[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

@@ -758,6 +758,15 @@ static Domain getDomain(const ConstantRange &CR) {
   return Domain::Unknown;
 }
 
+/// Returns the optimal bit width for the narrowed type.
+/// If desiredBitWidth is greater that 32 returns max int and thus effectively
+/// turns of the narrowing. For desiredBitWidth <= 32 returns i8, i16, i32.
+static unsigned getNarrowedWidth(unsigned desiredBitWidth) {
+  return desiredBitWidth <= 32
+             ? std::max<unsigned>(PowerOf2Ceil(desiredBitWidth), 8)
+             : std::numeric_limits<unsigned>::max();
+}
+
 /// Try to shrink a sdiv/srem's width down to the smallest power of two that's
 /// sufficient to contain its operands.
 static bool narrowSDivOrSRem(BinaryOperator *Instr, const ConstantRange &LCR,
@@ -781,7 +790,7 @@ static bool narrowSDivOrSRem(BinaryOperator *Instr, const ConstantRange &LCR,
     ++MinSignedBits;
 
   // Don't shrink below 8 bits wide.
-  unsigned NewWidth = std::max<unsigned>(PowerOf2Ceil(MinSignedBits), 8);
+  unsigned NewWidth = getNarrowedWidth(MinSignedBits);
 
   // NewWidth might be greater than OrigWidth if OrigWidth is not a power of
   // two.
@@ -899,7 +908,7 @@ static bool narrowUDivOrURem(BinaryOperator *Instr, const ConstantRange &XCR,
   // of both of the operands?
   unsigned MaxActiveBits = std::max(XCR.getActiveBits(), YCR.getActiveBits());
   // Don't shrink below 8 bits wide.
-  unsigned NewWidth = std::max<unsigned>(PowerOf2Ceil(MaxActiveBits), 8);
+  unsigned NewWidth = getNarrowedWidth(MaxActiveBits);
 
   // NewWidth might be greater than OrigWidth if OrigWidth is not a power of
   // two.
@@ -1156,7 +1165,7 @@ static bool narrowLShr(BinaryOperator *LShr, LazyValueInfo *LVI) {
 
   // What is the smallest bit width that can accommodate the entire value ranges
   // of both of the operands? Don't shrink below 8 bits wide.
-  unsigned NewWidth = std::max<unsigned>(PowerOf2Ceil(MaxActiveBits), 8);
+  unsigned NewWidth = getNarrowedWidth(MaxActiveBits);
 
   // NewWidth might be greater than OrigWidth if OrigWidth is not a power of
   // two.