bjoernager/rust
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Auto merge of #133984 - DaniPopes:scmp-ucmp, r=scottmcm

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Lower BinOp::Cmp to llvm.{s,u}cmp.* intrinsics

Lowers `mir::BinOp::Cmp` (`three_way_compare` intrinsic) to the corresponding LLVM `llvm.{s,u}cmp.i8.*` intrinsics.

These are the intrinsics mentioned in https://github.com/rust-lang/rust/pull/118310, which are now available in LLVM 19.

I couldn't find any follow-up PRs/discussions about this, please let me know if I missed something.

r? `@scottmcm`
This commit is contained in:
bors 2025-03-24 22:53:12 +00:00
commit 1df5affaca
8 changed files with 124 additions and 42 deletions
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3
compiler/rustc_codegen_ssa/src/mir/rvalue.rs
View file

@ -988,6 +988,9 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
mir::BinOp::Cmp => {
use std::cmp::Ordering;
assert!(!is_float);
if let Some(value) = bx.three_way_compare(lhs_ty, lhs, rhs) {
return value;
}
let pred = |op| base::bin_op_to_icmp_predicate(op, is_signed);
if bx.cx().tcx().sess.opts.optimize == OptLevel::No {
// FIXME: This actually generates tighter assembly, and is a classic trick

12
compiler/rustc_codegen_ssa/src/traits/builder.rs
View file

@ -397,6 +397,18 @@ pub trait BuilderMethods<'a, 'tcx>:
fn icmp(&mut self, op: IntPredicate, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
fn fcmp(&mut self, op: RealPredicate, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
/// Returns `-1` if `lhs < rhs`, `0` if `lhs == rhs`, and `1` if `lhs > rhs`.
// FIXME: Move the default implementation from `codegen_scalar_binop` into this method and
// remove the `Option` return once LLVM 20 is the minimum version.
fn three_way_compare(
&mut self,
_ty: Ty<'tcx>,
_lhs: Self::Value,
_rhs: Self::Value,
) -> Option<Self::Value> {
None
}
fn memcpy(
&mut self,
dst: Self::Value,