    -59,972  -17.70%   x 1.22 
 tests::medium::fill_only          229,274                                184,658                                    -44,616  -19.46%   x 1.24 
 tests::medium::insert_random      372,288                                308,971                                    -63,317  -17.01%   x 1.20 
 tests::medium::lookup_all         343,882                                299,799                                    -44,083  -12.82%   x 1.15 
 tests::medium::lookup_missing     339,559                                290,781                                    -48,778  -14.37%   x 1.17 
 tests::medium::lookup_one         230,504                                187,103                                    -43,401  -18.83%   x 1.23 
 tests::medium::lookup_random      351,303                                306,155                                    -45,148  -12.85%   x 1.15 
 tests::medium::random_gen         4,091                                  4,125                                           34    0.83%   x 0.99 
 tests::medium::traversal          229,764                                187,875                                    -41,889  -18.23%   x 1.22 
 tests::small::clone               24,523                                 22,276                                      -2,247   -9.16%   x 1.10 
 tests::small::copy_element_wise   26,569                                 23,911                                      -2,658  -10.00%   x 1.11 
 tests::small::fill_only           19,684                                 17,451                                      -2,233  -11.34%   x 1.13 
 tests::small::insert_random       35,241                                 29,824                                      -5,417  -15.37%   x 1.18 
 tests::small::lookup_all          31,017                                 27,736                                      -3,281  -10.58%   x 1.12 
 tests::small::lookup_missing      31,904                                 28,895                                      -3,009   -9.43%   x 1.10 
 tests::small::lookup_one          19,634                                 17,228                                      -2,406  -12.25%   x 1.14 
 tests::small::lookup_random       32,051                                 28,830                                      -3,221  -10.05%   x 1.11 
 tests::small::random_gen          734                                    741                                              7    0.95%   x 0.99 
 tests::small::traversal           19,894                                 17,342                                      -2,552  -12.83%   x 1.15 
 tests::tiny::clone                2,149                                  1,854                                         -295  -13.73%   x 1.16 
 tests::tiny::copy_element_wise    1,941                                  2,182                                          241   12.42%   x 0.89 
 tests::tiny::fill_only            1,474                                  1,491                                           17    1.15%   x 0.99 
 tests::tiny::insert_random        2,831                                  2,759                                          -72   -2.54%   x 1.03 
 tests::tiny::lookup_all           2,378                                  2,517                                          139    5.85%   x 0.94 
 tests::tiny::lookup_missing       2,576                                  2,626                                           50    1.94%   x 0.98 
 tests::tiny::lookup_one           1,549                                  1,597                                           48    3.10%   x 0.97 
 tests::tiny::lookup_random        2,570                                  2,679                                          109    4.24%   x 0.96 
 tests::tiny::random_gen           128                                    129                                              1    0.78%   x 0.99 
 tests::tiny::traversal            1,522                                  1,532                                           10    0.66%   x 0.99 

