diff options
Diffstat (limited to 'compiler/rustc_codegen_ssa/src')
| -rw-r--r-- | compiler/rustc_codegen_ssa/src/mir/operand.rs | 48 |
1 files changed, 46 insertions, 2 deletions
diff --git a/compiler/rustc_codegen_ssa/src/mir/operand.rs b/compiler/rustc_codegen_ssa/src/mir/operand.rs index 9910cb5469f..6a3fdb6ede1 100644 --- a/compiler/rustc_codegen_ssa/src/mir/operand.rs +++ b/compiler/rustc_codegen_ssa/src/mir/operand.rs @@ -517,8 +517,52 @@ 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 { - // The special cases don't apply, so we'll have to go with - // the general algorithm. + // With multiple niched variants we'll have to actually compute + // the variant index from the stored tag. + // + // However, there's still one small optimization we can often do for + // determining *whether* a tag value is a natural value or a niched + // variant. The general algorithm involves a subtraction that often + // wraps in practice, making it tricky to analyse. However, in cases + // where there are few enough possible values of the tag that it doesn't + // need to wrap around, we can instead just look for the contiguous + // tag values on the end of the range with a single comparison. + // + // For example, take the type `enum Demo { A, B, Untagged(bool) }`. + // The `bool` is {0, 1}, and the two other variants are given the + // tags {2, 3} respectively. That means the `tag_range` is + // `[0, 3]`, which doesn't wrap as unsigned (nor as signed), so + // we can test for the niched variants with just `>= 2`. + // + // That means we're looking either for the niche values *above* + // the natural values of the untagged variant: + // + // niche_start niche_end + // | | + // v v + // MIN -------------+---------------------------+---------- MAX + // ^ | is niche | + // | +---------------------------+ + // | | + // tag_range.start tag_range.end + // + // Or *below* the natural values: + // + // niche_start niche_end + // | | + // v v + // MIN ----+-----------------------+---------------------- MAX + // | is niche | ^ + // +-----------------------+ | + // | | + // tag_range.start tag_range.end + // + // With those two options and having the flexibility to choose + // between a signed or unsigned comparison on the tag, that + // covers most realistic scenarios. The tests have a (contrived) + // example of a 1-byte enum with over 128 niched variants which + // wraps both as signed as unsigned, though, and for something + // like that we're stuck with the general algorithm. let tag_range = tag_scalar.valid_range(&dl); let tag_size = tag_scalar.size(&dl); |