[CIR][Lowering] Fix Vector Comparison Lowering with -fno-signed-char/unsigned operand

Author: RiverDave

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

@@ -2044,6 +2044,15 @@ mlir::LogicalResult CIRToLLVMVecCreateOpLowering::matchAndRewrite(
   return mlir::success();
 }
 
+static bool isCIRZeroVector(mlir::Value value) {
+  if (auto constantOp = value.getDefiningOp<cir::ConstantOp>()) {
+    if (auto zeroAttr = mlir::dyn_cast<cir::ZeroAttr>(constantOp.getValue())) {
+      return true;
+    }
+  }
+  return false;
+}
+
 mlir::LogicalResult CIRToLLVMVecCmpOpLowering::matchAndRewrite(
     cir::VecCmpOp op, OpAdaptor adaptor,
     mlir::ConversionPatternRewriter &rewriter) const {
@@ -2052,9 +2061,16 @@ mlir::LogicalResult CIRToLLVMVecCmpOpLowering::matchAndRewrite(
   auto elementType = elementTypeIfVector(op.getLhs().getType());
   mlir::Value bitResult;
   if (auto intType = mlir::dyn_cast<cir::IntType>(elementType)) {
+
+    bool shouldUseSigned = intType.isSigned();
+    // Special treatment For sign-bit extraction patterns (lt comparison with
+    // zero), always use signed comparison to preserve the semantic intent
+    if (op.getKind() == cir::CmpOpKind::lt && isCIRZeroVector(op.getRhs())) {
+      shouldUseSigned = true;
+    }
     bitResult = rewriter.create<mlir::LLVM::ICmpOp>(
         op.getLoc(),
-        convertCmpKindToICmpPredicate(op.getKind(), intType.isSigned()),
+        convertCmpKindToICmpPredicate(op.getKind(), shouldUseSigned),
         adaptor.getLhs(), adaptor.getRhs());
   } else if (mlir::isa<cir::FPTypeInterface>(elementType)) {
     bitResult = rewriter.create<mlir::LLVM::FCmpOp>(

clang/test/CIR/Lowering/vec-cmp.cir

@@ -26,4 +26,4 @@ cir.func @vec_cmp_zero(%0: !cir.vector<!u8i x 16>) -> () {
 
 // MLIR: llvm.func @vec_cmp_zero
 // MLIR: %{{[0-9]+}} = llvm.icmp "slt" %arg0, %{{[0-9]+}} : vector<16xi8>
-// MLIR-NEXT: %{{[0-9]+}} = llvm.bitcast %{{[0-9]+}} : vector<16xi1> to i16
+// MLIR-NEXT: %{{[0-9]+}} = llvm.bitcast %{{[0-9]+}} : vector<16xi1> to i16