Collect VTable stats & add `-Zprint-vtable-sizes` #112400

WaffleLapkin · 2023-06-07T18:50:42Z

This is a bit hacky/buggy, but I'm not entirely sure how to fix it, so I want to ask reviewers for help...

To try this, use either of those:

cargo clean && RUSTFLAGS="-Zprint-vtable-sizes" cargo +toolchain b
cargo clean && cargo rustc +toolchain -Zprint-vtable-sizes
rustc +toolchain -Zprint-vtable-sizes ./file.rs

rustbot · 2023-06-07T18:50:47Z

r? @b-naber

(rustbot has picked a reviewer for you, use r? to override)

compiler/rustc_interface/src/passes.rs

compiler/rustc_session/src/code_stats.rs

WaffleLapkin · 2023-06-07T18:53:20Z

r? @compiler-errors

compiler/rustc_interface/src/passes.rs

…=Nilstrieb Don't `use compile_error as print` I've spent **1.5 hours** debugging this while trying to compile rust-lang#112400, if we use `compile_error!`, we should not just forward user input to it, but issue a reasonable error message. The better solution would be to use a lint like `clippy::print_stdout`, but since we don't have clippy in CI, let's at least make the macro error better. Also note that some functions called here actually do use `println` (see for example `print_type_sizes` function).

WaffleLapkin · 2023-06-12T16:17:41Z

So, status update:

I've fixed both problematic examples I noted above
I've also removed vtable_entries_no_resolve in favor of counting entries by hand, as suggested by @compiler-errors
I've added a skip of object-unsafe traits
I've also squashed all the fixes together

With this I think this is ready for review.

Some things we might want to fix:

I've made some things pub which maybe shouldn't be pub
Naming is somewhat off, for example size_words_without_upcasting is ehhh
There are no tests. There probably should be at least some tests.

compiler/rustc_interface/src/passes.rs

compiler-errors · 2023-06-12T16:35:33Z

compiler/rustc_interface/src/passes.rs

+                        let existential_trait_ref = trait_ref.map_bound(|trait_ref| {
+                            ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
+                        });
+
+                        // Lookup the shape of vtable for the trait.
+                        let own_existential_entries =
+                            tcx.own_existential_vtable_entries(existential_trait_ref.def_id());


Suggested change

let existential_trait_ref = trait_ref.map_bound(|trait_ref| {

ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)

});

// Lookup the shape of vtable for the trait.

let own_existential_entries =

tcx.own_existential_vtable_entries(existential_trait_ref.def_id());

// Lookup the shape of vtable for the trait.

let own_existential_entries =

tcx.own_existential_vtable_entries(trait_ref.def_id());

compiler/rustc_interface/src/passes.rs

compiler-errors · 2023-06-12T16:37:31Z

compiler/rustc_interface/src/passes.rs

+                    size_words_without_upcasting: unupcasted_cost,
+                    size_words_with_upcasting: unupcasted_cost + upcast_cost,
+                    difference_words: upcast_cost,
+                    difference_percent: upcast_cost as f64 / unupcasted_cost as f64 * 100.,


why isnt this (size_words_without_upcasting + size_words_with_upcasting) / size_words_without_upcasting... not sure if i totally understand the math here, that could use some explanation i think 😅

nvm, I think I get it. This creates a "how much % larger is the new vtable compared to the old table". could use doc tho

compiler-errors · 2023-06-12T16:38:48Z

compiler/rustc_interface/src/passes.rs

+                        let own_entries = own_existential_entries.iter().copied().map(|_def_id| {
+                            // The original code here ignores the method if its predicates are impossible.
+                            // We can't really do that as, for example, all not trivial bounds on generic
+                            // parameters are impossible (since we don't know the parameters...),
+                            // see the comment above.
+
+                            1
+                        });
+
+                        unupcasted_cost += own_entries.sum::<usize>();


