https://github.com/HendrikHuebner created 
https://github.com/llvm/llvm-project/pull/169963

This PR upstreams the three way compare op from the incubator repo

From 32285f45356c6d4d1986e0ee144b6821e992f4c3 Mon Sep 17 00:00:00 2001
From: hhuebner <[email protected]>
Date: Fri, 28 Nov 2025 23:41:30 +0100
Subject: [PATCH] [CIR] Upstream three way compare op

---
 .../include/clang/CIR/Dialect/IR/CIRAttrs.td  |  62 ++++
 clang/include/clang/CIR/Dialect/IR/CIROps.td  |  65 ++++
 clang/lib/CIR/CodeGen/CIRGenBuilder.h         |  31 ++
 clang/lib/CIR/CodeGen/CIRGenExprAggregate.cpp |  58 +++-
 clang/lib/CIR/Dialect/IR/CIRAttrs.cpp         |  54 +++
 clang/lib/CIR/Dialect/IR/CIRDialect.cpp       |  20 ++
 .../Dialect/Transforms/LoweringPrepare.cpp    |  39 ++-
 .../CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp |  53 +++
 clang/test/CIR/CodeGen/Inputs/std-compare.h   | 307 ++++++++++++++++++
 clang/test/CIR/CodeGen/three-way-cmp.cpp      |  72 ++++
 10 files changed, 759 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/CIR/CodeGen/Inputs/std-compare.h
 create mode 100644 clang/test/CIR/CodeGen/three-way-cmp.cpp

diff --git a/clang/include/clang/CIR/Dialect/IR/CIRAttrs.td 
b/clang/include/clang/CIR/Dialect/IR/CIRAttrs.td
index 12bc9cf7b5b04..0fce5b8bc6b72 100644
--- a/clang/include/clang/CIR/Dialect/IR/CIRAttrs.td
+++ b/clang/include/clang/CIR/Dialect/IR/CIRAttrs.td
@@ -447,6 +447,68 @@ def CIR_ConstPtrAttr : CIR_Attr<"ConstPtr", "ptr", 
[TypedAttrInterface]> {
   }];
 }
 
+//===----------------------------------------------------------------------===//
+// CmpThreeWayInfoAttr
+//===----------------------------------------------------------------------===//
+
+def CIR_CmpOrdering : CIR_I32EnumAttr<
+  "CmpOrdering", "three-way comparison ordering kind", [
+    I32EnumAttrCase<"Strong", 0, "strong">,
+    I32EnumAttrCase<"Partial", 1, "partial">
+]> {
+  let genSpecializedAttr = 0;
+}
+
+def CIR_CmpThreeWayInfoAttr : CIR_Attr<"CmpThreeWayInfo", "cmp3way_info"> {
+  let summary = "Holds information about a three-way comparison operation";
+  let description = [{
+    The `#cmp3way_info` attribute contains information about a three-way
+    comparison operation `cir.cmp3way`.
+
+    The `ordering` parameter gives the ordering kind of the three-way 
comparison
+    operation. It may be either strong ordering or partial ordering.
+
+    Given the two input operands of the three-way comparison operation `lhs` 
and
+    `rhs`, the `lt`, `eq`, `gt`, and `unordered` parameters gives the result
+    value that should be produced by the three-way comparison operation when 
the
+    ordering between `lhs` and `rhs` is `lhs < rhs`, `lhs == rhs`, `lhs > rhs`,
+    or neither, respectively.
+  }];
+
+  let parameters = (ins
+    EnumParameter<CIR_CmpOrdering>:$ordering,
+    "int64_t":$lt, "int64_t":$eq, "int64_t":$gt,
+    OptionalParameter<"std::optional<int64_t>">:$unordered
+  );
+
+  let builders = [
+    AttrBuilder<(ins "int64_t":$lt, "int64_t":$eq, "int64_t":$gt), [{
+      return $_get($_ctxt, CmpOrdering::Strong, lt, eq, gt, std::nullopt);
+    }]>,
+    AttrBuilder<(ins "int64_t":$lt, "int64_t":$eq, "int64_t":$gt,
+                     "int64_t":$unordered), [{
+      return $_get($_ctxt, CmpOrdering::Partial, lt, eq, gt, unordered);
+    }]>,
+  ];
+
+  let extraClassDeclaration = [{
+    /// Get attribute alias name for this attribute.
+    std::string getAlias() const;
+  }];
+
+  let assemblyFormat = [{
+    `<`
+      $ordering `,`
+      `lt` `=` $lt `,`
+      `eq` `=` $eq `,`
+      `gt` `=` $gt
+      (`,` `unordered` `=` $unordered^)?
+    `>`
+  }];
+
+  let genVerifyDecl = 1;
+}
+
 
//===----------------------------------------------------------------------===//
 // GlobalViewAttr
 
//===----------------------------------------------------------------------===//
diff --git a/clang/include/clang/CIR/Dialect/IR/CIROps.td 
b/clang/include/clang/CIR/Dialect/IR/CIROps.td
index a19c4f951fff9..411ee8dc5984a 100644
--- a/clang/include/clang/CIR/Dialect/IR/CIROps.td
+++ b/clang/include/clang/CIR/Dialect/IR/CIROps.td
@@ -958,6 +958,71 @@ def CIR_ScopeOp : CIR_Op<"scope", [
   let hasLLVMLowering = false;
 }
 
