[CVP] Implement type narrowing for LShr

Implements type narrowing for LShr. The treatment is analogous to the type narrowing of UDiv. Since LShr is a relatively cheap instruction, the narrowing occurs only if the following conditions hold: i) all the users of the LShr instruction are already TruncInst; ii) the narrowing is carried out to the largest TruncInst following the LShr instruction. Additionally, the function optimizes the cases where the result of the LShr instruction is guaranteed to vanish or be equal to poison.

Author: adam-bzowski

llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

@@ -1103,10 +1103,8 @@ static bool narrowLShr(BinaryOperator *LShr, LazyValueInfo *LVI) {
 
   unsigned OrigWidth = RetTy->getScalarSizeInBits();
   unsigned MaxActiveBitsInArg = ArgRange.getActiveBits();
-  uint64_t MinShiftValue64 = ShiftRange.getUnsignedMin().getZExtValue();
-  unsigned MinShiftValue =
-      MinShiftValue64 < std::numeric_limits<unsigned>::max()
-          ? static_cast<unsigned>(MinShiftValue64)
+  unsigned MinShiftValue = ShiftRange.getUnsignedMin().getActiveBits() <= 32
+          ? static_cast<unsigned>(ShiftRange.getUnsignedMin().getZExtValue())
           : std::numeric_limits<unsigned>::max();
 
   // First we deal with the cases where the result is guaranteed to vanish or be
@@ -1138,12 +1136,9 @@ static bool narrowLShr(BinaryOperator *LShr, LazyValueInfo *LVI) {
   // width of the argument, we must make sure that the maximal possible value
   // for the shift is larger than the new width after narrowing. Otherwise some
   // shifts that originally vanish would result in poison after the narrowing.
-  uint64_t MaxShiftValue64 = ShiftRange.getUnsignedMax().getZExtValue();
-  unsigned MaxShiftValue =
-      MaxShiftValue64 < std::numeric_limits<unsigned>::max()
-          ? static_cast<unsigned>(MaxShiftValue64)
+  unsigned MaxShiftValue = ShiftRange.getUnsignedMax().getActiveBits() <= 32
+          ? static_cast<unsigned>(ShiftRange.getUnsignedMax().getZExtValue())
           : std::numeric_limits<unsigned>::max();
-
   if (OrigWidth <= MaxShiftValue)
     return false;