optimise for size when compiling rust

[tomcraven]

optimising for size with rust produces a significant difference, comparable to nim on my machine

$ rustc -O fib.rs && hyperfine ./fib
Benchmark #1: ./fib
  Time (mean ± σ):      6.878 s ±  0.036 s    [User: 6.862 s, System: 0.009 s]
  Range (min … max):    6.822 s …  6.948 s

$ rustc -C opt-level=s fib.rs && hyperfine ./fib
Benchmark #1: ./fib
  Time (mean ± σ):      5.334 s ±  0.035 s    [User: 5.320 s, System: 0.006 s]
  Range (min … max):    5.284 s …  5.386 s

$ nim cpp -d:release fib.nim && hyperfine ./fib
Hint: used config file '/<redacted>/.choosenim/toolchains/nim-0.19.0/config/nim.cfg' [Conf]
Hint: system [Processing]
Hint: fib [Processing]
Hint:  [Link]
Hint: operation successful (12272 lines compiled; 0.214 sec total; 13.191MiB peakmem; Release Build) [SuccessX]
Benchmark #1: ./fib
  Time (mean ± σ):      5.338 s ±  0.030 s    [User: 5.324 s, System: 0.007 s]
  Range (min … max):    5.286 s …  5.385 s

[CryZe]

The ASM between both opt-level=s and opt-level=3 (or 2) is exactly the same, so I'm not entirely sure where these performance differences come from

[CryZe]

Here's the comparison: https://godbolt.org/z/d6504t

[drujensen]

Let me know if this is a normal flag for Rust. I am ok with changing it. It shaved ~ 0.5s from the results on my machine. 5.817 vs 6.438

[CryZe]

Yeah they stabilized this flag a few releases ago. You can integrate it if you want.

[xeoneux]

Same for me 😄

[drujensen]

done. Although I think the upgrade to Mojave had an affect on the results. I plan on moving this to linux soon.

[tomcraven]

8e64856

The time for the rust version increased?

[drujensen]

yes. Crystal slowed by a whole second as well. not sure the cause. LLVM?

[drujensen]

maybe i should change it back?

$rustc -O fib.rs
drujensen:~/workspace/crystal/fib (master)
$time ./fib
2971215073

real	0m6.363s
user	0m6.352s
sys	0m0.006s
drujensen:~/workspace/crystal/fib (master)
$rustc -C opt-level=s fib.rs
drujensen:~/workspace/crystal/fib (master)
$time ./fib
2971215073

real	0m6.980s
user	0m6.970s
sys	0m0.006s

[tommket]

Why not use opt-level=3 for speed optimization? It makes more sense when trying to benchmark for speed.
On my machine it was with:
stable-x86_64-unknown-linux-gnu updated - rustc 1.33.0 (2aa4c46cf 2019-02-28)
opt-level=s - around 6.0 +- 0.1 seconds
opt-level=3 - around 5.0 +- 0.1 seconds
As I just checked on godbolt the resulting assemblies are different: godbolt comparison of opt-level=s and opt-level=3.