Auto merge of #118310 - scottmcm:three-way-compare, r=davidtwco

Add `Ord::cmp` for primitives as a `BinOp` in MIR

Update: most of this OP was written months ago.  See https://github.com/rust-lang/rust/pull/118310#issuecomment-2016940014 below for where we got to recently that made it ready for review.

---

There are dozens of reasonable ways to implement `Ord::cmp` for integers using comparison, bit-ops, and branches.  Those differences are irrelevant at the rust level, however, so we can make things better by adding `BinOp::Cmp` at the MIR level:

1. Exactly how to implement it is left up to the backends, so LLVM can use whatever pattern its optimizer best recognizes and cranelift can use whichever pattern codegens the fastest.
2. By not inlining those details for every use of `cmp`, we drastically reduce the amount of MIR generated for `derive`d `PartialOrd`, while also making it more amenable to MIR-level optimizations.

Having extremely careful `if` ordering to μoptimize resource usage on broadwell (#63767) is great, but it really feels to me like libcore is the wrong place to put that logic.  Similarly, using subtraction [tricks](https://graphics.stanford.edu/~seander/bithacks.html#CopyIntegerSign) (#105840) is arguably even nicer, but depends on the optimizer understanding it (https://github.com/llvm/llvm-project/issues/73417) to be practical.  Or maybe [bitor is better than add](https://discourse.llvm.org/t/representing-in-ir/67369/2?u=scottmcm)?  But maybe only on a future version that [has `or disjoint` support](https://discourse.llvm.org/t/rfc-add-or-disjoint-flag/75036?u=scottmcm)?  And just because one of those forms happens to be good for LLVM, there's no guarantee that it'd be the same form that GCC or Cranelift would rather see -- especially given their very different optimizers.  Not to mention that if LLVM gets a spaceship intrinsic -- [which it should](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Suboptimal.20inlining.20in.20std.20function.20.60binary_search.60/near/404250586) -- we'll need at least a rustc intrinsic to be able to call it.

As for simplifying it in Rust, we now regularly inline `{integer}::partial_cmp`, but it's quite a large amount of IR.  The best way to see that is with https://github.com/rust-lang/rust/commit/8811efa88b25b5e41d63850e6047e8257c677858#diff-d134c32d028fbe2bf835fef2df9aca9d13332dd82284ff21ee7ebf717bfa4765R113 -- I added a new pre-codegen MIR test for a simple 3-tuple struct, and this PR change it from 36 locals and 26 basic blocks down to 24 locals and 8 basic blocks.  Even better, as soon as the construct-`Some`-then-match-it-in-same-BB noise is cleaned up, this'll expose the `Cmp == 0` branches clearly in MIR, so that an InstCombine (#105808) can simplify that to just a `BinOp::Eq` and thus fix some of our generated code perf issues.  (Tracking that through today's `if a < b { Less } else if a == b { Equal } else { Greater }` would be *much* harder.)

---

r? `@ghost`
But first I should check that perf is ok with this
~~...and my true nemesis, tidy.~~

36 files changed, 601 insertions, 13 deletions

diff --git a/compiler/rustc_codegen_cranelift/src/codegen_i128.rs b/compiler/rustc_codegen_cranelift/src/codegen_i128.rs
index b2bc289a5b6..4a5ef352151 100644
--- a/compiler/rustc_codegen_cranelift/src/codegen_i128.rs
+++ b/compiler/rustc_codegen_cranelift/src/codegen_i128.rs
@@ -68,7 +68,7 @@ pub(crate) fn maybe_codegen<'tcx>(
                 Some(CValue::by_val(ret_val, lhs.layout()))
             }
         }
-        BinOp::Lt | BinOp::Le | BinOp::Eq | BinOp::Ge | BinOp::Gt | BinOp::Ne => None,
+        BinOp::Lt | BinOp::Le | BinOp::Eq | BinOp::Ge | BinOp::Gt | BinOp::Ne | BinOp::Cmp => None,
         BinOp::Shl | BinOp::ShlUnchecked | BinOp::Shr | BinOp::ShrUnchecked => None,
     }
 }
@@ -134,6 +134,7 @@ pub(crate) fn maybe_codegen_checked<'tcx>(
         BinOp::AddUnchecked | BinOp::SubUnchecked | BinOp::MulUnchecked => unreachable!(),
         BinOp::Offset => unreachable!("offset should only be used on pointers, not 128bit ints"),
         BinOp::Div | BinOp::Rem => unreachable!(),
+        BinOp::Cmp => unreachable!(),
         BinOp::Lt | BinOp::Le | BinOp::Eq | BinOp::Ge | BinOp::Gt | BinOp::Ne => unreachable!(),
         BinOp::Shl | BinOp::ShlUnchecked | BinOp::Shr | BinOp::ShrUnchecked => unreachable!(),
     }
diff --git a/compiler/rustc_codegen_cranelift/src/num.rs b/compiler/rustc_codegen_cranelift/src/num.rs
index 8992f40fb90..796182418ad 100644
--- a/compiler/rustc_codegen_cranelift/src/num.rs
+++ b/compiler/rustc_codegen_cranelift/src/num.rs
@@ -40,6 +40,22 @@ pub(crate) fn bin_op_to_intcc(bin_op: BinOp, signed: bool) -> Option<IntCC> {
     })
 }
 