+//===----------------------------------------------------------------------===//
+// CmpThreeWayOp
+//===----------------------------------------------------------------------===//
+
+def CIR_CmpThreeWayOp : CIR_Op<"cmp3way", [Pure, SameTypeOperands]> {
+  let summary = "Compare two values with C++ three-way comparison semantics";
+  let description = [{
+    The `cir.cmp3way` operation models the `<=>` operator in C++20. It takes 
two
+    operands with the same type and produces a result indicating the ordering
+    between the two input operands.
+
+    The result of the operation is a signed integer that indicates the ordering
+    between the two input operands.
+
+    There are two kinds of ordering: strong ordering and partial ordering.
+    Comparing different types of values yields different kinds of orderings.
+    The `info` parameter gives the ordering kind and other necessary 
information
+    about the comparison.
+
+    Example:
+
+    ```mlir
+    !s32i = !cir.int<s, 32>
+
+    #cmp3way_strong = #cmp3way_info<strong, lt = -1, eq = 0, gt = 1>
+    #cmp3way_partial = #cmp3way_info<strong, lt = -1, eq = 0, gt = 1, 
unordered = 2>
+
+    %0 = cir.const #cir.int<0> : !s32i
+    %1 = cir.const #cir.int<1> : !s32i
+    %2 = cir.cmp3way(%0 : !s32i, %1, #cmp3way_strong) : !s8i
+
+    %3 = cir.const #cir.fp<0.0> : !cir.float
+    %4 = cir.const #cir.fp<1.0> : !cir.float
+    %5 = cir.cmp3way(%3 : !cir.float, %4, #cmp3way_partial) : !s8i
+    ```
+  }];
+
+  let arguments = (ins
+    CIR_AnyType:$lhs,
+    CIR_AnyType:$rhs,
+    CIR_CmpThreeWayInfoAttr:$info
+  );
+
+  let results = (outs CIR_AnySIntType:$result);
+
+  let assemblyFormat = [{
+    `(` $lhs `:` type($lhs) `,` $rhs `,` qualified($info) `)`
+    `:` type($result) attr-dict
+  }];
+
+  let extraClassDeclaration = [{
+    /// Determine whether this three-way comparison produces a strong ordering.
+    bool isStrongOrdering() {
+      return getInfo().getOrdering() == cir::CmpOrdering::Strong;
+    }
+
+    /// Determine whether this three-way comparison compares integral operands.
+    bool isIntegralComparison() {
+      return mlir::isa<cir::IntType>(getLhs().getType());
+    }
+  }];
+
+  let hasVerifier = 1;
+}
+
 
//===----------------------------------------------------------------------===//
 // SwitchOp
 
//===----------------------------------------------------------------------===//
diff --git a/clang/lib/CIR/CodeGen/CIRGenBuilder.h 
b/clang/lib/CIR/CodeGen/CIRGenBuilder.h
index 85b38120169fd..c296449084614 100644
--- a/clang/lib/CIR/CodeGen/CIRGenBuilder.h
+++ b/clang/lib/CIR/CodeGen/CIRGenBuilder.h
@@ -567,6 +567,37 @@ class CIRGenBuilderTy : public cir::CIRBaseBuilderTy {
     return cir::StackRestoreOp::create(*this, loc, v);
   }
 
+  cir::CmpThreeWayOp createThreeWayCmpStrong(mlir::Location loc,
+                                             mlir::Value lhs, mlir::Value rhs,
+                                             const llvm::APSInt &ltRes,
+                                             const llvm::APSInt &eqRes,
+                                             const llvm::APSInt &gtRes) {
+    assert(ltRes.getBitWidth() == eqRes.getBitWidth() &&
+           ltRes.getBitWidth() == gtRes.getBitWidth() &&
+           "the three comparison results must have the same bit width");
+    cir::IntType cmpResultTy = getSIntNTy(ltRes.getBitWidth());
+    auto infoAttr = cir::CmpThreeWayInfoAttr::get(
+        getContext(), ltRes.getSExtValue(), eqRes.getSExtValue(), 
gtRes.getSExtValue());
+    return cir::CmpThreeWayOp::create(*this, loc, cmpResultTy, lhs, rhs,
+                                      infoAttr);
+  }
+
+  cir::CmpThreeWayOp
+  createThreeWayCmpPartial(mlir::Location loc, mlir::Value lhs, mlir::Value 
rhs,
+                           const llvm::APSInt &ltRes, const llvm::APSInt 
&eqRes,
+                           const llvm::APSInt &gtRes,
+                           const llvm::APSInt &unorderedRes) {
+    assert(ltRes.getBitWidth() == eqRes.getBitWidth() &&
+           ltRes.getBitWidth() == gtRes.getBitWidth() &&
+           ltRes.getBitWidth() == unorderedRes.getBitWidth() &&
+           "the four comparison results must have the same bit width");
+    auto cmpResultTy = getSIntNTy(ltRes.getBitWidth());
+    auto infoAttr = cir::CmpThreeWayInfoAttr::get(
+        getContext(), ltRes.getSExtValue(), eqRes.getSExtValue(), 
gtRes.getSExtValue(), unorderedRes.getSExtValue());
+    return cir::CmpThreeWayOp::create(*this, loc, cmpResultTy, lhs, rhs,
+                                      infoAttr);
+  }
+
   mlir::Value createSetBitfield(mlir::Location loc, mlir::Type resultType,
                                 Address dstAddr, mlir::Type storageType,
                                 mlir::Value src, const CIRGenBitFieldInfo 
&info,
diff --git a/clang/lib/CIR/CodeGen/CIRGenExprAggregate.cpp 
b/clang/lib/CIR/CodeGen/CIRGenExprAggregate.cpp
index 872fc8d14ad95..985da8283baac 100644
--- a/clang/lib/CIR/CodeGen/CIRGenExprAggregate.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenExprAggregate.cpp
@@ -18,6 +18,7 @@
 #include "clang/AST/Expr.h"
 #include "clang/AST/RecordLayout.h"
 #include "clang/AST/StmtVisitor.h"
+#include "llvm/IR/Value.h"
 #include <cstdint>
 
