Pass alloc-variant-zeroed to LLVM

[clubby789]

Makes use of llvm/llvm-project#138299 (once we pull in a version of LLVM with this attribute). Unfortunately also requires llvm/llvm-project#149336 to work.

Closes #104847

[bors]

☔ The latest upstream changes (presumably #144249) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

r? @fee1-dead

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[clubby789]

r? compiler

edit: oops I guess that's automatic 😅

[nikic]

@bors try @rust-timer queue

[rust-bors]

💔 Test for f993fda failed: CI. Failed jobs:

• try - dist-x86_64-linux (web logs, extended logs)

[nikic]

I think the alloc symbols are mangled nowadays, so we need to pass the symbols with the proper mangling.

[clubby789]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 2217f3f (2217f3f4ed5671c4c8e939f81fa2b0b1e68806d2, parent: 350d0ef0ec0493e6d21cfb265cb8211a0e74d766)

[rust-timer]

Finished benchmarking commit (2217f3f): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.1%, 14.7%]	67
Regressions ❌ (secondary)	0.3%	[0.1%, 1.2%]	29
Improvements ✅ (primary)	-0.5%	[-0.9%, -0.1%]	2
Improvements ✅ (secondary)	-0.3%	[-0.5%, -0.2%]	12
All ❌✅ (primary)	0.4%	[-0.9%, 14.7%]	69

Max RSS (memory usage)

Results (primary 0.7%, secondary 0.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.8%	[2.8%, 2.8%]	1
Regressions ❌ (secondary)	3.0%	[3.0%, 3.0%]	1
Improvements ✅ (primary)	-1.4%	[-1.4%, -1.4%]	1
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	0.7%	[-1.4%, 2.8%]	2

Cycles

Results (primary 13.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	13.6%	[13.6%, 13.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	13.6%	[13.6%, 13.6%]	1

Binary size

Results (primary -0.1%, secondary 0.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.3%	[0.1%, 1.7%]	7
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	3
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	2
All ❌✅ (primary)	-0.1%	[-0.1%, -0.1%]	3

Bootstrap: 468.291s -> 469.62s (0.28%)
Artifact size: 377.36 MiB -> 377.37 MiB (0.00%)

[clubby789]

Hmm, I'm surprised at the impact here, given that there isn't a hugely significant amount of work done on LLVM or Rust's side. Maybe this just causes the optimisation to fire much more often and/or the folding allows further optimisations