[ValueTracking] add basic handling of BITCAST nodes

Author: vortex73

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

@@ -23,8 +23,7 @@ define <4 x i2> @test_sext_bitcast(<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:    [[RESULT:%.*]] = ashr <4 x i2> [[SUB]], splat (i2 1)
-; CHECK-NEXT:    ret <4 x i2> [[RESULT]]
+; CHECK-NEXT:    ret <4 x i2> [[SUB]]
 ;
 entry:
   %cmp = icmp sgt <1 x i8> %a0, %a1