Consider empty-unreachable otherwise branches in MatchBranchSimplification

[clubby789]

Fixes #131219

[rustbot]

r? @jieyouxu

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[jieyouxu]

r? wg-mir-opt

[clubby789]

Hmm, while I think this change is useful (given the improvement in another test), while checking the diffs I noticed that (on current master) the motivating example emits

fn my_is_some(_1: &Option<T>) -> bool {
    debug bar => _1;
    let mut _0: bool;
    let mut _2: isize;

    bb0: {
        _2 = discriminant((*_1));
        switchInt(move _2) -> [1: bb2, otherwise: bb1];   // two-branch SwitchInt, not 3
    }

    bb1: {
        _0 = const false;
        goto -> bb3;
    }

    bb2: {
        _0 = const true;
        goto -> bb3;
    }

    bb3: {
        return;
    }
}

And is therefore simplified by MatchBranchSimplification to the Eq version
Ah I see. At normal opt-levels, UnreachableEnumBranching adds the unreachable branch which inhibits this opt. Presumably the test runs with no other opts enabled, so the two-branch switch is simplified to an Eq

[clubby789]

Custom MIR limitations meant I couldn't have the branches in the same order, so this creates an Ne instead (unlike an Eq like the original demo)

[clubby789]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 7cd33b8 with merge 20e541a...

[bors]

☀️ Try build successful - checks-actions
Build commit: 20e541a (20e541ae5fc472867330cb73999e8ec1ffcb7b83)

[rust-timer]

Finished benchmarking commit (20e541a): comparison URL.

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

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 the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.3%, 0.3%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.8%, -0.0%]	2
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	-0.1%	[-0.8%, 0.3%]	4

Max RSS (memory usage)

Results (primary 4.7%, secondary -3.8%)

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)	4.7%	[2.1%, 8.9%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.8%	[-3.8%, -3.8%]	1
All ❌✅ (primary)	4.7%	[2.1%, 8.9%]	3

Cycles

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

Binary size

Results (primary -0.0%, 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)	0.2%	[0.0%, 1.0%]	11
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.6%, -0.0%]	31
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.0%]	5
All ❌✅ (primary)	-0.0%	[-0.6%, 1.0%]	42

Bootstrap: 783.769s -> 780.785s (-0.38%)
Artifact size: 333.67 MiB -> 333.62 MiB (-0.02%)