+fn codegen_three_way_compare<'tcx>(
+    fx: &mut FunctionCx<'_, '_, 'tcx>,
+    signed: bool,
+    lhs: Value,
+    rhs: Value,
+) -> CValue<'tcx> {
+    // This emits `(lhs > rhs) - (lhs < rhs)`, which is cranelift's preferred form per
+    // <https://github.com/bytecodealliance/wasmtime/blob/8052bb9e3b792503b225f2a5b2ba3bc023bff462/cranelift/codegen/src/prelude_opt.isle#L41-L47>
+    let gt_cc = crate::num::bin_op_to_intcc(BinOp::Gt, signed).unwrap();
+    let lt_cc = crate::num::bin_op_to_intcc(BinOp::Lt, signed).unwrap();
+    let gt = fx.bcx.ins().icmp(gt_cc, lhs, rhs);
+    let lt = fx.bcx.ins().icmp(lt_cc, lhs, rhs);
+    let val = fx.bcx.ins().isub(gt, lt);
+    CValue::by_val(val, fx.layout_of(fx.tcx.ty_ordering_enum(Some(fx.mir.span))))
+}
+
 fn codegen_compare_bin_op<'tcx>(
     fx: &mut FunctionCx<'_, '_, 'tcx>,
     bin_op: BinOp,
@@ -47,6 +63,10 @@ fn codegen_compare_bin_op<'tcx>(
     lhs: Value,
     rhs: Value,
 ) -> CValue<'tcx> {
+    if bin_op == BinOp::Cmp {
+        return codegen_three_way_compare(fx, signed, lhs, rhs);
+    }
+
     let intcc = crate::num::bin_op_to_intcc(bin_op, signed).unwrap();
     let val = fx.bcx.ins().icmp(intcc, lhs, rhs);
     CValue::by_val(val, fx.layout_of(fx.tcx.types.bool))
@@ -59,7 +79,7 @@ pub(crate) fn codegen_binop<'tcx>(
     in_rhs: CValue<'tcx>,
 ) -> CValue<'tcx> {
     match bin_op {
-        BinOp::Eq | BinOp::Lt | BinOp::Le | BinOp::Ne | BinOp::Ge | BinOp::Gt => {
+        BinOp::Eq | BinOp::Lt | BinOp::Le | BinOp::Ne | BinOp::Ge | BinOp::Gt | BinOp::Cmp => {
             match in_lhs.layout().ty.kind() {
                 ty::Bool | ty::Uint(_) | ty::Int(_) | ty::Char => {
                     let signed = type_sign(in_lhs.layout().ty);
@@ -160,7 +180,7 @@ pub(crate) fn codegen_int_binop<'tcx>(
         }
         BinOp::Offset => unreachable!("Offset is not an integer operation"),
         // Compare binops handles by `codegen_binop`.
-        BinOp::Eq | BinOp::Ne | BinOp::Lt | BinOp::Le | BinOp::Gt | BinOp::Ge => {
+        BinOp::Eq | BinOp::Ne | BinOp::Lt | BinOp::Le | BinOp::Gt | BinOp::Ge | BinOp::Cmp => {
             unreachable!("{:?}({:?}, {:?})", bin_op, in_lhs.layout().ty, in_rhs.layout().ty);
         }
     };
diff --git a/compiler/rustc_codegen_gcc/src/common.rs b/compiler/rustc_codegen_gcc/src/common.rs
index d243d7088ad..78d943192db 100644
--- a/compiler/rustc_codegen_gcc/src/common.rs
+++ b/compiler/rustc_codegen_gcc/src/common.rs
@@ -94,6 +94,10 @@ impl<'gcc, 'tcx> ConstMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
         self.const_int(self.type_i32(), i as i64)
     }
 
+    fn const_i8(&self, i: i8) -> RValue<'gcc> {
+        self.const_int(self.type_i8(), i as i64)
+    }
+
     fn const_u32(&self, i: u32) -> RValue<'gcc> {
         self.const_uint(self.type_u32(), i as u64)
     }
diff --git a/compiler/rustc_codegen_llvm/src/common.rs b/compiler/rustc_codegen_llvm/src/common.rs
index 25cbd90460f..568fcc3f3cf 100644
--- a/compiler/rustc_codegen_llvm/src/common.rs
+++ b/compiler/rustc_codegen_llvm/src/common.rs
@@ -160,6 +160,10 @@ impl<'ll, 'tcx> ConstMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         self.const_int(self.type_i32(), i as i64)
     }
 
+    fn const_i8(&self, i: i8) -> &'ll Value {
+        self.const_int(self.type_i8(), i as i64)
+    }
+
     fn const_u32(&self, i: u32) -> &'ll Value {
         self.const_uint(self.type_i32(), i as u64)
     }
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;
diff --git a/compiler/rustc_const_eval/src/interpret/operand.rs b/compiler/rustc_const_eval/src/interpret/operand.rs
index dbc6a317640..831787a92c8 100644
--- a/compiler/rustc_const_eval/src/interpret/operand.rs
+++ b/compiler/rustc_const_eval/src/interpret/operand.rs
@@ -236,6 +236,13 @@ impl<'tcx, Prov: Provenance> ImmTy<'tcx, Prov> {
     }
 
     #[inline]
+    pub fn from_ordering(c: std::cmp::Ordering, tcx: TyCtxt<'tcx>) -> Self {
+        let ty = tcx.ty_ordering_enum(None);
+        let layout = tcx.layout_of(ty::ParamEnv::reveal_all().and(ty)).unwrap();
+        Self::from_scalar(Scalar::from_i8(c as i8), layout)
+    }
+
+    #[inline]
     pub fn to_const_int(self) -> ConstInt {
         assert!(self.layout.ty.is_integral());
         let int = self.to_scalar().assert_int();
diff --git a/compiler/rustc_const_eval/src/interpret/operator.rs b/compiler/rustc_const_eval/src/interpret/operator.rs
index 475c533077a..5665bb4999f 100644
--- a/compiler/rustc_const_eval/src/interpret/operator.rs
+++ b/compiler/rustc_const_eval/src/interpret/operator.rs
@@ -61,6 +61,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 }
 
 impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
+    fn three_way_compare<T: Ord>(&self, lhs: T, rhs: T) -> (ImmTy<'tcx, M::Provenance>, bool) {
+        let res = Ord::cmp(&lhs, &rhs);
+        return (ImmTy::from_ordering(res, *self.tcx), false);
+    }
+
     fn binary_char_op(
         &self,
         bin_op: mir::BinOp,
@@ -69,6 +74,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     ) -> (ImmTy<'tcx, M::Provenance>, bool) {
         use rustc_middle::mir::BinOp::*;
 
+        if bin_op == Cmp {
+            return self.three_way_compare(l, r);
+        }
+
         let res = match bin_op {
             Eq => l == r,
             Ne => l != r,
@@ -231,6 +240,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let r = self.sign_extend(r, right_layout) as i128;
                 return Ok((ImmTy::from_bool(op(&l, &r), *self.tcx), false));
             }
+            if bin_op == Cmp {
+                let l = self.sign_extend(l, left_layout) as i128;
+                let r = self.sign_extend(r, right_layout) as i128;
+                return Ok(self.three_way_compare(l, r));
+            }
             let op: Option<fn(i128, i128) -> (i128, bool)> = match bin_op {
                 Div if r == 0 => throw_ub!(DivisionByZero),
                 Rem if r == 0 => throw_ub!(RemainderByZero),
@@ -270,6 +284,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
         }
 
