diff options
Diffstat (limited to 'compiler/rustc_codegen_ssa/src/mir/operand.rs')
| -rw-r--r-- | compiler/rustc_codegen_ssa/src/mir/operand.rs | 43 |
1 files changed, 29 insertions, 14 deletions
diff --git a/compiler/rustc_codegen_ssa/src/mir/operand.rs b/compiler/rustc_codegen_ssa/src/mir/operand.rs index 5459f95c186..d851c332980 100644 --- a/compiler/rustc_codegen_ssa/src/mir/operand.rs +++ b/compiler/rustc_codegen_ssa/src/mir/operand.rs @@ -498,6 +498,35 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> { bx.cx().const_uint(cast_to, niche_variants.start().as_u32() as u64); (is_niche, tagged_discr, 0) } else { + // Thanks to parameter attributes and load metadata, LLVM already knows + // the general valid range of the tag. It's possible, though, for there + // to be an impossible value *in the middle*, which those ranges don't + // communicate, so it's worth an `assume` to let the optimizer know. + // Most importantly, this means when optimizing a variant test like + // `SELECT(is_niche, complex, CONST) == CONST` it's ok to simplify that + // to `!is_niche` because the `complex` part can't possibly match. + // + // This was previously asserted on `tagged_discr` below, where the + // impossible value is more obvious, but that caused an intermediate + // value to become multi-use and thus not optimize, so instead this + // assumes on the original input which is always multi-use. See + // <https://github.com/llvm/llvm-project/issues/134024#issuecomment-3131782555> + // + // FIXME: If we ever get range assume operand bundles in LLVM (so we + // don't need the `icmp`s in the instruction stream any more), it + // might be worth moving this back to being on the switch argument + // where it's more obviously applicable. + if niche_variants.contains(&untagged_variant) + && bx.cx().sess().opts.optimize != OptLevel::No + { + let impossible = niche_start + .wrapping_add(u128::from(untagged_variant.as_u32())) + .wrapping_sub(u128::from(niche_variants.start().as_u32())); + let impossible = bx.cx().const_uint_big(tag_llty, impossible); + let ne = bx.icmp(IntPredicate::IntNE, tag, impossible); + bx.assume(ne); + } + // With multiple niched variants we'll have to actually compute // the variant index from the stored tag. // @@ -588,20 +617,6 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> { let untagged_variant_const = bx.cx().const_uint(cast_to, u64::from(untagged_variant.as_u32())); - // Thanks to parameter attributes and load metadata, LLVM already knows - // the general valid range of the tag. It's possible, though, for there - // to be an impossible value *in the middle*, which those ranges don't - // communicate, so it's worth an `assume` to let the optimizer know. - // Most importantly, this means when optimizing a variant test like - // `SELECT(is_niche, complex, CONST) == CONST` it's ok to simplify that - // to `!is_niche` because the `complex` part can't possibly match. - if niche_variants.contains(&untagged_variant) - && bx.cx().sess().opts.optimize != OptLevel::No - { - let ne = bx.icmp(IntPredicate::IntNE, tagged_discr, untagged_variant_const); - bx.assume(ne); - } - let discr = bx.select(is_niche, tagged_discr, untagged_variant_const); // In principle we could insert assumes on the possible range of `discr`, but |