cmp_string_pad_string_value

 name                              fx_hash_string_pad_string_value ns/iter  hb_map_string_pad_string_value ns/iter  diff ns/iter   diff %  speedup 
 tests::big::clone                 31,074,259                               30,330,383                                  -743,876   -2.39%   x 1.02 
 tests::big::copy_element_wise     42,554,324                               28,954,902                               -13,599,422  -31.96%   x 1.47 
 tests::big::fill_only             24,946,656                               18,672,641                                -6,274,015  -25.15%   x 1.34 
 tests::big::insert_random         46,814,633                               41,961,645                                -4,852,988  -10.37%   x 1.12 
 tests::big::lookup_all            24,548,355                               20,463,636                                -4,084,719  -16.64%   x 1.20 
 tests::big::lookup_missing        26,458,442                               19,633,151                                -6,825,291  -25.80%   x 1.35 
 tests::big::lookup_one            25,317,900                               18,534,411                                -6,783,489  -26.79%   x 1.37 
 tests::big::lookup_random         24,982,509                               20,742,252                                -4,240,257  -16.97%   x 1.20 
 tests::big::random_gen            748,150                                  768,670                                       20,520    2.74%   x 0.97 
 tests::big::traversal             24,895,937                               18,558,928                                -6,337,009  -25.45%   x 1.34 
 tests::large::clone               2,546,891                                2,225,389                                   -321,502  -12.62%   x 1.14 
 tests::large::copy_element_wise   6,602,670                                2,090,669                                 -4,512,001  -68.34%   x 3.16 
 tests::large::fill_only           1,911,649                                1,573,061                                   -338,588  -17.71%   x 1.22 
 tests::large::insert_random       3,213,892                                2,934,292                                   -279,600   -8.70%   x 1.10 
 tests::large::lookup_all          1,956,880                                1,638,664                                   -318,216  -16.26%   x 1.19 
 tests::large::lookup_missing      2,102,272                                1,746,804                                   -355,468  -16.91%   x 1.20 
 tests::large::lookup_one          1,917,841                                1,590,708                                   -327,133  -17.06%   x 1.21 
 tests::large::lookup_random       2,070,013                                1,740,617                                   -329,396  -15.91%   x 1.19 
 tests::large::random_gen          113,749                                  114,620                                          871    0.77%   x 0.99 
 tests::large::traversal           1,919,731                                1,594,213                                   -325,518  -16.96%   x 1.20 
 tests::medium::clone              245,905                                  197,544                                      -48,361  -19.67%   x 1.24 
 tests::medium::copy_element_wise  292,915                                  185,138                                     -107,777  -36.79%   x 1.58 
 tests::medium::fill_only          188,804                                  142,932                                      -45,872  -24.30%   x 1.32 
 tests::medium::insert_random      300,795                                  255,091                                      -45,704  -15.19%   x 1.18 
 tests::medium::lookup_all         193,627                                  147,859                                      -45,768  -23.64%   x 1.31 
 tests::medium::lookup_missing     199,230                                  151,758                                      -47,472  -23.83%   x 1.31 
 tests::medium::lookup_one         190,595                                  145,656                                      -44,939  -23.58%   x 1.31 
 tests::medium::lookup_random      198,991                                  151,738                                      -47,253  -23.75%   x 1.31 
 tests::medium::random_gen         4,125                                    4,225                                            100    2.42%   x 0.98 
 tests::medium::traversal          189,972                                  145,092                                      -44,880  -23.62%   x 1.31 
 tests::small::clone               21,990                                   20,150                                        -1,840   -8.37%   x 1.09 
 tests::small::copy_element_wise   24,618                                   18,084                                        -6,534  -26.54%   x 1.36 
 tests::small::fill_only           16,559                                   14,775                                        -1,784  -10.77%   x 1.12 
 tests::small::insert_random       28,461                                   26,172                                        -2,289   -8.04%   x 1.09 
 tests::small::lookup_all          17,859                                   15,087                                        -2,772  -15.52%   x 1.18 
 tests::small::lookup_missing      19,293                                   15,817                                        -3,476  -18.02%   x 1.22 
 tests::small::lookup_one          17,866                                   14,861                                        -3,005  -16.82%   x 1.20 
 tests::small::lookup_random       17,916                                   16,016                                        -1,900  -10.61%   x 1.12 
 tests::small::random_gen          735                                      749                                               14    1.90%   x 0.98 
 tests::small::traversal           17,061                                   14,718                                        -2,343  -13.73%   x 1.16 
 tests::tiny::clone                1,948                                    1,664                                           -284  -14.58%   x 1.17 
 tests::tiny::copy_element_wise    1,732                                    1,674                                            -58   -3.35%   x 1.03 
 tests::tiny::fill_only            1,256                                    1,284                                             28    2.23%   x 0.98 
 tests::tiny::insert_random        2,576                                    2,450                                           -126   -4.89%   x 1.05 
 tests::tiny::lookup_all           1,307                                    1,337                                             30    2.30%   x 0.98 
 tests::tiny::lookup_missing       1,460                                    1,464                                              4    0.27%   x 1.00 
 tests::tiny::lookup_one           1,342                                    1,311                                            -31   -2.31%   x 1.02 
 tests::tiny::lookup_random        1,477                                    1,497                                             20    1.35%   x 0.99 
 tests::tiny::random_gen           129                                      128                                               -1   -0.78%   x 1.01 
 tests::tiny::traversal            1,373                                    1,297                                            -76   -5.54%   x 1.06 

