Tell LLVM about impossible niche tags #139098
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Mar 29, 2025
tests/codegen/enum/enum-match.rs
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| // CHECK-LABEL: define{{.+}}i1 @match4_is_c(i8{{.+}}%e) | ||
| // CHECK-NEXT: start | ||
| // CHECK-NEXT: %[[REL_VAR:.+]] = add nsw i8 %e, -2 | ||
| // CHECK-NEXT: %[[NOT_NICHE:.+]] = icmp ugt i8 %[[REL_VAR]], 4 | ||
| // CHECK-NEXT: %[[NOT_IMPOSSIBLE:.+]] = icmp ne i8 %[[REL_VAR]], 2 | ||
| // CHECK-NEXT: call void @llvm.assume(i1 %[[NOT_IMPOSSIBLE]]) | ||
| // CHECK-NEXT: ret i1 %[[NOT_NICHE]] | ||
| #[no_mangle] | ||
| pub fn match4_is_c(e: MiddleNiche) -> bool { | ||
| // Before #139098, this couldn't optimize out the `select` because it looked | ||
| // like it was possible for a `2` to be produced on both sides. | ||
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| std::intrinsics::discriminant_value(&e) == 2 | ||
| } |
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Compare on nightly, https://rust.godbolt.org/z/4bdKKjeG5
define noundef zeroext i1 @match4_is_c(i8 noundef range(i8 0, 7) %e) unnamed_addr {
start:
%0 = add nsw i8 %e, -2
%1 = icmp ugt i8 %0, 4
%_01 = icmp eq i8 %0, 2
%_0 = or i1 %1, %_01
ret i1 %_0
}
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I was trying to find a better way of emitting discriminant calculations, but sadly had no luck.
So here's a fairly small PR with the bits that did seem worth bothering:
As the
TagEncoding::Nichedocs describe, it's possible to end up with a dead value in the input that's not already communicated via the range parameter attribute nor the range load metadata attribute. So this adds anllvm.assumein non-debug mode to tell LLVM about that. (That way it can tell that the sides of theselecthave disjoint possible values.)I'd written a bunch more tests, or at least made them parameterized, in the process of trying things out, so this checks in those tests to hopefully help future people not trip on the same weird edge cases, like when the tag type is
i8but yet there's still a variant index and discriminant of258which doesn't fit in that tag type because the enum is really weird.