+        if bin_op == Cmp {
+            return Ok(self.three_way_compare(l, r));
+        }
+
         let val = match bin_op {
             Eq => ImmTy::from_bool(l == r, *self.tcx),
             Ne => ImmTy::from_bool(l != r, *self.tcx),
diff --git a/compiler/rustc_const_eval/src/transform/validate.rs b/compiler/rustc_const_eval/src/transform/validate.rs
index 378b168a50c..1388e08e2d5 100644
--- a/compiler/rustc_const_eval/src/transform/validate.rs
+++ b/compiler/rustc_const_eval/src/transform/validate.rs
@@ -986,6 +986,15 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                             )
                         }
                     }
+                    Cmp => {
+                        for x in [a, b] {
+                            check_kinds!(
+                                x,
+                                "Cannot three-way compare non-integer type {:?}",
+                                ty::Char | ty::Uint(..) | ty::Int(..)
+                            )
+                        }
+                    }
                     AddUnchecked | SubUnchecked | MulUnchecked | Shl | ShlUnchecked | Shr
                     | ShrUnchecked => {
                         for x in [a, b] {
diff --git a/compiler/rustc_const_eval/src/util/mod.rs b/compiler/rustc_const_eval/src/util/mod.rs
index a8060463b69..0c3b59a0e78 100644
--- a/compiler/rustc_const_eval/src/util/mod.rs
+++ b/compiler/rustc_const_eval/src/util/mod.rs
@@ -19,7 +19,7 @@ pub fn binop_left_homogeneous(op: mir::BinOp) -> bool {
     match op {
         Add | AddUnchecked | Sub | SubUnchecked | Mul | MulUnchecked | Div | Rem | BitXor
         | BitAnd | BitOr | Offset | Shl | ShlUnchecked | Shr | ShrUnchecked => true,
-        Eq | Ne | Lt | Le | Gt | Ge => false,
+        Eq | Ne | Lt | Le | Gt | Ge | Cmp => false,
     }
 }
 
@@ -30,7 +30,7 @@ pub fn binop_right_homogeneous(op: mir::BinOp) -> bool {
     use rustc_middle::mir::BinOp::*;
     match op {
         Add | AddUnchecked | Sub | SubUnchecked | Mul | MulUnchecked | Div | Rem | BitXor
-        | BitAnd | BitOr | Eq | Ne | Lt | Le | Gt | Ge => true,
+        | BitAnd | BitOr | Eq | Ne | Lt | Le | Gt | Ge | Cmp => true,
         Offset | Shl | ShlUnchecked | Shr | ShrUnchecked => false,
     }
 }
diff --git a/compiler/rustc_hir/src/lang_items.rs b/compiler/rustc_hir/src/lang_items.rs
index da592768570..2a796ca5465 100644
--- a/compiler/rustc_hir/src/lang_items.rs
+++ b/compiler/rustc_hir/src/lang_items.rs
@@ -226,6 +226,7 @@ language_item_table! {
     Unpin,                   sym::unpin,               unpin_trait,                Target::Trait,          GenericRequirement::None;
     Pin,                     sym::pin,                 pin_type,                   Target::Struct,         GenericRequirement::None;
 
+    OrderingEnum,            sym::Ordering,            ordering_enum,              Target::Enum,           GenericRequirement::Exact(0);
     PartialEq,               sym::eq,                  eq_trait,                   Target::Trait,          GenericRequirement::Exact(1);
     PartialOrd,              sym::partial_ord,         partial_ord_trait,          Target::Trait,          GenericRequirement::Exact(1);
     CVoid,                   sym::c_void,              c_void,                     Target::Enum,           GenericRequirement::None;
diff --git a/compiler/rustc_hir_analysis/src/check/intrinsic.rs b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
index f482ae4f5fa..93d71e4bfb9 100644
--- a/compiler/rustc_hir_analysis/src/check/intrinsic.rs
+++ b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
@@ -107,6 +107,7 @@ pub fn intrinsic_operation_unsafety(tcx: TyCtxt<'_>, intrinsic_id: LocalDefId) -
         | sym::cttz
         | sym::bswap
         | sym::bitreverse
+        | sym::three_way_compare
         | sym::discriminant_value
         | sym::type_id
         | sym::likely
@@ -418,6 +419,10 @@ pub fn check_intrinsic_type(
             | sym::bswap
             | sym::bitreverse => (1, 0, vec![param(0)], param(0)),
 
+            sym::three_way_compare => {
+                (1, 0, vec![param(0), param(0)], tcx.ty_ordering_enum(Some(span)))
+            }
+
             sym::add_with_overflow | sym::sub_with_overflow | sym::mul_with_overflow => {
                 (1, 0, vec![param(0), param(0)], Ty::new_tup(tcx, &[param(0), tcx.types.bool]))
             }
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index 2fe63fe4cb8..2d05af85249 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -1438,6 +1438,16 @@ pub enum BinOp {
     Ge,
     /// The `>` operator (greater than)
     Gt,
+    /// The `<=>` operator (three-way comparison, like `Ord::cmp`)
+    ///
+    /// This is supported only on the integer types and `char`, always returning
+    /// [`rustc_hir::LangItem::OrderingEnum`] (aka [`std::cmp::Ordering`]).
+    ///
+    /// [`Rvalue::BinaryOp`]`(BinOp::Cmp, A, B)` returns
+    /// - `Ordering::Less` (`-1_i8`, as a Scalar) if `A < B`
+    /// - `Ordering::Equal` (`0_i8`, as a Scalar) if `A == B`
+    /// - `Ordering::Greater` (`+1_i8`, as a Scalar) if `A > B`
+    Cmp,
     /// The `ptr.offset` operator
     Offset,
 }
diff --git a/compiler/rustc_middle/src/mir/tcx.rs b/compiler/rustc_middle/src/mir/tcx.rs
index 56a0a623397..74c2aef8fcc 100644
--- a/compiler/rustc_middle/src/mir/tcx.rs
+++ b/compiler/rustc_middle/src/mir/tcx.rs
@@ -276,6 +276,11 @@ impl<'tcx> BinOp {
             &BinOp::Eq | &BinOp::Lt | &BinOp::Le | &BinOp::Ne | &BinOp::Ge | &BinOp::Gt => {
                 tcx.types.bool
             }
+            &BinOp::Cmp => {
+                // these should be integer-like types of the same size.
+                assert_eq!(lhs_ty, rhs_ty);
+                tcx.ty_ordering_enum(None)
+            }
         }
     }
 }
@@ -312,7 +317,8 @@ impl BinOp {
             BinOp::Gt => hir::BinOpKind::Gt,
             BinOp::Le => hir::BinOpKind::Le,
             BinOp::Ge => hir::BinOpKind::Ge,
-            BinOp::AddUnchecked
+            BinOp::Cmp
+            | BinOp::AddUnchecked
             | BinOp::SubUnchecked
             | BinOp::MulUnchecked
             | BinOp::ShlUnchecked
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 188cb50849d..23881c4b124 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -956,6 +956,13 @@ impl<'tcx> TyCtxt<'tcx> {
         self.get_lang_items(())
     }
 
