avoid phi node for pointers flowing into Vec appends

[the8472]

related discussion: https://rust-lang.zulipchat.com/#narrow/stream/187780-t-compiler.2Fwg-llvm/topic/nocapture.20and.20allocation.20elimination

r? ghost

[the8472]

On its own I don't expect this to do much, we also need llvm/llvm-project#110280 to get memcpy propagation.

But lets see what the perf impact is without the LLVM changes.

@bors try @rust-timer queue

[bors]

⌛ Trying commit 9a03f37 with merge fa341e6...

[bors]

☀️ Try build successful - checks-actions
Build commit: fa341e6 (fa341e6bb26c4367ebce0bf9e5583eb3df53c79f)

[rust-timer]

Finished benchmarking commit (fa341e6): 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-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.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.3%, 0.9%]	5
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.3%]	2
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	0.3%	[-0.4%, 0.9%]	7

Max RSS (memory usage)

Results (primary -0.1%, secondary 0.5%)

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)	2.5%	[2.5%, 2.5%]	1
Regressions ❌ (secondary)	1.7%	[1.7%, 1.7%]	1
Improvements ✅ (primary)	-2.6%	[-2.6%, -2.6%]	1
Improvements ✅ (secondary)	-0.7%	[-0.7%, -0.7%]	1
All ❌✅ (primary)	-0.1%	[-2.6%, 2.5%]	2

Cycles

Results (secondary -0.3%)

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)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.1%	[-3.1%, -3.1%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.0%, secondary 0.0%)

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)	0.0%	[0.0%, 0.0%]	13
Regressions ❌ (secondary)	0.0%	[0.0%, 0.1%]	31
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[-0.1%, 0.0%]	14

Bootstrap: 769.092s -> 770.401s (0.17%)
Artifact size: 341.40 MiB -> 341.45 MiB (0.01%)

[the8472]

As expected it doesn't do much on its own, let's wait for the LLVM change.

[the8472]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[bors]

⌛ Trying commit c44e0f4 with merge fd59d69...

[bors]

☀️ Try build successful - checks-actions
Build commit: fd59d69 (fd59d692ac24ed3d88de1b531716c0c053cc7680)

[rust-timer]

Queued fd59d69 with parent fb4aebd, future comparison URL.
There are currently 0 preceding artifacts in the queue.
It will probably take at least ~1.3 hours until the benchmark run finishes.