[InstCombine] FoldShiftByConstant - use PatternMatch for logicalshift(trunc(shift(x,c1)),c2) fold. NFCI.

Author: RKSimon

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -683,25 +683,27 @@ Instruction *InstCombinerImpl::FoldShiftByConstant(Value *Op0, Constant *Op1,
     return FoldedShift;
 
   // Fold shift2(trunc(shift1(x,c1)), c2) -> trunc(shift2(shift1(x,c1),c2))
-  if (TruncInst *TI = dyn_cast<TruncInst>(Op0)) {
-    Instruction *TrOp = dyn_cast<Instruction>(TI->getOperand(0));
+  if (auto *TI = dyn_cast<TruncInst>(Op0)) {
     // If 'shift2' is an ashr, we would have to get the sign bit into a funny
     // place.  Don't try to do this transformation in this case.  Also, we
     // require that the input operand is a shift-by-constant so that we have
     // confidence that the shifts will get folded together.  We could do this
     // xform in more cases, but it is unlikely to be profitable.
-    if (TrOp && I.isLogicalShift() && TrOp->isShift() &&
-        isa<ConstantInt>(TrOp->getOperand(1))) {
+    if (I.isLogicalShift() &&
+        match(TI->getOperand(0), m_Shift(m_Value(), m_ConstantInt()))) {
+      auto *TrOp = cast<Instruction>(TI->getOperand(0));
+      Type *SrcTy = TrOp->getType();
+
       // Okay, we'll do this xform.  Make the shift of shift.
-      Constant *ShAmt = ConstantExpr::getZExt(Op1, TrOp->getType());
+      Constant *ShAmt = ConstantExpr::getZExt(Op1, SrcTy);
       // (shift2 (shift1 & 0x00FF), c2)
       Value *NSh = Builder.CreateBinOp(I.getOpcode(), TrOp, ShAmt, I.getName());
 
       // For logical shifts, the truncation has the effect of making the high
       // part of the register be zeros.  Emulate this by inserting an AND to
       // clear the top bits as needed.  This 'and' will usually be zapped by
       // other xforms later if dead.
-      unsigned SrcSize = TrOp->getType()->getScalarSizeInBits();
+      unsigned SrcSize = SrcTy->getScalarSizeInBits();
       unsigned DstSize = TI->getType()->getScalarSizeInBits();
       APInt MaskV(APInt::getLowBitsSet(SrcSize, DstSize));