+    /// Gets a `Ty` representing the [`LangItem::OrderingEnum`]
+    #[track_caller]
+    pub fn ty_ordering_enum(self, span: Option<Span>) -> Ty<'tcx> {
+        let ordering_enum = self.require_lang_item(hir::LangItem::OrderingEnum, span);
+        self.type_of(ordering_enum).no_bound_vars().unwrap()
+    }
+
     /// Obtain the given diagnostic item's `DefId`. Use `is_diagnostic_item` if you just want to
     /// compare against another `DefId`, since `is_diagnostic_item` is cheaper.
     pub fn get_diagnostic_item(self, name: Symbol) -> Option<DefId> {
diff --git a/compiler/rustc_mir_transform/src/lower_intrinsics.rs b/compiler/rustc_mir_transform/src/lower_intrinsics.rs
index 7d4c1b9c21a..7e8920604c1 100644
--- a/compiler/rustc_mir_transform/src/lower_intrinsics.rs
+++ b/compiler/rustc_mir_transform/src/lower_intrinsics.rs
@@ -90,6 +90,7 @@ impl<'tcx> MirPass<'tcx> for LowerIntrinsics {
                     sym::wrapping_add
                     | sym::wrapping_sub
                     | sym::wrapping_mul
+                    | sym::three_way_compare
                     | sym::unchecked_add
                     | sym::unchecked_sub
                     | sym::unchecked_mul
@@ -109,6 +110,7 @@ impl<'tcx> MirPass<'tcx> for LowerIntrinsics {
                             sym::wrapping_add => BinOp::Add,
                             sym::wrapping_sub => BinOp::Sub,
                             sym::wrapping_mul => BinOp::Mul,
+                            sym::three_way_compare => BinOp::Cmp,
                             sym::unchecked_add => BinOp::AddUnchecked,
                             sym::unchecked_sub => BinOp::SubUnchecked,
                             sym::unchecked_mul => BinOp::MulUnchecked,
diff --git a/compiler/rustc_mir_transform/src/promote_consts.rs b/compiler/rustc_mir_transform/src/promote_consts.rs
index 2951897ebd6..d64cb9b12b2 100644
--- a/compiler/rustc_mir_transform/src/promote_consts.rs
+++ b/compiler/rustc_mir_transform/src/promote_consts.rs
@@ -525,6 +525,7 @@ impl<'tcx> Validator<'_, 'tcx> {
                     | BinOp::Lt
                     | BinOp::Ge
                     | BinOp::Gt
+                    | BinOp::Cmp
                     | BinOp::Offset
                     | BinOp::Add
                     | BinOp::AddUnchecked
diff --git a/compiler/rustc_smir/src/rustc_smir/convert/mir.rs b/compiler/rustc_smir/src/rustc_smir/convert/mir.rs
index b6a722da602..22e7e5323c7 100644
--- a/compiler/rustc_smir/src/rustc_smir/convert/mir.rs
+++ b/compiler/rustc_smir/src/rustc_smir/convert/mir.rs
@@ -493,6 +493,7 @@ impl<'tcx> Stable<'tcx> for mir::BinOp {
             BinOp::Ne => stable_mir::mir::BinOp::Ne,
             BinOp::Ge => stable_mir::mir::BinOp::Ge,
             BinOp::Gt => stable_mir::mir::BinOp::Gt,
+            BinOp::Cmp => stable_mir::mir::BinOp::Cmp,
             BinOp::Offset => stable_mir::mir::BinOp::Offset,
         }
     }
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 998b1a5c7ea..a94de14cb51 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -1813,6 +1813,7 @@ symbols! {
         thread,
         thread_local,
         thread_local_macro,
+        three_way_compare,
         thumb2,
         thumb_mode: "thumb-mode",
         tmm_reg,
diff --git a/compiler/rustc_ty_utils/src/consts.rs b/compiler/rustc_ty_utils/src/consts.rs
index 2a2e53a81ed..acbcc3918b2 100644
--- a/compiler/rustc_ty_utils/src/consts.rs
+++ b/compiler/rustc_ty_utils/src/consts.rs
@@ -82,7 +82,7 @@ fn check_binop(op: mir::BinOp) -> bool {
     match op {
         Add | AddUnchecked | Sub | SubUnchecked | Mul | MulUnchecked | Div | Rem | BitXor
         | BitAnd | BitOr | Shl | ShlUnchecked | Shr | ShrUnchecked | Eq | Lt | Le | Ne | Ge
-        | Gt => true,
+        | Gt | Cmp => true,
         Offset => false,
     }
 }
diff --git a/compiler/stable_mir/src/mir/body.rs b/compiler/stable_mir/src/mir/body.rs
index 7c536a3e914..636bf919f5c 100644
--- a/compiler/stable_mir/src/mir/body.rs
+++ b/compiler/stable_mir/src/mir/body.rs
@@ -329,6 +329,7 @@ pub enum BinOp {
     Ne,
     Ge,
     Gt,
+    Cmp,
     Offset,
 }
 