cmp_string_pad_string_key_string_value

 name                              fx_hash_string_pad_string_key_string_value ns/iter  hb_map_string_pad_string_key_string_value ns/iter  diff ns/iter   diff %  speedup 
 tests::big::clone                 68,565,017                                          72,475,248                                            3,910,231    5.70%   x 0.95 
 tests::big::copy_element_wise     65,421,891                                          75,619,984                                           10,198,093   15.59%   x 0.87 
 tests::big::fill_only             49,130,177                                          45,038,534                                           -4,091,643   -8.33%   x 1.09 
 tests::big::insert_random         93,900,269                                          84,954,632                                           -8,945,637   -9.53%   x 1.11 
 tests::big::lookup_all            68,206,790                                          64,794,378                                           -3,412,412   -5.00%   x 1.05 
 tests::big::lookup_missing        62,777,908                                          57,459,080                                           -5,318,828   -8.47%   x 1.09 
 tests::big::lookup_one            49,071,165                                          45,036,272                                           -4,034,893   -8.22%   x 1.09 
 tests::big::lookup_random         75,054,379                                          71,623,767                                           -3,430,612   -4.57%   x 1.05 
 tests::big::random_gen            764,731                                             749,908                                                 -14,823   -1.94%   x 1.02 
 tests::big::traversal             51,439,727                                          45,364,081                                           -6,075,646  -11.81%   x 1.13 
 tests::large::clone               5,004,835                                           4,213,762                                              -791,073  -15.81%   x 1.19 
 tests::large::copy_element_wise   5,447,282                                           4,220,486                                            -1,226,796  -22.52%   x 1.29 
 tests::large::fill_only           3,775,658                                           3,097,579                                              -678,079  -17.96%   x 1.22 
 tests::large::insert_random       6,499,488                                           5,580,395                                              -919,093  -14.14%   x 1.16 
 tests::large::lookup_all          5,040,883                                           4,317,939                                              -722,944  -14.34%   x 1.17 
 tests::large::lookup_missing      5,042,517                                           4,222,034                                              -820,483  -16.27%   x 1.19 
 tests::large::lookup_one          3,780,753                                           3,113,636                                              -667,117  -17.65%   x 1.21 
 tests::large::lookup_random       5,258,342                                           4,543,783                                              -714,559  -13.59%   x 1.16 
 tests::large::random_gen          114,543                                             114,491                                                     -52   -0.05%   x 1.00 
 tests::large::traversal           3,992,131                                           3,183,968                                              -808,163  -20.24%   x 1.25 
 tests::medium::clone              476,912                                             394,278                                                 -82,634  -17.33%   x 1.21 
 tests::medium::copy_element_wise  518,023                                             375,221                                                -142,802  -27.57%   x 1.38 
 tests::medium::fill_only          364,070                                             294,470                                                 -69,600  -19.12%   x 1.24 
 tests::medium::insert_random      597,360                                             505,532                                                 -91,828  -15.37%   x 1.18 
 tests::medium::lookup_all         480,594                                             403,716                                                 -76,878  -16.00%   x 1.19 
 tests::medium::lookup_missing     478,492                                             394,552                                                 -83,940  -17.54%   x 1.21 
 tests::medium::lookup_one         365,828                                             294,687                                                 -71,141  -19.45%   x 1.24 
 tests::medium::lookup_random      492,067                                             413,467                                                 -78,600  -15.97%   x 1.19 
 tests::medium::random_gen         4,281                                               4,056                                                      -225   -5.26%   x 1.06 
 tests::medium::traversal          385,080                                             305,391                                                 -79,689  -20.69%   x 1.26 
 tests::small::clone               40,645                                              37,194                                                   -3,451   -8.49%   x 1.09 
 tests::small::copy_element_wise   41,909                                              33,237                                                   -8,672  -20.69%   x 1.26 
 tests::small::fill_only           31,423                                              27,422                                                   -4,001  -12.73%   x 1.15 
 tests::small::insert_random       55,027                                              48,787                                                   -6,240  -11.34%   x 1.13 
 tests::small::lookup_all          42,130                                              37,757                                                   -4,373  -10.38%   x 1.12 
 tests::small::lookup_missing      43,490                                              38,896                                                   -4,594  -10.56%   x 1.12 
 tests::small::lookup_one          31,155                                              27,891                                                   -3,264  -10.48%   x 1.12 
 tests::small::lookup_random       43,896                                              39,166                                                   -4,730  -10.78%   x 1.12 
 tests::small::random_gen          743                                                 735                                                          -8   -1.08%   x 1.01 
 tests::small::traversal           33,762                                              28,222                                                   -5,540  -16.41%   x 1.20 
 tests::tiny::clone                3,634                                               3,012                                                      -622  -17.12%   x 1.21 
 tests::tiny::copy_element_wise    3,195                                               2,929                                                      -266   -8.33%   x 1.09 
 tests::tiny::fill_only            2,490                                               2,328                                                      -162   -6.51%   x 1.07 
 tests::tiny::insert_random        4,668                                               4,311                                                      -357   -7.65%   x 1.08 
 tests::tiny::lookup_all           3,453                                               3,309                                                      -144   -4.17%   x 1.04 
 tests::tiny::lookup_missing       3,601                                               3,425                                                      -176   -4.89%   x 1.05 
 tests::tiny::lookup_one           2,632                                               2,429                                                      -203   -7.71%   x 1.08 
 tests::tiny::lookup_random        3,601                                               3,453                                                      -148   -4.11%   x 1.04 
 tests::tiny::random_gen           128                                                 127                                                          -1   -0.78%   x 1.01 
 tests::tiny::traversal            2,700                                               2,342                                                      -358  -13.26%   x 1.15 

