diff options
| -rw-r--r-- | compiler/rustc_codegen_ssa/src/traits/builder.rs | 19 |
1 files changed, 11 insertions, 8 deletions
diff --git a/compiler/rustc_codegen_ssa/src/traits/builder.rs b/compiler/rustc_codegen_ssa/src/traits/builder.rs index 422fa715de1..4a5694e97fa 100644 --- a/compiler/rustc_codegen_ssa/src/traits/builder.rs +++ b/compiler/rustc_codegen_ssa/src/traits/builder.rs @@ -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( |