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

Author: RiverDave

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

@@ -2052,9 +2052,24 @@ mlir::LogicalResult CIRToLLVMVecCmpOpLowering::matchAndRewrite(
   auto elementType = elementTypeIfVector(op.getLhs().getType());
   mlir::Value bitResult;
   if (auto intType = mlir::dyn_cast<cir::IntType>(elementType)) {
+
+    auto 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;
+    };
+
+    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

@@ -3,6 +3,7 @@
 
 !s16i = !cir.int<s, 16>
 !u16i = !cir.int<u, 16>
+!u8i = !cir.int<u, 8>
 
 cir.func @vec_cmp(%0: !cir.vector<!s16i x 16>, %1: !cir.vector<!s16i x 16>) -> () {
   %2 = cir.vec.cmp(lt, %0, %1) : !cir.vector<!s16i x 16>, !cir.vector<!cir.int<u, 1> x 16> 
@@ -14,3 +15,15 @@ cir.func @vec_cmp(%0: !cir.vector<!s16i x 16>, %1: !cir.vector<!s16i x 16>) -> (
 // MLIR-NEXT: %{{[0-9]+}} = llvm.icmp "slt" %arg0, %arg1 : vector<16xi16>
 // MLIR-NEXT: %{{[0-9]+}} = llvm.bitcast %{{[0-9]+}} : vector<16xi1> to i16
 // MLIR-NEXT: llvm.return
+
+cir.func @vec_cmp_zero(%0: !cir.vector<!u8i x 16>) -> () {
+  %1 = cir.const #cir.zero : !cir.vector<!u8i x 16> 
+  %2 = cir.vec.cmp(lt, %0, %1) : !cir.vector<!u8i x 16>, !cir.vector<!cir.int<u, 1> x 16> 
+  %3 = cir.cast(bitcast, %2 : !cir.vector<!cir.int<u, 1> x 16>), !cir.int<u, 16>
+
+  cir.return
+}
+
+// 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