[arthurprs]

To be clear I'm not saying it's going to be slower or anything like that. We just need to look into the worst case.

[michaelwoerister]

These results look pretty good!

Is there a reason why the new hashmap impl can't cache hash values?

[Gankro]

One of the major improvements of hashbrown over our current design is to truncate the hash array to be byte entries instead of 8 bytes. This significantly improves cache locality of the hash search under high load factors, reduces the memory footprint of all our hashtables, and allows 4, 8, or 16 hashes to be compared at once (on 32-bit, 64-bit, and sse2 platforms respectively).

This comes at the cost of only storing 7 bits of the hash (you still need one bit for empty/full). So in theory we could avoid rehashing up to a capacity of 128, but past that we need to rehash to retrieve the higher bits.

[Amanieu]

This comes at the cost of only storing 7 bits of the hash (you still need one bit for empty/full). So in theory we could avoid rehashing up to a capacity of 128, but past that we need to rehash to retrieve the higher bits.

The 7 bits stored in the table are taken from the top bits of the hash code, while the bits used to determine the initial search position in the array are taken from the bottom bits. This is important since it means that when we find a matching control byte in the table, we have actually matched both the top and bottom bits of the hash code, which significantly decreases the chance of false positives.

However this means that these 7 bits are completely useless when rehashing, since they don't contain the bits used for determining the position in the array.

[Gankro]

Ah dang, well in my defense I haven't gotten to that part of the code review yet :)

[michaelwoerister]

Ah, interesting. Thanks for the info!