Preprocess and cache dominator tree

[cjgillot]

Preprocessing dominators has a very strong effect for #111344.
That pass checks that assignments dominate their uses repeatedly. Using the unprocessed dominator tree caused a quadratic runtime (number of bbs x depth of the dominator tree).

This PR also caches the dominator tree and the pre-processed dominators in the MIR cfg cache.

Rebase of #107157
cc @tmiasko

[rustbot]

r? @oli-obk

(rustbot has picked a reviewer for you, use r? to override)

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[tmiasko]

r=cjgillot,tmiasko after addressing remaining nits, unless you think it deserves another look given that it was coauthored.

[oli-obk]

r? @tmiasko

[oli-obk]

@rustbot author

[cjgillot]

@bors r=cjgillot,tmiasko

[bors]

📌 Commit 7c8f29f has been approved by cjgillot,tmiasko

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7c8f29f with merge 97d3280...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot,tmiasko
Pushing 97d3280 to master...

[rust-timer]

Finished benchmarking commit (97d3280): 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)	0.3%	[0.2%, 0.4%]	3
Regressions ❌ (secondary)	0.3%	[0.2%, 0.4%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.2%	[-1.2%, -1.2%]	3
All ❌✅ (primary)	0.3%	[0.2%, 0.4%]	3

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)	1.4%	[1.0%, 2.8%]	7
Regressions ❌ (secondary)	1.9%	[1.1%, 2.9%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.4%	[1.0%, 2.8%]	7

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)	-2.4%	[-2.7%, -2.1%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 645.989s -> 647.059s (0.17%)

[pnkfelix]

• primary benchmarks libc {check, debug}-full and bitmaps check-full regressed a little.
• this pays a bit of upfront cost to avoid a O(n^2) pathology in other planned code.
• marked as triaged.

@rustbot label: perf-regression-triaged