 using namespace clang;
@@ -298,8 +299,63 @@ class AggExprEmitter : public StmtVisitor<AggExprEmitter> {
     Visit(e->getRHS());
   }
   void VisitBinCmp(const BinaryOperator *e) {
-    cgf.cgm.errorNYI(e->getSourceRange(), "AggExprEmitter: VisitBinCmp");
+    assert(cgf.getContext().hasSameType(e->getLHS()->getType(),
+                                        e->getRHS()->getType()));
+    const ComparisonCategoryInfo &cmpInfo =
+        cgf.getContext().CompCategories.getInfoForType(e->getType());
+    assert(cmpInfo.Record->isTriviallyCopyable() &&
+          "cannot copy non-trivially copyable aggregate");
+
+    QualType argTy = e->getLHS()->getType();
+
+    if (!argTy->isIntegralOrEnumerationType() && !argTy->isRealFloatingType() 
&&
+        !argTy->isNullPtrType() && !argTy->isPointerType() &&
+        !argTy->isMemberPointerType() && !argTy->isAnyComplexType())
+      llvm_unreachable("aggregate three-way comparison");
+
+    mlir::Location loc = cgf.getLoc(e->getSourceRange());
+    CIRGenBuilderTy builder = cgf.getBuilder();
+
+    if (e->getType()->isAnyComplexType())
+      llvm_unreachable("NYI");
+
+    mlir::Value lhs = cgf.emitAnyExpr(e->getLHS()).getValue();
+    mlir::Value rhs = cgf.emitAnyExpr(e->getRHS()).getValue();
+
+    mlir::Value resultScalar;
+    if (argTy->isNullPtrType()) {
+      resultScalar =
+        builder.getConstInt(loc, cmpInfo.getEqualOrEquiv()->getIntValue());
+    } else {
+      llvm::APSInt ltRes = cmpInfo.getLess()->getIntValue();
+      llvm::APSInt eqRes = cmpInfo.getEqualOrEquiv()->getIntValue();
+      llvm::APSInt gtRes = cmpInfo.getGreater()->getIntValue();
+      if (!cmpInfo.isPartial()) {
+        // Strong ordering.
+        resultScalar = builder.createThreeWayCmpStrong(loc, lhs, rhs, ltRes,
+                                                          eqRes, gtRes);
+      } else {
+        // Partial ordering.
+        llvm::APSInt unorderedRes = cmpInfo.getUnordered()->getIntValue();
+        resultScalar = builder.createThreeWayCmpPartial(
+            loc, lhs, rhs, ltRes, eqRes, gtRes, unorderedRes);
+      }
+    }
+
+    // Create the return value in the destination slot.
+    ensureDest(loc, e->getType());
+    LValue destLVal = cgf.makeAddrLValue(dest.getAddress(), e->getType());
+
+    // Emit the address of the first (and only) field in the comparison 
category
+    // type, and initialize it from the constant integer value produced above.
+    const FieldDecl *resultField = *cmpInfo.Record->field_begin();
+    LValue fieldLVal = cgf.emitLValueForFieldInitialization(destLVal, 
resultField,
+                                                          
resultField->getName());
+    cgf.emitStoreThroughLValue(RValue::get(resultScalar), fieldLVal);
+
+    // All done! The result is in the dest slot.
   }
+
   void VisitCXXRewrittenBinaryOperator(CXXRewrittenBinaryOperator *e) {
     cgf.cgm.errorNYI(e->getSourceRange(),
                      "AggExprEmitter: VisitCXXRewrittenBinaryOperator");
diff --git a/clang/lib/CIR/Dialect/IR/CIRAttrs.cpp 
b/clang/lib/CIR/Dialect/IR/CIRAttrs.cpp
index 64ac97025e7c7..70e483de91d8d 100644
--- a/clang/lib/CIR/Dialect/IR/CIRAttrs.cpp
+++ b/clang/lib/CIR/Dialect/IR/CIRAttrs.cpp
@@ -267,6 +267,60 @@ LogicalResult 
FPAttr::verify(function_ref<InFlightDiagnostic()> emitError,
   return success();
 }
 
+
+//===----------------------------------------------------------------------===//
+// CmpThreeWayInfoAttr definitions
+//===----------------------------------------------------------------------===//
+
+std::string CmpThreeWayInfoAttr::getAlias() const {
+  std::string alias = "cmp3way_info";
+
+  if (getOrdering() == CmpOrdering::Strong)
+    alias.append("_strong_");
+  else
+    alias.append("_partial_");
+
+  auto appendInt = [&](int64_t value) {
+    if (value < 0) {
+      alias.push_back('n');
+      value = -value;
+    }
+    alias.append(std::to_string(value));
+  };
+
+  alias.append("lt");
+  appendInt(getLt());
+  alias.append("eq");
+  appendInt(getEq());
+  alias.append("gt");
+  appendInt(getGt());
+
+  if (std::optional<int> unordered = getUnordered()) {
+    alias.append("un");
+    appendInt(unordered.value());
+  }
+
+  return alias;
+}
+
+LogicalResult
+CmpThreeWayInfoAttr::verify(function_ref<InFlightDiagnostic()> emitError,
+                            CmpOrdering ordering, int64_t lt, int64_t eq,
+                            int64_t gt, std::optional<int64_t> unordered) {
+  // The presense of unordered must match the value of ordering.
+  if (ordering == CmpOrdering::Strong && unordered) {
+    emitError() << "strong ordering does not include unordered ordering";
+    return failure();
+  }
+  if (ordering == CmpOrdering::Partial && !unordered) {
+    emitError() << "partial ordering lacks unordered ordering";
+    return failure();
+  }
+
+  return success();
+}
+
+
 
//===----------------------------------------------------------------------===//
 // ConstComplexAttr definitions
 
