[perf experiments] Clone all MIR bodies

[saethlin]

We keep saying things like cloning all the MIR would be expensive, but... how expensive actually?

Based on this PR, it looks like the literal clone is cheap enough to be considered free, but running basically any MIR transform on the cloned MIR is a measurable slowdown. The hope here was that we could do optimizations on monomorphized MIR. I think ther eare two problems with that:

The first problem is this:

rust/compiler/rustc_mir_transform/src/add_call_guards.rs

Line 28 in 032be6f

* NOTE: Simplify CFG will happily undo most of the work this pass does.

LLVM needs a certain structure around calls, which is generally wasteful in terms of the MIR that it takes to generate. We currently cope with this by creating the required structure at the end of the optimized_mir passes. This is very problematic, because the most obvious post-mono optimization is running something like SimplifyCfg to remove if true and if false and the resultant goto chains, but that deletes all the work of AddCallGuard, so now you need to run at least two post-mono MIR transforms.

The second problem is that naively monomorphizing the entire Body doesn't skip unreachable blocks. I tried adding some code that uses the same reachability analysis to skip dead code, but something subtle happens that causes one of our secondary benchmarks to regress a lot. That's this perf run: #125025 (comment). I never figured out what causes this slowdown. The later perf runs are me trying to figure out what that's from.

[rustbot]

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[saethlin]

Oops, sorry about that.

@bors try @rust-timer queue

[bors]

⌛ Trying commit bedbc3c with merge c0bcf54...

[bors]

☀️ Try build successful - checks-actions
Build commit: c0bcf54 (c0bcf54db18d2b0d37a4f4b8b4bf1fca822c49f8)

[rust-timer]

Finished benchmarking commit (c0bcf54): comparison URL.

Overall result: no relevant changes - 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 benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

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

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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 673.703s -> 675.385s (0.25%)
Artifact size: 315.98 MiB -> 315.88 MiB (-0.03%)

[saethlin]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 940cb4a with merge f0a6c88...

[bors]

☀️ Try build successful - checks-actions
Build commit: f0a6c88 (f0a6c88ce9ed0e6f6081dedbc069ad2802ba903c)

[rust-timer]

Finished benchmarking commit (f0a6c88): comparison URL.

Overall result: no relevant changes - 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 benchmark run did not return any relevant results for this metric.

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)	4.0%	[4.0%, 4.0%]	1
Regressions ❌ (secondary)	2.8%	[2.8%, 2.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.0%	[4.0%, 4.0%]	1

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)	0.9%	[0.9%, 0.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.1%	[-2.1%, -2.1%]	1
All ❌✅ (primary)	0.9%	[0.9%, 0.9%]	1

Binary size

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

Bootstrap: 676.399s -> 675.548s (-0.13%)
Artifact size: 315.85 MiB -> 316.04 MiB (0.06%)

[RalfJung]

Hm... so maybe Miri should clone the body and apply the substitution once, rather than applying it on each statement? For hot loops that should help a lot. But for big functions where we only run a small part of the code, less so.

[rust-timer]

Finished benchmarking commit (6fd1dde): comparison URL.

Overall result: ❌ regressions - 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)	0.6%	[0.2%, 1.6%]	205
Regressions ❌ (secondary)	0.8%	[0.1%, 18.7%]	82
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.2%, 1.6%]	205

Max RSS (memory usage)

Results (primary -2.3%, secondary 1.2%)

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)	2.9%	[2.4%, 3.5%]	2
Improvements ✅ (primary)	-2.3%	[-4.6%, -0.7%]	4
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	-2.3%	[-4.6%, -0.7%]	4

Cycles

Results (primary 1.1%, secondary -1.1%)

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)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	14.7%	[14.7%, 14.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-5.9%, -2.2%]	7
All ❌✅ (primary)	1.1%	[1.1%, 1.1%]	1

Binary size

Results (primary -0.4%, secondary -0.2%)

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)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.7%, -0.1%]	65
Improvements ✅ (secondary)	-0.2%	[-0.5%, -0.1%]	18
All ❌✅ (primary)	-0.4%	[-0.7%, -0.1%]	65

Bootstrap: 671.52s -> 677.804s (0.94%)
Artifact size: 320.35 MiB -> 927.58 MiB (189.55%)

[saethlin]

@bors try @rust-timer queue

[bors]

⌛ Trying commit c49fd8f with merge 7da58b9...

[bors]

💔 Test failed - checks-actions

[saethlin]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 17184f8 with merge e881ea0...

[bors]

☀️ Try build successful - checks-actions
Build commit: e881ea0 (e881ea00203b2885924cf09b9387f3d24b6b6143)

[rust-timer]

Finished benchmarking commit (e881ea0): comparison URL.

Overall result: ❌ regressions - 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%	[0.2%, 1.6%]	168
Regressions ❌ (secondary)	0.7%	[0.1%, 18.5%]	73
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[0.2%, 1.6%]	168

Max RSS (memory usage)

Results (primary -3.2%, secondary 2.5%)

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)	-	-	0
Regressions ❌ (secondary)	2.5%	[2.1%, 3.2%]	3
Improvements ✅ (primary)	-3.2%	[-4.4%, -2.6%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.2%	[-4.4%, -2.6%]	4

Cycles

Results (secondary 3.0%)

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
Regressions ❌ (secondary)	8.5%	[2.7%, 14.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-2.8%, -2.3%]	2
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.4%, secondary -0.2%)

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)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.7%, -0.1%]	65
Improvements ✅ (secondary)	-0.2%	[-0.5%, -0.1%]	18
All ❌✅ (primary)	-0.4%	[-0.7%, -0.1%]	65

Bootstrap: 671.997s -> 675.131s (0.47%)
Artifact size: 320.43 MiB -> 923.77 MiB (188.29%)

[saethlin]

@bors try

[bors]

⌛ Trying commit 740c135 with merge b3d0a6a...

[bors]

☀️ Try build successful - checks-actions
Build commit: b3d0a6a (b3d0a6a5f12f66b25f0266e1af9dfe665f1acbc4)

[saethlin]

I've updated the PR description with my conclusions from these experiments.