[codegen] assume the tag, not the relative discriminant #144764
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nikic
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Aug 1, 2025
| // CHECK: tail call void @llvm.assume(i1 %[[A_NOT_HOLE]]) | ||
| // CHECK: %[[A_DISCR:.+]] = select i1 %[[A_IS_NICHE]], i64 %[[A_REL_DISCR]], i64 1 | ||
| // LLVM20: %[[A_DISCR:.+]] = select i1 %[[A_IS_NICHE]], i64 %[[A_REL_DISCR]], i64 1 | ||
| // LLVM21: %[[A_MODIFIED_TAG:.+]] = select i1 %[[A_IS_NICHE]], i64 %[[A_TRUNC]], i64 6 |
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Wouldn't this select be at the bottom for LLVM 21?
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Oops, I completely missed that! Thanks.
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This looks like a reasonable thing to do to me, but I'm probably the wrong person to check whether the logic is correct. The surrounding niche/discriminant logic looks kinda tricky. |
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scottmcm
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Aug 1, 2025
| if niche_variants.contains(&untagged_variant) | ||
| && bx.cx().sess().opts.optimize != OptLevel::No | ||
| { | ||
| let ne = bx.icmp(IntPredicate::IntNE, tagged_discr, untagged_variant_const); |
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In case this helps review, here's how you can see this just from the diff, rather than from https://doc.rust-lang.org/nightly/nightly-rustc/rustc_abi/enum.TagEncoding.html#variant.Niche:
Previously we were doing tagged_discr != untagged_variant here.
But
relative_discr = tag - niche_start
delta = niche_variants.start()
tagged_discr = relative_discr + delta
so
tagged_discr != untagged_variant
=> relative_discr + delta != untagged_variant
=> (tag - niche_start) + niche_variants.start() != untagged_variant
=> tag != niche_start + untagged_variant - niche_variants.start()
which is the calculation on line 522.
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Address the issue mentioned in llvm/llvm-project#134024 (comment) by changing discriminant calculation to
assumeon the originally-loadedtag, rather than oncast(tag)-OFFSET.The previous way does make the purpose of the assume clearer, IMHO, since you see
assume(x != 4); if p { x } else { 4 }, but doing it this way instead means that theadds optimize away in LLVM21, which is more important. And this new way is still easily thought of as being like metadata on the load saying specifically which value is impossible.Demo of the LLVM20 vs LLVM21 difference: https://llvm.godbolt.org/z/n54x5Mq1T
r? @nikic