@@ -368,6 +369,9 @@ impl BinOp {
                 assert!(lhs_kind.is_primitive() || lhs_kind.is_raw_ptr() || lhs_kind.is_fn_ptr());
                 Ty::bool_ty()
             }
+            BinOp::Cmp => {
+                unimplemented!("Should cmp::Ordering be a RigidTy?");
+            }
         }
     }
 }
diff --git a/library/core/src/cmp.rs b/library/core/src/cmp.rs
index b0c5021022f..fa218600ed9 100644
--- a/library/core/src/cmp.rs
+++ b/library/core/src/cmp.rs
@@ -376,6 +376,10 @@ pub struct AssertParamIsEq<T: Eq + ?Sized> {
 /// ```
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
 #[stable(feature = "rust1", since = "1.0.0")]
+// This is a lang item only so that `BinOp::Cmp` in MIR can return it.
+// It has no special behaviour, but does require that the three variants
+// `Less`/`Equal`/`Greater` remain `-1_i8`/`0_i8`/`+1_i8` respectively.
+#[cfg_attr(not(bootstrap), lang = "Ordering")]
 #[repr(i8)]
 pub enum Ordering {
     /// An ordering where a compared value is less than another.
@@ -1554,7 +1558,14 @@ mod impls {
             impl PartialOrd for $t {
                 #[inline]
                 fn partial_cmp(&self, other: &$t) -> Option<Ordering> {
-                    Some(self.cmp(other))
+                    #[cfg(bootstrap)]
+                    {
+                        Some(self.cmp(other))
+                    }
+                    #[cfg(not(bootstrap))]
+                    {
+                        Some(crate::intrinsics::three_way_compare(*self, *other))
+                    }
                 }
                 #[inline(always)]
                 fn lt(&self, other: &$t) -> bool { (*self) < (*other) }
@@ -1570,11 +1581,18 @@ mod impls {
             impl Ord for $t {
                 #[inline]
                 fn cmp(&self, other: &$t) -> Ordering {
-                    // The order here is important to generate more optimal assembly.
-                    // See <https://github.com/rust-lang/rust/issues/63758> for more info.
-                    if *self < *other { Less }
-                    else if *self == *other { Equal }
-                    else { Greater }
+                    #[cfg(bootstrap)]
+                    {
+                        // The order here is important to generate more optimal assembly.
+                        // See <https://github.com/rust-lang/rust/issues/63758> for more info.
+                        if *self < *other { Less }
+                        else if *self == *other { Equal }
+                        else { Greater }
+                    }
+                    #[cfg(not(bootstrap))]
+                    {
+                        crate::intrinsics::three_way_compare(*self, *other)
+                    }
                 }
             }
         )*)
diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs
index 26e813346be..879ee5c5182 100644
--- a/library/core/src/intrinsics.rs
+++ b/library/core/src/intrinsics.rs
@@ -2146,6 +2146,18 @@ extern "rust-intrinsic" {
     #[rustc_nounwind]
     pub fn bitreverse<T: Copy>(x: T) -> T;
 