//===----------------------------------------------------------------------===//
diff --git a/clang/lib/CIR/Dialect/IR/CIRDialect.cpp 
b/clang/lib/CIR/Dialect/IR/CIRDialect.cpp
index 6bf543cf794b7..cb90dbeda7abf 100644
--- a/clang/lib/CIR/Dialect/IR/CIRDialect.cpp
+++ b/clang/lib/CIR/Dialect/IR/CIRDialect.cpp
@@ -78,6 +78,11 @@ struct CIROpAsmDialectInterface : public 
OpAsmDialectInterface {
       os << dynCastInfoAttr.getAlias();
       return AliasResult::FinalAlias;
     }
+    if (auto cmpThreeWayInfoAttr =
+            mlir::dyn_cast<cir::CmpThreeWayInfoAttr>(attr)) {
+      os << cmpThreeWayInfoAttr.getAlias();
+      return AliasResult::FinalAlias;
+    }
     return AliasResult::NoAlias;
   }
 };
@@ -1132,6 +1137,21 @@ Block 
*cir::BrCondOp::getSuccessorForOperands(ArrayRef<Attribute> operands) {
   return nullptr;
 }
 
+
+//===----------------------------------------------------------------------===//
+// CmpThreeWayOp
+//===----------------------------------------------------------------------===//
+
+mlir::LogicalResult CmpThreeWayOp::verify() {
+  // Type of the result must be a signed integer type.
+  if (!getType().isSigned()) {
+    emitOpError() << "result type of cir.cmp3way must be a signed integer 
type";
+    return failure();
+  }
+
+  return success();
+}
+
 
//===----------------------------------------------------------------------===//
 // CaseOp
 
//===----------------------------------------------------------------------===//
diff --git a/clang/lib/CIR/Dialect/Transforms/LoweringPrepare.cpp 
b/clang/lib/CIR/Dialect/Transforms/LoweringPrepare.cpp
index f8f354c2d1072..14ecd7013afeb 100644
--- a/clang/lib/CIR/Dialect/Transforms/LoweringPrepare.cpp
+++ b/clang/lib/CIR/Dialect/Transforms/LoweringPrepare.cpp
@@ -71,6 +71,7 @@ struct LoweringPreparePass
   void lowerComplexMulOp(cir::ComplexMulOp op);
   void lowerUnaryOp(cir::UnaryOp op);
   void lowerGlobalOp(cir::GlobalOp op);
+  void lowerThreeWayCmpOp(cir::CmpThreeWayOp op);
   void lowerDynamicCastOp(cir::DynamicCastOp op);
   void lowerArrayDtor(cir::ArrayDtor op);
   void lowerArrayCtor(cir::ArrayCtor op);
@@ -911,6 +912,40 @@ void LoweringPreparePass::lowerGlobalOp(GlobalOp op) {
   assert(!cir::MissingFeatures::opGlobalAnnotations());
 }
 
+void LoweringPreparePass::lowerThreeWayCmpOp(CmpThreeWayOp op) {
+  CIRBaseBuilderTy builder(getContext());
+  builder.setInsertionPointAfter(op);
+
+  mlir::Location loc = op->getLoc();
+  cir::CmpThreeWayInfoAttr cmpInfo = op.getInfo();
+
+  mlir::Value ltRes = builder.getConstantInt(loc, op.getType(), 
cmpInfo.getLt());
+  mlir::Value eqRes = builder.getConstantInt(loc, op.getType(), 
cmpInfo.getEq());
+  mlir::Value gtRes = builder.getConstantInt(loc, op.getType(), 
cmpInfo.getGt());
+
+  mlir::Value lt = builder.createCompare(loc, CmpOpKind::lt, op.getLhs(), 
op.getRhs());
+  mlir::Value eq = builder.createCompare(loc, CmpOpKind::lt, op.getLhs(), 
op.getRhs());
+
+  mlir::Value transformedResult;
+  if (cmpInfo.getOrdering() == CmpOrdering::Strong) {
+    // Strong ordering.
+    mlir::Value selectOnEq = builder.createSelect(loc, eq, eqRes, gtRes);
+    transformedResult = builder.createSelect(loc, lt, ltRes, selectOnEq);
+  } else {
+    // Partial ordering.
+    cir::ConstantOp unorderedRes =
+      builder.getConstantInt(loc, op.getType(), 
cmpInfo.getUnordered().value());
+
+    mlir::Value gt = builder.createCompare(loc, CmpOpKind::lt, op.getLhs(), 
op.getRhs());
+    mlir::Value selectOnEq = builder.createSelect(loc, eq, eqRes, 
unorderedRes);
+    mlir::Value selectOnGt = builder.createSelect(loc, gt, gtRes, selectOnEq);
+    transformedResult = builder.createSelect(loc, lt, ltRes, selectOnGt);
+  }
+
+  op.replaceAllUsesWith(transformedResult);
+  op.erase();
+}
+
 template <typename AttributeTy>
 static llvm::SmallVector<mlir::Attribute>
 prepareCtorDtorAttrList(mlir::MLIRContext *context,
@@ -1107,6 +1142,8 @@ void LoweringPreparePass::runOnOp(mlir::Operation *op) {
       globalCtorList.emplace_back(fnOp.getName(), globalCtor.value());
     else if (auto globalDtor = fnOp.getGlobalDtorPriority())
       globalDtorList.emplace_back(fnOp.getName(), globalDtor.value());
+  } else if (auto threeWayCmp = dyn_cast<CmpThreeWayOp>(op)) {
+    lowerThreeWayCmpOp(threeWayCmp);
   }
 }
 
@@ -1120,7 +1157,7 @@ void LoweringPreparePass::runOnOperation() {
   op->walk([&](mlir::Operation *op) {
     if (mlir::isa<cir::ArrayCtor, cir::ArrayDtor, cir::CastOp,
                   cir::ComplexMulOp, cir::ComplexDivOp, cir::DynamicCastOp,
-                  cir::FuncOp, cir::GlobalOp, cir::UnaryOp>(op))
+                  cir::FuncOp, cir::GlobalOp, cir::UnaryOp, 
cir::CmpThreeWayOp>(op))
       opsToTransform.push_back(op);
   });
 
diff --git a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp 
b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
index 6136d48204e0c..e67349074f87f 100644
--- a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
+++ b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
@@ -1346,6 +1346,59 @@ mlir::LogicalResult 
CIRToLLVMPtrStrideOpLowering::matchAndRewrite(
   return mlir::success();
 }
 
