[InstCombine] Add new combine to add folding

(X | C1) + C2 --> (X | C1) ^ C1 iff (C1 == -C2)

I verified the correctness using Alive:
https://rise4fun.com/Alive/YNV

This transform enables the following transform that already exists in
instcombine:

(X | Y) ^ Y --> X & ~Y

As a result, the full expected transform is:

(X | C1) + C2 --> X & ~C1 iff (C1 == -C2)

There already exists the transform in the sub case:

(X | Y) - Y --> X & ~Y

However this does not trigger in the case where Y is constant due to an earlier
transform:

X - (-C) --> X + C

With this new add fold, both the add and sub constant cases are handled.

Patch by Chris Dawson.

Differential Revision: https://reviews.llvm.org/D61517


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@360185 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -892,6 +892,11 @@ Instruction *InstCombiner::foldAddWithConstant(BinaryOperator &Add) {
   if (!match(Op1, m_APInt(C)))
     return nullptr;
 
+  // (X | C2) + C --> (X | C2) ^ C2 iff (C2 == -C)
+  const APInt *C2;
+  if (match(Op0, m_Or(m_Value(), m_APInt(C2))) && *C2 == -*C)
+    return BinaryOperator::CreateXor(Op0, ConstantInt::get(Add.getType(), *C2));
+
   if (C->isSignMask()) {
     // If wrapping is not allowed, then the addition must set the sign bit:
     // X + (signmask) --> X | signmask
@@ -906,7 +911,6 @@ Instruction *InstCombiner::foldAddWithConstant(BinaryOperator &Add) {
   // Is this add the last step in a convoluted sext?
   // add(zext(xor i16 X, -32768), -32768) --> sext X
   Type *Ty = Add.getType();
-  const APInt *C2;
   if (match(Op0, m_ZExt(m_Xor(m_Value(X), m_APInt(C2)))) &&
       C2->isMinSignedValue() && C2->sext(Ty->getScalarSizeInBits()) == *C)
     return CastInst::Create(Instruction::SExt, X, Ty);