[InstCombine] Simplify 'xor' to 'or' if no common bits are set.

Summary:
In order to get the whole fold as specified in [[ https://bugs.llvm.org/show_bug.cgi?id=6773 | PR6773 ]],
let's first handle the simple straight-forward things.
Let's start with the `and` -> `or` simplification.

The one obvious thing missing here: the constant mask is not handled.
I have an idea how to handle it, but it will require some thinking,
and is not strictly required here, so i've left that for later.

https://rise4fun.com/Alive/Pkmg

Reviewers: spatel, craig.topper, eli.friedman, jingyue

Reviewed By: spatel

Subscribers: llvm-commits

Was reviewed as part of https://reviews.llvm.org/D45631

llvm-svn: 330103

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index 9ba92f7a637..28d68a38b2b 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -2446,6 +2446,10 @@ Instruction *InstCombiner::visitXor(BinaryOperator &I) {
   if (Value *V = SimplifyBSwap(I, Builder))
     return replaceInstUsesWith(I, V);
 
+  // A^B --> A|B iff A and B have no bits set in common.
+  if (haveNoCommonBitsSet(Op0, Op1, DL, &AC, &I, &DT))
+    return BinaryOperator::CreateOr(Op0, Op1);
+
   // Apply DeMorgan's Law for 'nand' / 'nor' logic with an inverted operand.
   Value *X, *Y;