Can you uplift this comment and just use count 😅

WaffleLapkin · 2023-06-13T16:37:34Z

@compiler-errors I've simplified the code, added some more docs, renamed some variables to make sense and finally added a test. I think this should be ready

compiler-errors · 2023-06-13T17:55:43Z

@bors r+

bors · 2023-06-13T17:55:47Z

📌 Commit af4631a has been approved by compiler-errors

It is now in the queue for this repository.

bors · 2023-06-14T11:24:45Z

⌛ Testing commit af4631a with merge 7b0eac4...

bors · 2023-06-14T14:20:52Z

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing 7b0eac4 to master...

rust-timer · 2023-06-14T16:35:14Z

Finished benchmarking commit (7b0eac4): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.9%	[0.9%, 0.9%]	1
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.3%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.3%	[-0.3%, -0.3%]	2

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.7%	[3.7%, 3.7%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.8%	[-0.1%, 3.7%]	2

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	5.4%	[5.2%, 5.6%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 648.428s -> 648.095s (-0.05%)

lqd · 2023-06-14T18:59:41Z

I'm pretty sure the perf changes are noise 😅

kornelski · 2023-06-22T02:57:44Z

Is this meant to display sizes for traits that are never used with dyn? I have a crate-private trait that is only ever used with a monomorphic implementation, but I still see print-vtable-sizes line for it.

WaffleLapkin · 2023-06-22T07:46:17Z

@kornelski yes, this is expected. theoretically it'd be possible to filter out private & not used with dyn traits, but it's not trivial...

rustbot assigned b-naber Jun 7, 2023

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 7, 2023

WaffleLapkin commented Jun 7, 2023

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rustbot assigned compiler-errors and unassigned b-naber Jun 7, 2023

This was referenced Jun 7, 2023

add feature gate to permit measuring memory usage of dyn upcasting #112355

Closed

Don't use compile_error as print #112401

Merged

compiler-errors reviewed Jun 7, 2023

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rust-cloud-vms bot force-pushed the vtable_stats branch from ff566a0 to 88095f5 Compare June 12, 2023 15:49

Collect VTable stats & add -Zprint-vtable-sizes

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rust-cloud-vms bot force-pushed the vtable_stats branch from 88095f5 to f2545fb Compare June 12, 2023 15:58

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WaffleLapkin added 3 commits June 13, 2023 11:46

Simplify code as suggested by the review

5008a08

Tweak names and docs for vtable stats

8e6a193

Tweak the sort of vtable sizes

dc0fba0

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Add a test for -Zprint-vtable-sizes

af4631a

rust-cloud-vms bot force-pushed the vtable_stats branch from 2bef9e9 to af4631a Compare June 13, 2023 15:17

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bors removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jun 13, 2023

bors added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Jun 13, 2023

bors added the merged-by-bors This PR was explicitly merged by bors. label Jun 14, 2023

bors merged commit 7b0eac4 into rust-lang:master Jun 14, 2023

rustbot added this to the 1.72.0 milestone Jun 14, 2023

lqd mentioned this pull request Jun 14, 2023

Add support for targets without unwinding in mir-opt, and improve --bless for it #112418

Merged

WaffleLapkin deleted the vtable_stats branch June 15, 2023 10:46

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Collect VTable stats & add `-Zprint-vtable-sizes` #112400

Collect VTable stats & add `-Zprint-vtable-sizes` #112400

WaffleLapkin commented Jun 7, 2023

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kornelski commented Jun 22, 2023

WaffleLapkin commented Jun 22, 2023

Collect VTable stats & add -Zprint-vtable-sizes #112400

Collect VTable stats & add -Zprint-vtable-sizes #112400

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bors commented Jun 14, 2023

rust-timer commented Jun 14, 2023

Overall result: ❌✅ regressions and improvements - no action needed

lqd commented Jun 14, 2023

kornelski commented Jun 22, 2023

WaffleLapkin commented Jun 22, 2023

Collect VTable stats & add `-Zprint-vtable-sizes` #112400

Collect VTable stats & add `-Zprint-vtable-sizes` #112400