Perform SimplifyLocals before ConstProp.

[cjgillot]

MIR before ConstProp may have a lot of dead writes, this makes ConstProp do unnecessary work.

r? @ghost

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[cjgillot]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 188cf4001097ba58feedb4fd396ee1412b6a1bb6 with merge b7447ffcd791c86d0621a163caf0d6d11bbf3e65...

[bors]

☀️ Try build successful - checks-actions
Build commit: b7447ffcd791c86d0621a163caf0d6d11bbf3e65 (b7447ffcd791c86d0621a163caf0d6d11bbf3e65)

[rust-timer]

Finished benchmarking commit (b7447ffcd791c86d0621a163caf0d6d11bbf3e65): comparison URL.

Overall result: ✅ improvements - no 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.

@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.7%	[-0.9%, -0.6%]	4
Improvements ✅ (secondary)	-0.5%	[-0.6%, -0.5%]	3
All ❌✅ (primary)	-0.7%	[-0.9%, -0.6%]	4

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.

	mean	range	count
Regressions ❌ (primary)	2.8%	[2.6%, 3.0%]	2
Regressions ❌ (secondary)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-1.6%	[-3.2%, -0.0%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[-3.2%, 3.0%]	4

Cycles

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

[cjgillot]

r? compiler

[davidtwco]

@bors r+

[bors]

📌 Commit a48127f has been approved by davidtwco

It is now in the queue for this repository.

[bors]

⌛ Testing commit a48127f with merge b1691f6...

[bors]

☀️ Test successful - checks-actions
Approved by: davidtwco
Pushing b1691f6 to master...

[rust-timer]

Finished benchmarking commit (b1691f6): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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)	0.7%	[0.2%, 1.6%]	4
Regressions ❌ (secondary)	0.6%	[0.6%, 0.6%]	1
Improvements ✅ (primary)	-0.6%	[-0.8%, -0.5%]	4
Improvements ✅ (secondary)	-0.5%	[-0.5%, -0.5%]	1
All ❌✅ (primary)	0.0%	[-0.8%, 1.6%]	8

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.

	mean	range	count
Regressions ❌ (primary)	9.2%	[9.1%, 9.4%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.3%	[-2.3%, -2.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	5.4%	[-2.3%, 9.4%]	3

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.

	mean	range	count
Regressions ❌ (primary)	2.0%	[2.0%, 2.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.0%	[2.0%, 2.0%]	1

[Mark-Simulacrum]

Regression appears to not be noise (at least in serde-derive). The previous perf run didn't show that regression, so I wonder if there were perhaps changes made after it, @cjgillot, that may have led to a regression were there wasn't one before?

[saethlin]

I saw commentary about this in the t-compiler meeting today.

I have seen fluctuations of almost exactly that magnitude in serde_derive opt+full. The syn opt+full benchmark has a pretty incredible significance threshold, and I think whatever instability is driving that is bleeding into serde_derive.

[Mark-Simulacrum]

https://perf.rust-lang.org/index.html?start=2022-12-08&end=b1691f6413a4afd7529ab309965123cc63e82021&benchmark=serde_derive-1.0.136&profile=opt&scenario=full&stat=instructions%3Au&kind=percentrelative:

This last change is the last data point there -- it seems pretty clearly outside the normal fluctuation range. I don't see corresponding negative changes, so I think it's not just instability that settles back to net-zero after some time.

[saethlin]

Ah sorry I think I wasn't clear. I'm not saying this is noise. I'm saying that small perturbations to the input produce hard-to-predict swings of about this magnitude to this benchmark (examples of what I've seen below). And if that is the case, the regression in this benchmark is real, but the next perturbation to MIR optimizations is reasonably likely to accidentally cause the benchmark to swing back to its previous state.

https://perf.rust-lang.org/compare.html?start=7ac9572c48435b5342ad3550d6036bde835d37dc&end=11d6ba903e6762c93f69ccf2ece4c445cfc575ae&stat=instructions:u
https://perf.rust-lang.org/compare.html?start=7ac9572c48435b5342ad3550d6036bde835d37dc&end=ebf0d2d3df7ddf24af902c6cb1401231a299dc2d&stat=instructions:u
https://perf.rust-lang.org/compare.html?start=cc47b069983292e4ee8982d5dabe6301452c5f25&end=0c68d2596c4617e0a308139af6ad56d4ac858bf1&stat=instructions:u
https://perf.rust-lang.org/compare.html?start=657eefe2dcf18f76ac67a39945810128e101178c&end=8e6760e82a081fb624e777d52ef744c1def2f593&stat=instructions:u

---

But also squinting at this graph isn't exactly a statistical test, so who knows, this might actually be noise.