2 files changed, 31 insertions, 0 deletions
diff --git a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
index 0af84ff067a..f6596980f4a 100644
--- a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
@@ -7,6 +7,7 @@ use crate::common::{self, IntPredicate};
 use crate::traits::*;
 use crate::MemFlags;
 
+use rustc_hir as hir;
 use rustc_middle::mir;
 use rustc_middle::mir::Operand;
 use rustc_middle::ty::cast::{CastTy, IntTy};
@@ -882,6 +883,35 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     bx.icmp(base::bin_op_to_icmp_predicate(op.to_hir_binop(), is_signed), lhs, rhs)
                 }
             }
+            mir::BinOp::Cmp => {
+                use std::cmp::Ordering;
+                debug_assert!(!is_float);
+                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
+                    // <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.
+                    let is_gt = bx.icmp(pred(hir::BinOpKind::Gt), lhs, rhs);
+                    let gtext = bx.zext(is_gt, bx.type_i8());
+                    let is_lt = bx.icmp(pred(hir::BinOpKind::Lt), lhs, rhs);
+                    let ltext = bx.zext(is_lt, bx.type_i8());
+                    bx.unchecked_ssub(gtext, ltext)
+                } else {
+                    // These operations are those expected by `tests/codegen/integer-cmp.rs`,
+                    // from <https://github.com/rust-lang/rust/pull/63767>.
+                    let is_lt = bx.icmp(pred(hir::BinOpKind::Lt), lhs, rhs);
+                    let is_ne = bx.icmp(pred(hir::BinOpKind::Ne), lhs, rhs);
+                    let ge = bx.select(
+                        is_ne,
+                        bx.cx().const_i8(Ordering::Greater as i8),
+                        bx.cx().const_i8(Ordering::Equal as i8),
+                    );
+                    bx.select(is_lt, bx.cx().const_i8(Ordering::Less as i8), ge)
+                }
+            }
         }
     }
 
diff --git a/compiler/rustc_codegen_ssa/src/traits/consts.rs b/compiler/rustc_codegen_ssa/src/traits/consts.rs
index 4dff9c7684f..8cb17a5b37a 100644
--- a/compiler/rustc_codegen_ssa/src/traits/consts.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/consts.rs
@@ -19,6 +19,7 @@ pub trait ConstMethods<'tcx>: BackendTypes {
     fn const_bool(&self, val: bool) -> Self::Value;
     fn const_i16(&self, i: i16) -> Self::Value;
     fn const_i32(&self, i: i32) -> Self::Value;
+    fn const_i8(&self, i: i8) -> Self::Value;
     fn const_u32(&self, i: u32) -> Self::Value;
     fn const_u64(&self, i: u64) -> Self::Value;
     fn const_u128(&self, i: u128) -> Self::Value;