[ValueTracking] ComputeNumSignBitsImpl - add basic handling of BITCAST nodes

llvm/lib/Analysis/ValueTracking.cpp

@@ -3922,6 +3922,35 @@ 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();
+      Type *SrcScalarTy = SrcTy->getScalarType();
+
+      if (!SrcScalarTy->isIntegerTy())
+        break;
+
+      unsigned SrcBits = SrcTy->getScalarSizeInBits();
+
+      if ((SrcBits % TyBits) != 0)
+        break;
+
+      if (auto *DstVTy = dyn_cast<FixedVectorType>(Ty)) {
+        unsigned Scale = SrcBits / TyBits;
+
+        APInt SrcDemandedElts =
+            APInt::getSplat(DstVTy->getNumElements() / Scale, APInt(1, 1));
+
+        Tmp = ComputeNumSignBits(Src, SrcDemandedElts, Depth + 1, Q);
+        if (Tmp == SrcBits)
+          return TyBits;
+      } else {
+        Tmp = ComputeNumSignBits(Src, APInt(1, 1), 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 +1,34 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -passes=instcombine -S < %s | FileCheck %s
+
+define i32 @test_compute_sign_bits() {
+; CHECK-LABEL: define i32 @test_compute_sign_bits() {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    ret i32 -1
+;
+entry:
+  %a = add i8 -1, 0
+  %b = bitcast i8 %a to <4 x i2>
+  %c = ashr <4 x i2> %b, <i2 1, i2 1, i2 1, i2 1>
+  %d = bitcast <4 x i2> %c to i8
+  %e = sext i8 %d to i32
+  ret i32 %e
+}
+
+; Test with sign extension to ensure proper sign bit tracking
+define <4 x i2> @test_sext_bitcast(<1 x i8> %a0, <1 x i8> %a1) {
+; CHECK-LABEL: define <4 x i2> @test_sext_bitcast(
+; CHECK-SAME: <1 x i8> [[A0:%.*]], <1 x i8> [[A1:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; 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]]
+;
+entry:
+  %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>
+  %result = ashr <4 x i2> %sub, <i2 1, i2 1, i2 1, i2 1>
+  ret <4 x i2> %result
+}