[perf experiment] Don't emit noalias for box when compiling rustc itself

[Nilstrieb]

This compiles rustc without noalias on box to evaluate the performance impact of such a change on a large scale project like rustc.

Only add it for stage1 compiling stage2, since beta doesn't have the flag yet and stage2 is running the benchmarks.

Most importantly, this does NOT change the emitting of noalias when compiling crates other than rustc, to avoid perf impacts from LLVM having to process less attributes like in #98017.

r? @RalfJung

[RalfJung]

I guess the "r?" is a request for a perf run?
@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit fd92015 with merge 0f733da0e9625176a5a6f82c73e1f78d08c44b88...

[Nilstrieb]

Thanks! Let's see where this goes and whether there are regressions (or if I messed up the bootstrap :D)

[bors]

☀️ Try build successful - checks-actions
Build commit: 0f733da0e9625176a5a6f82c73e1f78d08c44b88 (0f733da0e9625176a5a6f82c73e1f78d08c44b88)

[rust-timer]

Queued 0f733da0e9625176a5a6f82c73e1f78d08c44b88 with parent a7468c6, future comparison URL.

[rust-timer]

Finished benchmarking commit (0f733da0e9625176a5a6f82c73e1f78d08c44b88): comparison url.

Instruction count

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

Max RSS (memory usage)

Results

• Primary benchmarks: 🎉 relevant improvements found
• Secondary benchmarks: 🎉 relevant improvements found

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	N/A	N/A	0
Improvements 🎉 (primary)	-4.0%	-5.4%	2
Improvements 🎉 (secondary)	-2.9%	-3.9%	3
All 😿🎉 (primary)	-4.0%	-5.4%	2

Cycles

Results

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: 😿 relevant regression found

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	1.9%	1.9%	1
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	N/A	N/A	0
All 😿🎉 (primary)	N/A	N/A	0

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf -perf-regression

Footnotes

1. the arithmetic mean of the percent change ↩ ↩²

2. number of relevant changes ↩ ↩²

[Nilstrieb]

That's pretty cool. Now we need to know whether there are benefits for other crates, I'd encourage anyone willing to help to run some ecosystem crates benchmarks with the -Zbox-noalias=no RUSTFLAGS and see whether anything changes. If it doesn't, it seems like the way forward is to least disable noalias to mitigate any potential LLVM UB by currently invalid box usage, and consider this for the discussion around Boxes place in SB.

[Nilstrieb]

I will now close this, as this was only a perf experiment.