[InstSimplify] add folds for constant mask of value shifted by constant

We would eventually catch these via demanded bits and computing known bits in InstCombine,
but I think it's better to handle the simple cases as soon as possible as a matter of efficiency.

This fold allows further simplifications based on distributed ops transforms. eg:
  %a = lshr i8 %x, 7
  %b = or i8 %a, 2
  %c = and i8 %b, 1

InstSimplify can directly fold this now:
  %a = lshr i8 %x, 7

Differential Revision: https://reviews.llvm.org/D33221

llvm-svn: 303213

1 files changed, 18 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 5728887cc1e..5652248a60c 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -1752,6 +1752,24 @@ static Value *SimplifyAndInst(Value *Op0, Value *Op1, const SimplifyQuery &Q,
       (A == Op0 || B == Op0))
     return Op0;
 
+  // A mask that only clears known zeros of a shifted value is a no-op.
+  Value *X;
+  const APInt *Mask;
+  const APInt *ShAmt;
+  if (match(Op1, m_APInt(Mask))) {
+    // If all bits in the inverted and shifted mask are clear:
+    // and (shl X, ShAmt), Mask --> shl X, ShAmt
+    if (match(Op0, m_Shl(m_Value(X), m_APInt(ShAmt))) &&
+        (~(*Mask)).lshr(*ShAmt).isNullValue())
+      return Op0;
+
+    // If all bits in the inverted and shifted mask are clear:
+    // and (lshr X, ShAmt), Mask --> lshr X, ShAmt
+    if (match(Op0, m_LShr(m_Value(X), m_APInt(ShAmt))) &&
+        (~(*Mask)).shl(*ShAmt).isNullValue())
+      return Op0;
+  }
+
   // A & (-A) = A if A is a power of two or zero.
   if (match(Op0, m_Neg(m_Specific(Op1))) ||
       match(Op1, m_Neg(m_Specific(Op0)))) {