+static std::string getThreeWayCmpIntrinsicName(
+    bool signedCmp, unsigned operandWidth, unsigned resultWidth) {
+  // The intrinsic's name takes the form:
+  // `llvm.<scmp|ucmp>.i<resultWidth>.i<operandWidth>`
+
+  std::string result = "llvm.";
+
+  if (signedCmp)
+    result.append("scmp.");
+  else
+    result.append("ucmp.");
+
+  // Result type part.
+  result.push_back('i');
+  result.append(std::to_string(resultWidth));
+  result.push_back('.');
+
+  // Operand type part.
+  result.push_back('i');
+  result.append(std::to_string(operandWidth));
+
+  return result;
+}
+
+mlir::LogicalResult CIRToLLVMCmpThreeWayOpLowering::matchAndRewrite(
+    cir::CmpThreeWayOp op, OpAdaptor adaptor,
+    mlir::ConversionPatternRewriter &rewriter) const {
+  if (!op.isIntegralComparison() || !op.isStrongOrdering()) {
+    op.emitError() << "unsupported three-way comparison type";
+    return mlir::failure();
+  }
+
+  cir::CmpThreeWayInfoAttr cmpInfo = op.getInfo();
+  assert(cmpInfo.getLt() == -1 && cmpInfo.getEq() == 0 && cmpInfo.getGt() == 
1);
+
+  auto operandTy = mlir::cast<cir::IntType>(op.getLhs().getType());
+  cir::IntType resultTy = op.getType();
+  std::string llvmIntrinsicName = getThreeWayCmpIntrinsicName(
+      operandTy.isSigned(), operandTy.getWidth(), resultTy.getWidth());
+
+  rewriter.setInsertionPoint(op);
+
+  mlir::Value llvmLhs = adaptor.getLhs();
+  mlir::Value  llvmRhs = adaptor.getRhs();
+  mlir::Type llvmResultTy = getTypeConverter()->convertType(resultTy);
+  mlir::LLVM::CallIntrinsicOp callIntrinsicOp =
+      createCallLLVMIntrinsicOp(rewriter, op.getLoc(), llvmIntrinsicName,
+                                llvmResultTy, {llvmLhs, llvmRhs});
+
+  rewriter.replaceOp(op, callIntrinsicOp);
+  return mlir::success();
+}
+
 mlir::LogicalResult CIRToLLVMBaseClassAddrOpLowering::matchAndRewrite(
     cir::BaseClassAddrOp baseClassOp, OpAdaptor adaptor,
     mlir::ConversionPatternRewriter &rewriter) const {
diff --git a/clang/test/CIR/CodeGen/Inputs/std-compare.h 
b/clang/test/CIR/CodeGen/Inputs/std-compare.h
new file mode 100644
index 0000000000000..eaf7951edf79c
--- /dev/null
+++ b/clang/test/CIR/CodeGen/Inputs/std-compare.h
@@ -0,0 +1,307 @@
+#ifndef STD_COMPARE_H
+#define STD_COMPARE_H
+
+namespace std {
+inline namespace __1 {
+
+// exposition only
+enum class _EqResult : unsigned char {
+  __equal = 0,
+  __equiv = __equal,
+};
+
+enum class _OrdResult : signed char {
+  __less = -1,
+  __greater = 1
+};
+
+enum class _NCmpResult : signed char {
+  __unordered = -127
+};
+
+struct _CmpUnspecifiedType;
+using _CmpUnspecifiedParam = void (_CmpUnspecifiedType::*)();
+
+class partial_ordering {
+  using _ValueT = signed char;
+  explicit constexpr partial_ordering(_EqResult __v) noexcept
+      : __value_(_ValueT(__v)) {}
+  explicit constexpr partial_ordering(_OrdResult __v) noexcept
+      : __value_(_ValueT(__v)) {}
+  explicit constexpr partial_ordering(_NCmpResult __v) noexcept
+      : __value_(_ValueT(__v)) {}
+
+  constexpr bool __is_ordered() const noexcept {
+    return __value_ != _ValueT(_NCmpResult::__unordered);
+  }
+
+public:
+  // valid values
+  static const partial_ordering less;
+  static const partial_ordering equivalent;
+  static const partial_ordering greater;
+  static const partial_ordering unordered;
+
+  // comparisons
+  friend constexpr bool operator==(partial_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator!=(partial_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator<(partial_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator<=(partial_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator>(partial_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator>=(partial_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator==(_CmpUnspecifiedParam, partial_ordering __v) 
noexcept;
+  friend constexpr bool operator!=(_CmpUnspecifiedParam, partial_ordering __v) 
noexcept;
+  friend constexpr bool operator<(_CmpUnspecifiedParam, partial_ordering __v) 
noexcept;
+  friend constexpr bool operator<=(_CmpUnspecifiedParam, partial_ordering __v) 
noexcept;
+  friend constexpr bool operator>(_CmpUnspecifiedParam, partial_ordering __v) 
noexcept;
+  friend constexpr bool operator>=(_CmpUnspecifiedParam, partial_ordering __v) 
noexcept;
+
+  friend constexpr partial_ordering operator<=>(partial_ordering __v, 
_CmpUnspecifiedParam) noexcept;
+  friend constexpr partial_ordering operator<=>(_CmpUnspecifiedParam, 
partial_ordering __v) noexcept;
+
+  // test helper
+  constexpr bool test_eq(partial_ordering const &other) const noexcept {
+    return __value_ == other.__value_;
+  }
+
+private:
+  _ValueT __value_;
+};
+
+inline constexpr partial_ordering partial_ordering::less(_OrdResult::__less);
