match lowering: sort Eq candidates in the failure case too

[Nadrieril]

This is a slight tweak to MIR gen of matches. Take a match like:

match (s, flag) {
    ("a", _) if foo() => 1,
    ("b", true) => 2,
    ("a", false) => 3,
    (_, true) => 4,
    _ => 5,
}

If we switch on s == "a", the first candidate matches, and we learn almost nothing about the second candidate. So there's a choice:

1. (what we do today) stop sorting candidates, keep the "b" case grouped with everything below. This could allow us to be clever here and test on flag == true next.
2. (what this PR does) sort "b" into the failure case. The "b" will be alone (fewer opportunities for picking a good test), but that means the two "a" cases require a single test.

Today, we aren't clever in which tests we pick, so this is an unambiguous win. In a future where we pick tests better, idk. Grouping tests as much as possible feels like a generally good strategy.

This was proposed in #29623 (9 years ago :D)

[rustbot]

r? @petrochenkov

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They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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[petrochenkov]

fully_matched = test_val == case_val;
Some(if fully_matched { TestBranch::Success } else { TestBranch::Failure })

[Nadrieril]

I prefer keeping them separate because the fully_matched logic is already not easy to follow across the function

[petrochenkov]

Is there anybody who reviews pattern matching changes regularly?
I have no idea what "sorting" and "grouping" mean in this context, and what are implications of the change, so it's probably better to reassign.
@rustbot author

[Nadrieril]

I know one but I was hoping I could give the easy changes to others. Let's see...

r? @oli-obk do you follow the match lowering logic?

[oli-obk]

Since you're looking for specific patterns, it may be good to use filecheck to ensure it doesn't get blessed away in some larger change

[Nadrieril]

@bors r=@oli-obk

[bors]

📌 Commit 65efa5b has been approved by oli-obk

It is now in the queue for this repository.

[bors]

⌛ Testing commit 65efa5b with merge 5baf1e1...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 5baf1e1 to master...

[rust-timer]

Finished benchmarking commit (5baf1e1): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -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)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.2%, 0.2%]	1

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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-5.9%	[-5.9%, -5.9%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-5.9%	[-5.9%, -5.9%]	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
Regressions ❌ (secondary)	3.4%	[2.4%, 4.7%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.3%	[-0.3%, -0.3%]	1

Bootstrap: 667.618s -> 670.332s (0.41%)
Artifact size: 315.66 MiB -> 315.71 MiB (0.02%)