[SimplifyCFG] Don't create unnecessary PHIs

In conditional store merging, we were creating PHIs when we didn't
need to. If the value to be predicated isn't defined in the block
we're predicating, then it doesn't need a PHI at all (because we only
deal with triangles and diamonds, any value not in the predicated BB
must dominate the predicated BB).

This fixes a large code size increase in some benchmarks in a popular embedded benchmark suite.

Now with a fix (and fixed tests) for the conformance issue seen in Chromium.

llvm-svn: 255767

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index ab281ef1edb..d0932f834cf 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2419,6 +2419,11 @@ static Value *ensureValueAvailableInSuccessor(Value *V, BasicBlock *BB,
   if (PHI)
     return PHI;
 
+  // If V is not an instruction defined in BB, just return it.
+  if (!AlternativeV &&
+      (!isa<Instruction>(V) || cast<Instruction>(V)->getParent() != BB))
+    return V;
+
   PHI = PHINode::Create(V->getType(), 2, "simplifycfg.merge", &Succ->front());
   PHI->addIncoming(V, BB);
   for (BasicBlock *PredBB : predecessors(Succ))