Compute jump threading opportunities in a single pass

[cjgillot]

The current implementation of jump threading walks MIR CFG backwards from each SwitchInt terminator. This PR replaces this by a single postorder traversal of MIR. In theory, we could do a full fixpoint dataflow analysis, but this has low returns as we forbid threading through a loop header, and we do not merge TOs yet.

The second commit in this PR modifies the carried state to a lighter data structure. The current implementation uses some kind of IndexVec<ValueIndex, &[Condition]>. This is needlessly heavy, as the state rarely ever carries more than a few Conditions. The first commit replaces this state with a simpler &[Condition], and puts the corresponding ValueIndex inside Condition.

The last commit is the main change. It needs a fair amount of data structure tweaks, as each condition now needs to carry its chain of blocks with it.

[rustbot]

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rust-log-analyzer]

The job aarch64-gnu-llvm-19-1 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

test [crashes] tests/crashes/131295.rs ... ok
test [crashes] tests/crashes/131347.rs ... ok
test [crashes] tests/crashes/131373.rs ... ok
test [crashes] tests/crashes/131406.rs ... ok
2025-06-21T11:51:03.317242Z ERROR compiletest::runtest: fatal error, panic: "crashtest no longer crashes/triggers ICE, hooray! Please give it a meaningful name, add a doc-comment to the start of the test explaining why it exists and move it to tests/ui or wherever you see fit. Adding 'Fixes #<issueNr>' to your PR description ensures that the corresponding ticket is auto-closed upon merge. If you want to see verbose output, set `COMPILETEST_VERBOSE_CRASHES=1`."
test [crashes] tests/crashes/131451.rs ... FAILED
test [crashes] tests/crashes/131507.rs ... ok
test [crashes] tests/crashes/131534.rs ... ok
test [crashes] tests/crashes/131342.rs ... ok
test [crashes] tests/crashes/131762.rs ... ok
---
failures:

---- [crashes] tests/crashes/131451.rs stdout ----

error: crashtest no longer crashes/triggers ICE, hooray! Please give it a meaningful name, add a doc-comment to the start of the test explaining why it exists and move it to tests/ui or wherever you see fit. Adding 'Fixes #<issueNr>' to your PR description ensures that the corresponding ticket is auto-closed upon merge. If you want to see verbose output, set `COMPILETEST_VERBOSE_CRASHES=1`.

thread '[crashes] tests/crashes/131451.rs' panicked at src/tools/compiletest/src/runtest/crashes.rs:17:18:
fatal error
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[oli-obk]

@bors try @rust-timer queue

[bors]

⌛ Trying commit bdf9d85 with merge d27b44e...

[bors]

☀️ Try build successful - checks-actions
Build commit: d27b44e (d27b44ea37402dbd9462aa3b8eee9ff6d5e4c007)

[rust-timer]

Finished benchmarking commit (d27b44e): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.1%, 3.1%]	54
Regressions ❌ (secondary)	1.4%	[0.2%, 6.0%]	17
Improvements ✅ (primary)	-0.4%	[-0.8%, -0.2%]	8
Improvements ✅ (secondary)	-0.4%	[-0.6%, -0.2%]	8
All ❌✅ (primary)	0.4%	[-0.8%, 3.1%]	62

Max RSS (memory usage)

Results (primary -1.9%, secondary -3.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	5.5%	[5.5%, 5.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.7%	[-4.7%, -2.7%]	4
Improvements ✅ (secondary)	-3.6%	[-3.6%, -3.6%]	1
All ❌✅ (primary)	-1.9%	[-4.7%, 5.5%]	5

Cycles

Results (primary 1.6%, secondary 2.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.6%	[0.9%, 2.9%]	3
Regressions ❌ (secondary)	2.4%	[1.7%, 3.1%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.6%	[0.9%, 2.9%]	3

Binary size

Results (primary -0.1%, secondary -0.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 1.7%]	24
Regressions ❌ (secondary)	0.2%	[0.0%, 0.6%]	8
Improvements ✅ (primary)	-0.3%	[-0.8%, -0.0%]	49
Improvements ✅ (secondary)	-0.6%	[-13.0%, -0.1%]	86
All ❌✅ (primary)	-0.1%	[-0.8%, 1.7%]	73

Bootstrap: 689.042s -> 688.964s (-0.01%)
Artifact size: 371.85 MiB -> 371.64 MiB (-0.06%)