[ValueTracking] ComputeNumSignBitsImpl - add basic handling of BITCAST nodes

llvm/lib/Analysis/ValueTracking.cpp

@@ -3922,6 +3922,31 @@ static unsigned ComputeNumSignBitsImpl(const Value *V,
   if (auto *U = dyn_cast<Operator>(V)) {
     switch (Operator::getOpcode(V)) {
     default: break;
+    case Instruction::BitCast: {
+      Value *Src = U->getOperand(0);
+      Type *SrcTy = Src->getType();
+
+      // Skip if the source type is not an integer or integer vector type
+      // This ensures we only process integer-like types
+      if (!SrcTy->isIntOrIntVectorTy())
+        break;
+
+      unsigned SrcBits = SrcTy->getScalarSizeInBits();
+
+      // Bitcast 'large element' scalar/vector to 'small element' vector.
+      if ((SrcBits % TyBits) != 0)
+        break;
+
+      // Only proceed if the destination type is a fixed-size vector
+      if (isa<FixedVectorType>(Ty)) {
+        // Fast case - sign splat can be simply split across the small elements.
+        // This works for both vector and scalar sources
+        Tmp = ComputeNumSignBits(Src, Depth + 1, Q);
+        if (Tmp == SrcBits)
+          return TyBits;
+      }
+      break;
+    }
     case Instruction::SExt:
       Tmp = TyBits - U->getOperand(0)->getType()->getScalarSizeInBits();
       return ComputeNumSignBits(U->getOperand(0), DemandedElts, Depth + 1, Q) +

llvm/test/Transforms/InstCombine/compute-sign-bits-bitcast.ll

@@ -0,0 +1,61 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -passes=instcombine -S < %s | FileCheck %s
+
+; Case 1: Vector to Vector bitcast
+define <4 x i2> @test_vector_to_vector(<1 x i8> %a0, <1 x i8> %a1) {
+; CHECK-LABEL: define <4 x i2> @test_vector_to_vector(
+; CHECK-SAME: <1 x i8> [[A0:%.*]], <1 x i8> [[A1:%.*]]) {
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt <1 x i8> [[A0]], [[A1]]
+; CHECK-NEXT:    [[EXT:%.*]] = sext <1 x i1> [[CMP]] to <1 x i8>
+; CHECK-NEXT:    [[SUB:%.*]] = bitcast <1 x i8> [[EXT]] to <4 x i2>
+; CHECK-NEXT:    ret <4 x i2> [[SUB]]
+;
+  %cmp = icmp sgt <1 x i8> %a0, %a1
+  %ext = sext <1 x i1> %cmp to <1 x i8>
+  %sub = bitcast <1 x i8> %ext to <4 x i2>
+  %sra = ashr <4 x i2> %sub, <i2 1, i2 1, i2 1, i2 1>
+  ret <4 x i2> %sra
+}
+
+; Case 2: Scalar to Vector bitcast
+define <2 x i16> @test_scalar_to_vector(i1 %cond) {
+; CHECK-LABEL: define <2 x i16> @test_scalar_to_vector(
+; CHECK-SAME: i1 [[COND:%.*]]) {
+; CHECK-NEXT:    [[EXT:%.*]] = sext i1 [[COND]] to i32
+; CHECK-NEXT:    [[BC:%.*]] = bitcast i32 [[EXT]] to <2 x i16>
+; CHECK-NEXT:    ret <2 x i16> [[BC]]
+;
+  %ext = sext i1 %cond to i32
+  %bc = bitcast i32 %ext to <2 x i16>
+  %sra = ashr <2 x i16> %bc, <i16 8, i16 8>
+  ret <2 x i16> %sra
+}
+
+
+; Case 3: Multiple right shifts
+define <8 x i8> @test_multiple_shifts(i1 %cond) {
+; CHECK-LABEL: define <8 x i8> @test_multiple_shifts(
+; CHECK-SAME: i1 [[COND:%.*]]) {
+; CHECK-NEXT:    [[EXT:%.*]] = sext i1 [[COND]] to i64
+; CHECK-NEXT:    [[BC:%.*]] = bitcast i64 [[EXT]] to <8 x i8>
+; CHECK-NEXT:    ret <8 x i8> [[BC]]
+;
+  %ext = sext i1 %cond to i64
+  %bc = bitcast i64 %ext to <8 x i8>
+  %sra1 = ashr <8 x i8> %bc, <i8 1, i8 2, i8 3, i8 4, i8 5, i8 6, i8 7, i8 1>
+  %sra2 = ashr <8 x i8> %sra1, <i8 2, i8 1, i8 1, i8 1, i8 1, i8 1, i8 1, i8 1>
+  ret <8 x i8> %sra2
+}
+
+; (Negative) Case 4: Test with non-sign-extended source
+define <4 x i8> @test_non_sign_extended(i32 %val) {
+; CHECK-LABEL: define <4 x i8> @test_non_sign_extended(
+; CHECK-SAME: i32 [[VAL:%.*]]) {
+; CHECK-NEXT:    [[BC:%.*]] = bitcast i32 [[VAL]] to <4 x i8>
+; CHECK-NEXT:    [[SRA:%.*]] = ashr <4 x i8> [[BC]], splat (i8 1)
+; CHECK-NEXT:    ret <4 x i8> [[SRA]]
+;
+  %bc = bitcast i32 %val to <4 x i8>
+  %sra = ashr <4 x i8> %bc, <i8 1, i8 1, i8 1, i8 1>
+  ret <4 x i8> %sra
+}