+    /// Does a three-way comparison between the two integer arguments.
+    ///
+    /// This is included as an intrinsic as it's useful to let it be one thing
+    /// in MIR, rather than the multiple checks and switches that make its IR
+    /// large and difficult to optimize.
+    ///
+    /// The stabilized version of this intrinsic is [`Ord::cmp`].
+    #[cfg(not(bootstrap))]
+    #[rustc_const_unstable(feature = "const_three_way_compare", issue = "none")]
+    #[rustc_safe_intrinsic]
+    pub fn three_way_compare<T: Copy>(lhs: T, rhs: T) -> crate::cmp::Ordering;
+
     /// Performs checked integer addition.
     ///
     /// Note that, unlike most intrinsics, this is safe to call;
diff --git a/library/core/tests/intrinsics.rs b/library/core/tests/intrinsics.rs
index 740565d0df6..eb1e1a0b9b1 100644
--- a/library/core/tests/intrinsics.rs
+++ b/library/core/tests/intrinsics.rs
@@ -99,3 +99,30 @@ fn test_const_deallocate_at_runtime() {
         const_deallocate(core::ptr::null_mut(), 1, 1); // nop
     }
 }
+
+#[cfg(not(bootstrap))]
+#[test]
+fn test_three_way_compare_in_const_contexts() {
+    use core::cmp::Ordering::{self, *};
+    use core::intrinsics::three_way_compare;
+
+    const UNSIGNED_LESS: Ordering = three_way_compare(123_u16, 456);
+    const UNSIGNED_EQUAL: Ordering = three_way_compare(456_u16, 456);
+    const UNSIGNED_GREATER: Ordering = three_way_compare(789_u16, 456);
+    const CHAR_LESS: Ordering = three_way_compare('A', 'B');
+    const CHAR_EQUAL: Ordering = three_way_compare('B', 'B');
+    const CHAR_GREATER: Ordering = three_way_compare('C', 'B');
+    const SIGNED_LESS: Ordering = three_way_compare(123_i64, 456);
+    const SIGNED_EQUAL: Ordering = three_way_compare(456_i64, 456);
+    const SIGNED_GREATER: Ordering = three_way_compare(789_i64, 456);
+
+    assert_eq!(UNSIGNED_LESS, Less);
+    assert_eq!(UNSIGNED_EQUAL, Equal);
+    assert_eq!(UNSIGNED_GREATER, Greater);
+    assert_eq!(CHAR_LESS, Less);
+    assert_eq!(CHAR_EQUAL, Equal);
+    assert_eq!(CHAR_GREATER, Greater);
+    assert_eq!(SIGNED_LESS, Less);
+    assert_eq!(SIGNED_EQUAL, Equal);
+    assert_eq!(SIGNED_GREATER, Greater);
+}
diff --git a/library/core/tests/lib.rs b/library/core/tests/lib.rs
index 52d2b798c91..175c27662d0 100644
--- a/library/core/tests/lib.rs
+++ b/library/core/tests/lib.rs
@@ -22,6 +22,7 @@
 #![feature(const_pointer_is_aligned)]
 #![feature(const_ptr_as_ref)]
 #![feature(const_ptr_write)]
+#![cfg_attr(not(bootstrap), feature(const_three_way_compare))]
 #![feature(const_trait_impl)]
 #![feature(const_likely)]
 #![feature(const_location_fields)]
diff --git a/tests/assembly/x86_64-cmp.rs b/tests/assembly/x86_64-cmp.rs
new file mode 100644
index 00000000000..31efdda1bfa
--- /dev/null
+++ b/tests/assembly/x86_64-cmp.rs
@@ -0,0 +1,51 @@
+//@ revisions: DEBUG OPTIM
+//@ [DEBUG] compile-flags: -C opt-level=0
+//@ [OPTIM] compile-flags: -C opt-level=3
+//@ assembly-output: emit-asm
+//@ compile-flags: --crate-type=lib -C llvm-args=-x86-asm-syntax=intel
+//@ only-x86_64
+//@ ignore-sgx
+
+#![feature(core_intrinsics)]
+
+use std::intrinsics::three_way_compare;
+
+#[no_mangle]
+// CHECK-LABEL: signed_cmp:
+pub fn signed_cmp(a: i16, b: i16) -> std::cmp::Ordering {
+    // DEBUG: cmp
+    // DEBUG: setg
+    // DEBUG: and
+    // DEBUG: cmp
+    // DEBUG: setl
+    // DEBUG: and
+    // DEBUG: sub
+
+    // OPTIM: xor
+    // OPTIM: cmp
+    // OPTIM: setne
+    // OPTIM: mov
+    // OPTIM: cmovge
+    // OPTIM: ret
+    three_way_compare(a, b)
+}
+
+#[no_mangle]
+// CHECK-LABEL: unsigned_cmp:
+pub fn unsigned_cmp(a: u16, b: u16) -> std::cmp::Ordering {
+    // DEBUG: cmp
+    // DEBUG: seta
+    // DEBUG: and
+    // DEBUG: cmp
+    // DEBUG: setb
+    // DEBUG: and
+    // DEBUG: sub
+
+    // OPTIM: xor
+    // OPTIM: cmp
+    // OPTIM: setne
+    // OPTIM: mov
+    // OPTIM: cmovae
+    // OPTIM: ret
+    three_way_compare(a, b)
+}
diff --git a/tests/codegen/intrinsics/three_way_compare.rs b/tests/codegen/intrinsics/three_way_compare.rs
new file mode 100644
index 00000000000..f3b631abc22
--- /dev/null
+++ b/tests/codegen/intrinsics/three_way_compare.rs
@@ -0,0 +1,47 @@
+//@ revisions: DEBUG OPTIM
+//@ [DEBUG] compile-flags: -C opt-level=0
+//@ [OPTIM] compile-flags: -C opt-level=3
+//@ compile-flags: -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+#![feature(core_intrinsics)]
+
+use std::intrinsics::three_way_compare;
+
+#[no_mangle]
+// CHECK-LABEL: @signed_cmp
+// DEBUG-SAME: (i16 %a, i16 %b)
+// OPTIM-SAME: (i16 noundef %a, i16 noundef %b)
+pub fn signed_cmp(a: i16, b: i16) -> std::cmp::Ordering {
+    // DEBUG: %[[GT:.+]] = icmp sgt i16 %a, %b
+    // DEBUG: %[[ZGT:.+]] = zext i1 %[[GT]] to i8
+    // DEBUG: %[[LT:.+]] = icmp slt i16 %a, %b
+    // DEBUG: %[[ZLT:.+]] = zext i1 %[[LT]] to i8
+    // DEBUG: %[[R:.+]] = sub nsw i8 %[[ZGT]], %[[ZLT]]
+
+    // OPTIM: %[[LT:.+]] = icmp slt i16 %a, %b
+    // OPTIM: %[[NE:.+]] = icmp ne i16 %a, %b
+    // OPTIM: %[[CGE:.+]] = select i1 %[[NE]], i8 1, i8 0
+    // OPTIM: %[[CGEL:.+]] = select i1 %[[LT]], i8 -1, i8 %[[CGE]]
+    // OPTIM: ret i8 %[[CGEL]]
+    three_way_compare(a, b)
+}
+
+#[no_mangle]
+// CHECK-LABEL: @unsigned_cmp
+// DEBUG-SAME: (i16 %a, i16 %b)
+// OPTIM-SAME: (i16 noundef %a, i16 noundef %b)
+pub fn unsigned_cmp(a: u16, b: u16) -> std::cmp::Ordering {
+    // DEBUG: %[[GT:.+]] = icmp ugt i16 %a, %b
+    // DEBUG: %[[ZGT:.+]] = zext i1 %[[GT]] to i8
+    // DEBUG: %[[LT:.+]] = icmp ult i16 %a, %b
+    // DEBUG: %[[ZLT:.+]] = zext i1 %[[LT]] to i8
+    // DEBUG: %[[R:.+]] = sub nsw i8 %[[ZGT]], %[[ZLT]]
+
+    // OPTIM: %[[LT:.+]] = icmp ult i16 %a, %b
+    // OPTIM: %[[NE:.+]] = icmp ne i16 %a, %b
+    // OPTIM: %[[CGE:.+]] = select i1 %[[NE]], i8 1, i8 0
+    // OPTIM: %[[CGEL:.+]] = select i1 %[[LT]], i8 -1, i8 %[[CGE]]
+    // OPTIM: ret i8 %[[CGEL]]
+    three_way_compare(a, b)
+}
diff --git a/tests/mir-opt/lower_intrinsics.rs b/tests/mir-opt/lower_intrinsics.rs
index 278ddfce1c3..e9eb392e795 100644
--- a/tests/mir-opt/lower_intrinsics.rs
+++ b/tests/mir-opt/lower_intrinsics.rs
@@ -229,3 +229,18 @@ pub unsafe fn ptr_offset(p: *const i32, d: isize) -> *const i32 {
 
