[EXPERIMENT] Use wider types in Layout multiplication

[scottmcm]

This lets us phrase it as just one check, rather than two, and might make it easier on LLVM to optimize. It still passes the codegen test from #99174 without needing the manual optimization anymore, so let's see whether perf likes it.

We've picked up llvm/llvm-project#56563, so LLVM is now smarter about optimizing mul nuw with constants, which is what this is frequently emitting (because the type size often comes from a generic type parameter).

The IR/ASM looks pretty good for this approach too. For comparisons, see

• https://rust.godbolt.org/z/ssjr1Yeas (64-bit)
• https://rust.godbolt.org/z/9f8c45b7W (32-bit)

cc @CAD97 & #99117

[rust-highfive]

r? @thomcc

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[scottmcm]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[bors]

⌛ Trying commit d8ed3d2 with merge 1bd20b5b3b52926ba90895b739c9b4b5472f4adf...

[bors]

☀️ Try build successful - checks-actions
Build commit: 1bd20b5b3b52926ba90895b739c9b4b5472f4adf (1bd20b5b3b52926ba90895b739c9b4b5472f4adf)

[rust-timer]

Queued 1bd20b5b3b52926ba90895b739c9b4b5472f4adf with parent d0ea1d7, future comparison URL.

[rust-timer]

Finished benchmarking commit (1bd20b5b3b52926ba90895b739c9b4b5472f4adf): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

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.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

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

	mean1	max	count2
Regressions ❌ (primary)	1.0%	2.1%	33
Regressions ❌ (secondary)	0.9%	1.7%	6
Improvements ✅ (primary)	-0.5%	-0.6%	5
Improvements ✅ (secondary)	-0.5%	-0.7%	20
All ❌✅ (primary)	0.8%	2.1%	38

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.

	mean1	max	count2
Regressions ❌ (primary)	2.1%	2.2%	3
Regressions ❌ (secondary)	2.2%	2.2%	1
Improvements ✅ (primary)	-7.1%	-14.8%	3
Improvements ✅ (secondary)	-2.1%	-3.3%	4
All ❌✅ (primary)	-2.5%	-14.8%	6

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.

	mean1	max	count2
Regressions ❌ (primary)	2.4%	2.9%	5
Regressions ❌ (secondary)	3.5%	5.1%	10
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	2.9%	5

Footnotes

1. the arithmetic mean of the percent change ↩ ↩² ↩³

2. number of relevant changes ↩ ↩² ↩³

[scottmcm]

Some surprising wins in doc builds, somehow, but clearly not a good change overall.