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Update the FIXME comments in the generic three_way_compare

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This commit is contained in:
Josh Stone 2025-08-08 14:20:56 -07:00
parent e54602c5bb
commit 88bef49646
1 changed files with 11 additions and 8 deletions
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19
compiler/rustc_codegen_ssa/src/traits/builder.rs
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@ -412,23 +412,26 @@ pub trait BuilderMethods<'a, 'tcx>:
lhs: Self::Value,
rhs: Self::Value,
) -> Self::Value {
// FIXME: This implementation was designed around LLVM's ability to optimize, but `cg_llvm`
// overrides this to just use `@llvm.scmp`/`ucmp` since LLVM 20. This default impl should be
// reevaluated with respect to the remaining backends like cg_gcc, whether they might use
// specialized implementations as well, or continue to use a generic implementation here.
use std::cmp::Ordering;
let pred = |op| crate::base::bin_op_to_icmp_predicate(op, ty.is_signed());
if self.cx().sess().opts.optimize == OptLevel::No {
// FIXME: This actually generates tighter assembly, and is a classic trick
// <https://graphics.stanford.edu/~seander/bithacks.html#CopyIntegerSign>
// However, as of 2023-11 it optimizes worse in things like derived
// `PartialOrd`, so only use it in debug for now. Once LLVM can handle it
// better (see <https://github.com/llvm/llvm-project/issues/73417>), it'll
// be worth trying it in optimized builds as well.
// This actually generates tighter assembly, and is a classic trick:
// <https://graphics.stanford.edu/~seander/bithacks.html#CopyIntegerSign>.
// However, as of 2023-11 it optimized worse in LLVM in things like derived
// `PartialOrd`, so we were only using it in debug. Since LLVM now uses its own
// intrinsics, it may be be worth trying it in optimized builds for other backends.
let is_gt = self.icmp(pred(mir::BinOp::Gt), lhs, rhs);
let gtext = self.zext(is_gt, self.type_i8());
let is_lt = self.icmp(pred(mir::BinOp::Lt), lhs, rhs);
let ltext = self.zext(is_lt, self.type_i8());
self.unchecked_ssub(gtext, ltext)
} else {
// These operations are those expected by `tests/codegen-llvm/integer-cmp.rs`,
// from <https://github.com/rust-lang/rust/pull/63767>.
// These operations were better optimized by LLVM, before `@llvm.scmp`/`ucmp` in 20.
// See <https://github.com/rust-lang/rust/pull/63767>.
let is_lt = self.icmp(pred(mir::BinOp::Lt), lhs, rhs);
let is_ne = self.icmp(pred(mir::BinOp::Ne), lhs, rhs);
let ge = self.select(