     core::intrinsics::offset(p, d)
 }
+
+// EMIT_MIR lower_intrinsics.three_way_compare_char.LowerIntrinsics.diff
+pub fn three_way_compare_char(a: char, b: char) {
+    let _x = core::intrinsics::three_way_compare(a, b);
+}
+
+// EMIT_MIR lower_intrinsics.three_way_compare_signed.LowerIntrinsics.diff
+pub fn three_way_compare_signed(a: i16, b: i16) {
+    core::intrinsics::three_way_compare(a, b);
+}
+
+// EMIT_MIR lower_intrinsics.three_way_compare_unsigned.LowerIntrinsics.diff
+pub fn three_way_compare_unsigned(a: u32, b: u32) {
+    let _x = core::intrinsics::three_way_compare(a, b);
+}
diff --git a/tests/mir-opt/lower_intrinsics.three_way_compare_char.LowerIntrinsics.panic-abort.diff b/tests/mir-opt/lower_intrinsics.three_way_compare_char.LowerIntrinsics.panic-abort.diff
new file mode 100644
index 00000000000..816d6209715
--- /dev/null
+++ b/tests/mir-opt/lower_intrinsics.three_way_compare_char.LowerIntrinsics.panic-abort.diff
@@ -0,0 +1,34 @@
+- // MIR for `three_way_compare_char` before LowerIntrinsics
++ // MIR for `three_way_compare_char` after LowerIntrinsics
+  
+  fn three_way_compare_char(_1: char, _2: char) -> () {
+      debug a => _1;
+      debug b => _2;
+      let mut _0: ();
+      let _3: std::cmp::Ordering;
+      let mut _4: char;
+      let mut _5: char;
+      scope 1 {
+          debug _x => _3;
+      }
+  
+      bb0: {
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = _1;
+          StorageLive(_5);
+          _5 = _2;
+-         _3 = three_way_compare::<char>(move _4, move _5) -> [return: bb1, unwind unreachable];
++         _3 = Cmp(move _4, move _5);
++         goto -> bb1;
+      }
+  
+      bb1: {
+          StorageDead(_5);
+          StorageDead(_4);
+          _0 = const ();
+          StorageDead(_3);
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/lower_intrinsics.three_way_compare_char.LowerIntrinsics.panic-unwind.diff b/tests/mir-opt/lower_intrinsics.three_way_compare_char.LowerIntrinsics.panic-unwind.diff
new file mode 100644
index 00000000000..80b4bd7a2be
--- /dev/null
+++ b/tests/mir-opt/lower_intrinsics.three_way_compare_char.LowerIntrinsics.panic-unwind.diff
@@ -0,0 +1,34 @@
+- // MIR for `three_way_compare_char` before LowerIntrinsics
++ // MIR for `three_way_compare_char` after LowerIntrinsics
+  
+  fn three_way_compare_char(_1: char, _2: char) -> () {
+      debug a => _1;
+      debug b => _2;
+      let mut _0: ();
+      let _3: std::cmp::Ordering;
+      let mut _4: char;
+      let mut _5: char;
+      scope 1 {
+          debug _x => _3;
+      }
+  
+      bb0: {
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = _1;
+          StorageLive(_5);
+          _5 = _2;
+-         _3 = three_way_compare::<char>(move _4, move _5) -> [return: bb1, unwind continue];
++         _3 = Cmp(move _4, move _5);
++         goto -> bb1;
+      }
+  
+      bb1: {
+          StorageDead(_5);
+          StorageDead(_4);
+          _0 = const ();
+          StorageDead(_3);
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/lower_intrinsics.three_way_compare_signed.LowerIntrinsics.panic-abort.diff b/tests/mir-opt/lower_intrinsics.three_way_compare_signed.LowerIntrinsics.panic-abort.diff
new file mode 100644
index 00000000000..05c20aaa09a
--- /dev/null
+++ b/tests/mir-opt/lower_intrinsics.three_way_compare_signed.LowerIntrinsics.panic-abort.diff
@@ -0,0 +1,31 @@
+- // MIR for `three_way_compare_signed` before LowerIntrinsics
++ // MIR for `three_way_compare_signed` after LowerIntrinsics
+  
+  fn three_way_compare_signed(_1: i16, _2: i16) -> () {
+      debug a => _1;
+      debug b => _2;
+      let mut _0: ();
+      let _3: std::cmp::Ordering;
+      let mut _4: i16;
+      let mut _5: i16;
+  
+      bb0: {
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = _1;
+          StorageLive(_5);
+          _5 = _2;
+-         _3 = three_way_compare::<i16>(move _4, move _5) -> [return: bb1, unwind unreachable];
++         _3 = Cmp(move _4, move _5);
++         goto -> bb1;
+      }
+  
+      bb1: {
+          StorageDead(_5);
+          StorageDead(_4);
+          StorageDead(_3);
+          _0 = const ();
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/lower_intrinsics.three_way_compare_signed.LowerIntrinsics.panic-unwind.diff b/tests/mir-opt/lower_intrinsics.three_way_compare_signed.LowerIntrinsics.panic-unwind.diff
new file mode 100644
index 00000000000..8a254d02a47
--- /dev/null
+++ b/tests/mir-opt/lower_intrinsics.three_way_compare_signed.LowerIntrinsics.panic-unwind.diff
@@ -0,0 +1,31 @@
+- // MIR for `three_way_compare_signed` before LowerIntrinsics
++ // MIR for `three_way_compare_signed` after LowerIntrinsics
+  
+  fn three_way_compare_signed(_1: i16, _2: i16) -> () {
+      debug a => _1;
+      debug b => _2;
+      let mut _0: ();
+      let _3: std::cmp::Ordering;
+      let mut _4: i16;
+      let mut _5: i16;
+  
+      bb0: {
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = _1;
+          StorageLive(_5);
+          _5 = _2;
+-         _3 = three_way_compare::<i16>(move _4, move _5) -> [return: bb1, unwind continue];
++         _3 = Cmp(move _4, move _5);
++         goto -> bb1;
+      }
+  
+      bb1: {
+          StorageDead(_5);
+          StorageDead(_4);
+          StorageDead(_3);
+          _0 = const ();