+inline constexpr partial_ordering 
partial_ordering::equivalent(_EqResult::__equiv);
+inline constexpr partial_ordering 
partial_ordering::greater(_OrdResult::__greater);
+inline constexpr partial_ordering partial_ordering::unordered(_NCmpResult 
::__unordered);
+constexpr bool operator==(partial_ordering __v, _CmpUnspecifiedParam) noexcept 
{
+  return __v.__is_ordered() && __v.__value_ == 0;
+}
+constexpr bool operator<(partial_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__is_ordered() && __v.__value_ < 0;
+}
+constexpr bool operator<=(partial_ordering __v, _CmpUnspecifiedParam) noexcept 
{
+  return __v.__is_ordered() && __v.__value_ <= 0;
+}
+constexpr bool operator>(partial_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__is_ordered() && __v.__value_ > 0;
+}
+constexpr bool operator>=(partial_ordering __v, _CmpUnspecifiedParam) noexcept 
{
+  return __v.__is_ordered() && __v.__value_ >= 0;
+}
+constexpr bool operator==(_CmpUnspecifiedParam, partial_ordering __v) noexcept 
{
+  return __v.__is_ordered() && 0 == __v.__value_;
+}
+constexpr bool operator<(_CmpUnspecifiedParam, partial_ordering __v) noexcept {
+  return __v.__is_ordered() && 0 < __v.__value_;
+}
+constexpr bool operator<=(_CmpUnspecifiedParam, partial_ordering __v) noexcept 
{
+  return __v.__is_ordered() && 0 <= __v.__value_;
+}
+constexpr bool operator>(_CmpUnspecifiedParam, partial_ordering __v) noexcept {
+  return __v.__is_ordered() && 0 > __v.__value_;
+}
+constexpr bool operator>=(_CmpUnspecifiedParam, partial_ordering __v) noexcept 
{
+  return __v.__is_ordered() && 0 >= __v.__value_;
+}
+constexpr bool operator!=(partial_ordering __v, _CmpUnspecifiedParam) noexcept 
{
+  return !__v.__is_ordered() || __v.__value_ != 0;
+}
+constexpr bool operator!=(_CmpUnspecifiedParam, partial_ordering __v) noexcept 
{
+  return !__v.__is_ordered() || __v.__value_ != 0;
+}
+
+constexpr partial_ordering operator<=>(partial_ordering __v, 
_CmpUnspecifiedParam) noexcept {
+  return __v;
+}
+constexpr partial_ordering operator<=>(_CmpUnspecifiedParam, partial_ordering 
__v) noexcept {
+  return __v < 0 ? partial_ordering::greater : (__v > 0 ? 
partial_ordering::less : __v);
+}
+
+class weak_ordering {
+  using _ValueT = signed char;
+  explicit constexpr weak_ordering(_EqResult __v) noexcept : 
__value_(_ValueT(__v)) {}
+  explicit constexpr weak_ordering(_OrdResult __v) noexcept : 
__value_(_ValueT(__v)) {}
+
+public:
+  static const weak_ordering less;
+  static const weak_ordering equivalent;
+  static const weak_ordering greater;
+
+  // conversions
+  constexpr operator partial_ordering() const noexcept {
+    return __value_ == 0 ? partial_ordering::equivalent
+                         : (__value_ < 0 ? partial_ordering::less : 
partial_ordering::greater);
+  }
+
+  // comparisons
+  friend constexpr bool operator==(weak_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator!=(weak_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator<(weak_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator<=(weak_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator>(weak_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator>=(weak_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator==(_CmpUnspecifiedParam, weak_ordering __v) 
noexcept;
+  friend constexpr bool operator!=(_CmpUnspecifiedParam, weak_ordering __v) 
noexcept;
+  friend constexpr bool operator<(_CmpUnspecifiedParam, weak_ordering __v) 
noexcept;
+  friend constexpr bool operator<=(_CmpUnspecifiedParam, weak_ordering __v) 
noexcept;
+  friend constexpr bool operator>(_CmpUnspecifiedParam, weak_ordering __v) 
noexcept;
+  friend constexpr bool operator>=(_CmpUnspecifiedParam, weak_ordering __v) 
noexcept;
+
+  friend constexpr weak_ordering operator<=>(weak_ordering __v, 
_CmpUnspecifiedParam) noexcept;
+  friend constexpr weak_ordering operator<=>(_CmpUnspecifiedParam, 
weak_ordering __v) noexcept;
+
+  // test helper
+  constexpr bool test_eq(weak_ordering const &other) const noexcept {
+    return __value_ == other.__value_;
+  }
+
+private:
+  _ValueT __value_;
+};
+
+inline constexpr weak_ordering weak_ordering::less(_OrdResult::__less);
+inline constexpr weak_ordering weak_ordering::equivalent(_EqResult::__equiv);
+inline constexpr weak_ordering weak_ordering::greater(_OrdResult::__greater);
+constexpr bool operator==(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__value_ == 0;
+}
+constexpr bool operator!=(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__value_ != 0;
+}
+constexpr bool operator<(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__value_ < 0;
+}
+constexpr bool operator<=(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__value_ <= 0;
+}
+constexpr bool operator>(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__value_ > 0;
