EXPERIMENT [MIR-OPT] Add a pass that replicates simple branches into predecessors

[scottmcm]

This means fewer blocks, so is hopefully a net win.

Draft until #105808 can simplify the discriminant checks in here.

r? @ghost

[scottmcm]

Perf queue is currently empty, so out of curiosity for whether this is horrible,
@bors try @rust-timer queue

[bors]

⌛ Trying commit 9fea3d8 with merge 8f21ec12447a8995bb7bb565c500ee52c5f36110...

[bors]

☀️ Try build successful - checks-actions
Build commit: 8f21ec12447a8995bb7bb565c500ee52c5f36110 (8f21ec12447a8995bb7bb565c500ee52c5f36110)

[rust-timer]

Finished benchmarking commit (8f21ec12447a8995bb7bb565c500ee52c5f36110): 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)	1.3%	[0.8%, 1.9%]	4
Regressions ❌ (secondary)	2.3%	[0.7%, 5.9%]	8
Improvements ✅ (primary)	-0.5%	[-1.2%, -0.3%]	72
Improvements ✅ (secondary)	-0.5%	[-1.1%, -0.2%]	44
All ❌✅ (primary)	-0.4%	[-1.2%, 1.9%]	76

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)	4.1%	[2.9%, 5.5%]	3
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-2.5%	[-2.5%, -2.5%]	1
Improvements ✅ (secondary)	-1.9%	[-1.9%, -1.9%]	1
All ❌✅ (primary)	2.4%	[-2.5%, 5.5%]	4

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)	1.1%	[1.0%, 1.2%]	3
Regressions ❌ (secondary)	1.8%	[1.6%, 2.1%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.1%	[1.0%, 1.2%]	3

Binary size

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)	0.2%	[0.0%, 0.9%]	76
Regressions ❌ (secondary)	0.5%	[0.1%, 1.2%]	87
Improvements ✅ (primary)	-0.2%	[-0.6%, -0.1%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[-0.6%, 0.9%]	80

Bootstrap: 642.634s -> 641.068s (-0.24%)

[cjgillot]

What is the difference with the SeparateConstSwitch pass?