[InstSimplify] simplify power-of-2 (single bit set) sequences

As discussed in PR42314:
https://bugs.llvm.org/show_bug.cgi?id=42314

Improving the canonicalization for these patterns:
rL363956
...means we should adjust/enhance the related simplification.

https://rise4fun.com/Alive/w1cp

  Name: isPow2 or zero
  %x = and i32 %xx, 2048
  %a = add i32 %x, -1
  %r = and i32 %a, %x
  =>
  %r = i32 0

llvm-svn: 363997

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index ba76c5b0479..ce0b8a0fe64 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -1845,6 +1845,16 @@ static Value *SimplifyAndInst(Value *Op0, Value *Op1, const SimplifyQuery &Q,
       return Op1;
   }
 
+  // This is a similar pattern used for checking if a value is a power-of-2:
+  // (A - 1) & A --> 0 (if A is a power-of-2 or 0)
+  // A & (A - 1) --> 0 (if A is a power-of-2 or 0)
+  if (match(Op0, m_Add(m_Specific(Op1), m_AllOnes())) &&
+      isKnownToBeAPowerOfTwo(Op1, Q.DL, /*OrZero*/ true, 0, Q.AC, Q.CxtI, Q.DT))
+    return Constant::getNullValue(Op1->getType());
+  if (match(Op1, m_Add(m_Specific(Op0), m_AllOnes())) &&
+      isKnownToBeAPowerOfTwo(Op0, Q.DL, /*OrZero*/ true, 0, Q.AC, Q.CxtI, Q.DT))
+    return Constant::getNullValue(Op0->getType());
+
   if (Value *V = simplifyAndOrOfCmps(Q, Op0, Op1, true))
     return V;