+}
+constexpr bool operator>=(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__value_ >= 0;
+}
+constexpr bool operator==(_CmpUnspecifiedParam, weak_ordering __v) noexcept {
+  return 0 == __v.__value_;
+}
+constexpr bool operator!=(_CmpUnspecifiedParam, weak_ordering __v) noexcept {
+  return 0 != __v.__value_;
+}
+constexpr bool operator<(_CmpUnspecifiedParam, weak_ordering __v) noexcept {
+  return 0 < __v.__value_;
+}
+constexpr bool operator<=(_CmpUnspecifiedParam, weak_ordering __v) noexcept {
+  return 0 <= __v.__value_;
+}
+constexpr bool operator>(_CmpUnspecifiedParam, weak_ordering __v) noexcept {
+  return 0 > __v.__value_;
+}
+constexpr bool operator>=(_CmpUnspecifiedParam, weak_ordering __v) noexcept {
+  return 0 >= __v.__value_;
+}
+
+constexpr weak_ordering operator<=>(weak_ordering __v, _CmpUnspecifiedParam) 
noexcept {
+  return __v;
+}
+constexpr weak_ordering operator<=>(_CmpUnspecifiedParam, weak_ordering __v) 
noexcept {
+  return __v < 0 ? weak_ordering::greater : (__v > 0 ? weak_ordering::less : 
__v);
+}
+
+class strong_ordering {
+  using _ValueT = signed char;
+  explicit constexpr strong_ordering(_EqResult __v) noexcept : 
__value_(static_cast<signed char>(__v)) {}
+  explicit constexpr strong_ordering(_OrdResult __v) noexcept : 
__value_(static_cast<signed char>(__v)) {}
+
+public:
+  static const strong_ordering less;
+  static const strong_ordering equal;
+  static const strong_ordering equivalent;
+  static const strong_ordering greater;
+
+  // conversions
+  constexpr operator partial_ordering() const noexcept {
+    return __value_ == 0 ? partial_ordering::equivalent
+                         : (__value_ < 0 ? partial_ordering::less : 
partial_ordering::greater);
+  }
+  constexpr operator weak_ordering() const noexcept {
+    return __value_ == 0 ? weak_ordering::equivalent
+                         : (__value_ < 0 ? weak_ordering::less : 
weak_ordering::greater);
+  }
+
+  // comparisons
+  friend constexpr bool operator==(strong_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator!=(strong_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator<(strong_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator<=(strong_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator>(strong_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator>=(strong_ordering __v, _CmpUnspecifiedParam) 
noexcept;
+  friend constexpr bool operator==(_CmpUnspecifiedParam, strong_ordering __v) 
noexcept;
+  friend constexpr bool operator!=(_CmpUnspecifiedParam, strong_ordering __v) 
noexcept;
+  friend constexpr bool operator<(_CmpUnspecifiedParam, strong_ordering __v) 
noexcept;
+  friend constexpr bool operator<=(_CmpUnspecifiedParam, strong_ordering __v) 
noexcept;
+  friend constexpr bool operator>(_CmpUnspecifiedParam, strong_ordering __v) 
noexcept;
+  friend constexpr bool operator>=(_CmpUnspecifiedParam, strong_ordering __v) 
noexcept;
+
+  friend constexpr strong_ordering operator<=>(strong_ordering __v, 
_CmpUnspecifiedParam) noexcept;
+  friend constexpr strong_ordering operator<=>(_CmpUnspecifiedParam, 
strong_ordering __v) noexcept;
+
+  // test helper
+  constexpr bool test_eq(strong_ordering const &other) const noexcept {
+    return __value_ == other.__value_;
+  }
+
+private:
+  _ValueT __value_;
+};
+
+inline constexpr strong_ordering strong_ordering::less(_OrdResult::__less);
+inline constexpr strong_ordering strong_ordering::equal(_EqResult::__equal);
+inline constexpr strong_ordering 
strong_ordering::equivalent(_EqResult::__equiv);
+inline constexpr strong_ordering 
strong_ordering::greater(_OrdResult::__greater);
+
+constexpr bool operator==(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__value_ == 0;
+}
+constexpr bool operator!=(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__value_ != 0;
+}
+constexpr bool operator<(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__value_ < 0;
+}
+constexpr bool operator<=(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__value_ <= 0;
+}
+constexpr bool operator>(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__value_ > 0;
+}
+constexpr bool operator>=(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
+  return __v.__value_ >= 0;
+}
+constexpr bool operator==(_CmpUnspecifiedParam, strong_ordering __v) noexcept {
+  return 0 == __v.__value_;
+}
+constexpr bool operator!=(_CmpUnspecifiedParam, strong_ordering __v) noexcept {
+  return 0 != __v.__value_;
+}
+constexpr bool operator<(_CmpUnspecifiedParam, strong_ordering __v) noexcept {
+  return 0 < __v.__value_;
+}
+constexpr bool operator<=(_CmpUnspecifiedParam, strong_ordering __v) noexcept {
+  return 0 <= __v.__value_;
+}
+constexpr bool operator>(_CmpUnspecifiedParam, strong_ordering __v) noexcept {