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/lower_intrinsics.three_way_compare_unsigned.LowerIntrinsics.panic-abort.diff b/tests/mir-opt/lower_intrinsics.three_way_compare_unsigned.LowerIntrinsics.panic-abort.diff
new file mode 100644
index 00000000000..437614ec673
--- /dev/null
+++ b/tests/mir-opt/lower_intrinsics.three_way_compare_unsigned.LowerIntrinsics.panic-abort.diff
@@ -0,0 +1,34 @@
+- // MIR for `three_way_compare_unsigned` before LowerIntrinsics
++ // MIR for `three_way_compare_unsigned` after LowerIntrinsics
+  
+  fn three_way_compare_unsigned(_1: u32, _2: u32) -> () {
+      debug a => _1;
+      debug b => _2;
+      let mut _0: ();
+      let _3: std::cmp::Ordering;
+      let mut _4: u32;
+      let mut _5: u32;
+      scope 1 {
+          debug _x => _3;
+      }
+  
+      bb0: {
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = _1;
+          StorageLive(_5);
+          _5 = _2;
+-         _3 = three_way_compare::<u32>(move _4, move _5) -> [return: bb1, unwind unreachable];
++         _3 = Cmp(move _4, move _5);
++         goto -> bb1;
+      }
+  
+      bb1: {
+          StorageDead(_5);
+          StorageDead(_4);
+          _0 = const ();
+          StorageDead(_3);
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/lower_intrinsics.three_way_compare_unsigned.LowerIntrinsics.panic-unwind.diff b/tests/mir-opt/lower_intrinsics.three_way_compare_unsigned.LowerIntrinsics.panic-unwind.diff
new file mode 100644
index 00000000000..7d6137979c8
--- /dev/null
+++ b/tests/mir-opt/lower_intrinsics.three_way_compare_unsigned.LowerIntrinsics.panic-unwind.diff
@@ -0,0 +1,34 @@
+- // MIR for `three_way_compare_unsigned` before LowerIntrinsics
++ // MIR for `three_way_compare_unsigned` after LowerIntrinsics
+  
+  fn three_way_compare_unsigned(_1: u32, _2: u32) -> () {
+      debug a => _1;
+      debug b => _2;
+      let mut _0: ();
+      let _3: std::cmp::Ordering;
+      let mut _4: u32;
+      let mut _5: u32;
+      scope 1 {
+          debug _x => _3;
+      }
+  
+      bb0: {
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = _1;
+          StorageLive(_5);
+          _5 = _2;
+-         _3 = three_way_compare::<u32>(move _4, move _5) -> [return: bb1, unwind continue];
++         _3 = Cmp(move _4, move _5);
++         goto -> bb1;
+      }
+  
+      bb1: {
+          StorageDead(_5);
+          StorageDead(_4);
+          _0 = const ();
+          StorageDead(_3);
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/pre-codegen/derived_ord.rs b/tests/mir-opt/pre-codegen/derived_ord.rs
new file mode 100644
index 00000000000..bad751edf84
--- /dev/null
+++ b/tests/mir-opt/pre-codegen/derived_ord.rs
@@ -0,0 +1,9 @@
+// skip-filecheck
+//@ compile-flags: -O -Zmir-opt-level=2 -Cdebuginfo=0
+
+#![crate_type = "lib"]
+
+#[derive(PartialOrd, PartialEq)]
+pub struct MultiField(char, i16);
+
+// EMIT_MIR derived_ord.{impl#0}-partial_cmp.PreCodegen.after.mir
diff --git a/tests/mir-opt/pre-codegen/derived_ord.{impl#0}-partial_cmp.PreCodegen.after.mir b/tests/mir-opt/pre-codegen/derived_ord.{impl#0}-partial_cmp.PreCodegen.after.mir
new file mode 100644
index 00000000000..a6c64425912
--- /dev/null
+++ b/tests/mir-opt/pre-codegen/derived_ord.{impl#0}-partial_cmp.PreCodegen.after.mir
@@ -0,0 +1,78 @@
+// MIR for `<impl at $DIR/derived_ord.rs:6:10: 6:20>::partial_cmp` after PreCodegen
+
+fn <impl at $DIR/derived_ord.rs:6:10: 6:20>::partial_cmp(_1: &MultiField, _2: &MultiField) -> Option<std::cmp::Ordering> {
+    debug self => _1;
+    debug other => _2;
+    let mut _0: std::option::Option<std::cmp::Ordering>;
+    let mut _3: &char;
+    let mut _4: &char;
+    let mut _8: std::option::Option<std::cmp::Ordering>;
+    let mut _9: i8;
+    let mut _10: &i16;
+    let mut _11: &i16;
+    scope 1 {
+        debug cmp => _8;
+    }
+    scope 2 (inlined std::cmp::impls::<impl PartialOrd for char>::partial_cmp) {
+        debug self => _3;
+        debug other => _4;
+        let mut _5: char;
+        let mut _6: char;
+        let mut _7: std::cmp::Ordering;
+    }
+    scope 3 (inlined std::cmp::impls::<impl PartialOrd for i16>::partial_cmp) {
+        debug self => _10;
+        debug other => _11;
+        let mut _12: i16;
+        let mut _13: i16;
+        let mut _14: std::cmp::Ordering;
+    }
+
+    bb0: {
+        StorageLive(_3);
+        _3 = &((*_1).0: char);
+        StorageLive(_4);
+        _4 = &((*_2).0: char);
+        StorageLive(_5);
+        _5 = ((*_1).0: char);
+        StorageLive(_6);
+        _6 = ((*_2).0: char);
+        _7 = Cmp(move _5, move _6);
+        StorageDead(_6);
+        StorageDead(_5);
+        _8 = Option::<std::cmp::Ordering>::Some(_7);
+        StorageDead(_4);
+        StorageDead(_3);
+        _9 = discriminant(_7);
+        switchInt(move _9) -> [0: bb1, otherwise: bb2];
+    }
+
+    bb1: {
+        StorageLive(_10);
+        _10 = &((*_1).1: i16);
+        StorageLive(_11);
+        _11 = &((*_2).1: i16);
+        StorageLive(_14);
+        StorageLive(_12);
+        _12 = ((*_1).1: i16);
+        StorageLive(_13);
+        _13 = ((*_2).1: i16);
+        _14 = Cmp(move _12, move _13);
+        StorageDead(_13);
+        StorageDead(_12);
+        _0 = Option::<std::cmp::Ordering>::Some(move _14);
+        StorageDead(_14);
+        StorageDead(_11);
+        StorageDead(_10);
+        goto -> bb3;
+    }
+
+    bb2: {
+        _0 = _8;
+        goto -> bb3;
+    }
+
+    bb3: {
+        return;
+    }
+}