[SimpleLoopUnswitch] Early check exit for trivial unswitch with Memor…

…ySSA.

Summary:
If MemorySSA is avaiable, we can skip checking all instructions if block has any Defs.
(volatile loads are also Defs).
We still need to check all instructions for "canThrow", even if no Defs are found.

Reviewers: chandlerc

Subscribers: sanjoy, jlebar, Prazek, george.burgess.iv, llvm-commits

Differential Revision: https://reviews.llvm.org/D57129

llvm-svn: 352393

llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp

@@ -847,6 +847,10 @@ static bool unswitchAllTrivialConditions(Loop &L, DominatorTree &DT,
     // Check if there are any side-effecting instructions (e.g. stores, calls,
     // volatile loads) in the part of the loop that the code *would* execute
     // without unswitching.
+    if (MSSAU) // Possible early exit with MSSA
+      if (auto *Defs = MSSAU->getMemorySSA()->getBlockDefs(CurrentBB))
+        if (!isa<MemoryPhi>(*Defs->begin()) || (++Defs->begin() != Defs->end()))
+          return Changed;
     if (llvm::any_of(*CurrentBB,
                      [](Instruction &I) { return I.mayHaveSideEffects(); }))
       return Changed;