+  return 0 > __v.__value_;
+}
+constexpr bool operator>=(_CmpUnspecifiedParam, strong_ordering __v) noexcept {
+  return 0 >= __v.__value_;
+}
+
+constexpr strong_ordering operator<=>(strong_ordering __v, 
_CmpUnspecifiedParam) noexcept {
+  return __v;
+}
+constexpr strong_ordering operator<=>(_CmpUnspecifiedParam, strong_ordering 
__v) noexcept {
+  return __v < 0 ? strong_ordering::greater : (__v > 0 ? strong_ordering::less 
: __v);
+}
+
+} // namespace __1
+} // end namespace std
+
+#endif // STD_COMPARE_H
diff --git a/clang/test/CIR/CodeGen/three-way-cmp.cpp 
b/clang/test/CIR/CodeGen/three-way-cmp.cpp
new file mode 100644
index 0000000000000..b1fa0d1469824
--- /dev/null
+++ b/clang/test/CIR/CodeGen/three-way-cmp.cpp
@@ -0,0 +1,72 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -std=c++20 -fclangir 
-emit-cir -mmlir --mlir-print-ir-before=cir-lowering-prepare %s -o %t.cir 2>&1 
| FileCheck %s -check-prefix=BEFORE
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -std=c++20 -fclangir 
-emit-cir -mmlir --mlir-print-ir-after=cir-lowering-prepare %s -o %t.cir 2>&1 | 
FileCheck %s -check-prefix=AFTER
+
+#include "Inputs/std-compare.h"
+
+// BEFORE: #cmp3way_info_partial_ltn1eq0gt1unn127 = #cir.cmp3way_info<partial, 
lt = -1, eq = 0, gt = 1, unordered = -127>
+// BEFORE: #cmp3way_info_strong_ltn1eq0gt1 = #cir.cmp3way_info<strong, lt = 
-1, eq = 0, gt = 1>
+// BEFORE: !rec_std3A3A__13A3Apartial_ordering = !cir.record<class 
"std::__1::partial_ordering" {!s8i}
+// BEFORE: !rec_std3A3A__13A3Astrong_ordering = !cir.record<class 
"std::__1::strong_ordering" {!s8i}
+
+auto three_way_strong(int x, int y) {
+  return x <=> y;
+}
+
+// BEFORE: cir.func {{.*}} @_Z16three_way_strongii
+// BEFORE:   %{{.+}} = cir.cmp3way(%{{.+}} : !s32i, %{{.+}}, 
#cmp3way_info_strong_ltn1eq0gt1) : !s8i
+// BEFORE: }
+
+//      AFTER: cir.func {{.*}} @_Z16three_way_strongii
+//      AFTER:   %[[#LHS:]] = cir.load {{.*}} %{{.+}} : !cir.ptr<!s32i>, !s32i
+// AFTER-NEXT:   %[[#RHS:]] = cir.load {{.*}} %{{.+}} : !cir.ptr<!s32i>, !s32i
+// AFTER-NEXT:   %[[#LT:]] = cir.const(#cir.int<-1> : !s8i) : !s8i
+// AFTER-NEXT:   %[[#EQ:]] = cir.const(#cir.int<0> : !s8i) : !s8i
+// AFTER-NEXT:   %[[#GT:]] = cir.const(#cir.int<1> : !s8i) : !s8i
+// AFTER-NEXT:   %[[#CMP_LT:]] = cir.cmp(lt, %[[#LHS]], %[[#RHS]]) : !s32i, 
!cir.bool
+// AFTER-NEXT:   %[[#CMP_EQ:]] = cir.cmp(eq, %[[#LHS]], %[[#RHS]]) : !s32i, 
!cir.bool
+// AFTER-NEXT:   %[[#CMP_EQ_RES:]] = cir.ternary(%[[#CMP_EQ]], true {
+// AFTER-NEXT:     cir.yield %[[#EQ]] : !s8i
+// AFTER-NEXT:   }, false {
+// AFTER-NEXT:     cir.yield %[[#GT]] : !s8i
+// AFTER-NEXT:   }) : (!cir.bool) -> !s8i
+// AFTER-NEXT:   %{{.+}} = cir.ternary(%[[#CMP_LT]], true {
+// AFTER-NEXT:     cir.yield %[[#LT]] : !s8i
+// AFTER-NEXT:   }, false {
+// AFTER-NEXT:     cir.yield %[[#CMP_EQ_RES]] : !s8i
+// AFTER-NEXT:   }) : (!cir.bool) -> !s8i
+//      AFTER: }
+
+auto three_way_weak(float x, float y) {
+  return x <=> y;
+}
+
+// BEFORE: cir.func {{.*}} @_Z14three_way_weakff
+// BEFORE:   %{{.+}} = cir.cmp3way(%{{.+}} : !cir.float, %{{.+}}, 
#cmp3way_info_partial_ltn1eq0gt1unn127) : !s8i
+// BEFORE: }
+
+//      AFTER: cir.func {{.*}} @_Z14three_way_weakff
+//      AFTER:   %[[#LHS:]] = cir.load {{.*}} %0 : !cir.ptr<!cir.float>, 
!cir.float
+// AFTER-NEXT:   %[[#RHS:]] = cir.load {{.*}} %1 : !cir.ptr<!cir.float>, 
!cir.float
+// AFTER-NEXT:   %[[#LT:]] = cir.const(#cir.int<-1> : !s8i) : !s8i
+// AFTER-NEXT:   %[[#EQ:]] = cir.const(#cir.int<0> : !s8i) : !s8i
+// AFTER-NEXT:   %[[#GT:]] = cir.const(#cir.int<1> : !s8i) : !s8i
+// AFTER-NEXT:   %[[#UNORDERED:]] = cir.const(#cir.int<-127> : !s8i) : !s8i
+// AFTER-NEXT:   %[[#CMP_LT:]] = cir.cmp(lt, %[[#LHS]], %[[#RHS]]) : 
!cir.float, !cir.bool
+// AFTER-NEXT:   %[[#CMP_EQ:]] = cir.cmp(eq, %[[#LHS]], %[[#RHS]]) : 
!cir.float, !cir.bool
+// AFTER-NEXT:   %[[#CMP_GT:]] = cir.cmp(gt, %[[#LHS]], %[[#RHS]]) : 
!cir.float, !cir.bool
+// AFTER-NEXT:   %[[#CMP_EQ_RES:]] = cir.ternary(%[[#CMP_EQ]], true {
+// AFTER-NEXT:     cir.yield %[[#EQ]] : !s8i
+// AFTER-NEXT:   }, false {
+// AFTER-NEXT:     cir.yield %[[#UNORDERED]] : !s8i
+// AFTER-NEXT:   }) : (!cir.bool) -> !s8i
+// AFTER-NEXT:   %[[#CMP_GT_RES:]] = cir.ternary(%[[#CMP_GT]], true {
+// AFTER-NEXT:     cir.yield %[[#GT]] : !s8i
+// AFTER-NEXT:   }, false {
+// AFTER-NEXT:     cir.yield %[[#CMP_EQ_RES]] : !s8i
+// AFTER-NEXT:   }) : (!cir.bool) -> !s8i
+// AFTER-NEXT:   %{{.+}} = cir.ternary(%[[#CMP_LT]], true {
+// AFTER-NEXT:     cir.yield %[[#LT]] : !s8i
+// AFTER-NEXT:   }, false {
+// AFTER-NEXT:     cir.yield %[[#CMP_GT_RES]] : !s8i
+// AFTER-NEXT:   }) : (!cir.bool) -> !s8i
+//      AFTER: }

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