[InstCombine] Unfold masked merge with constant mask

Summary:
As discussed in D45733, we want to do this in InstCombine.

https://rise4fun.com/Alive/LGk

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: chandlerc, xbolva00, llvm-commits

Differential Revision: https://reviews.llvm.org/D45867

llvm-svn: 331205

1 files changed, 15 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index 5d9e6abf629..e389af2a2e3 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -2412,14 +2412,19 @@ Value *InstCombiner::foldXorOfICmps(ICmpInst *LHS, ICmpInst *RHS) {
 }
 
 /// If we have a masked merge, in the canonical form of:
+/// (assuming that A only has one use.)
 ///   |        A  |  |B|
 ///   ((x ^ y) & M) ^ y
 ///    |  D  |
 /// * If M is inverted:
 ///      |  D  |
 ///     ((x ^ y) & ~M) ^ y
-///   If A has one use, and, we want to canonicalize it to non-inverted mask:
+///   We can canonicalize by swapping the final xor operand
+///   to eliminate the 'not' of the mask.
 ///     ((x ^ y) & M) ^ x
+/// * If M is a constant, and D has one use, we transform to 'and' / 'or' ops
+///   because that shortens the dependency chain and improves analysis:
+///     (x & M) | (y & ~M)
 static Instruction *visitMaskedMerge(BinaryOperator &I,
                                      InstCombiner::BuilderTy &Builder) {
   Value *B, *X, *D;
@@ -2438,6 +2443,15 @@ static Instruction *visitMaskedMerge(BinaryOperator &I,
     return BinaryOperator::CreateXor(NewA, X);
   }
 
+  Constant *C;
+  if (D->hasOneUse() && match(M, m_Constant(C))) {
+    // Unfold.
+    Value *LHS = Builder.CreateAnd(X, C);
+    Value *NotC = Builder.CreateNot(C);
+    Value *RHS = Builder.CreateAnd(B, NotC);
+    return BinaryOperator::CreateOr(LHS, RHS);
+  }
+
   return nullptr;
 }