Teach ObjCARC optimizer about equivalent PHIs when eliminating autoreleaseRV/retainRV pairs

OptimizeAutoreleaseRVCall skips optimizing llvm.objc.autoreleaseReturnValue if it
sees a user which is llvm.objc.retainAutoreleasedReturnValue, and if they have
equivalent arguments (either identical or equivalent PHIs). It then assumes that
ObjCARCOpt::OptimizeRetainRVCall will optimize the pair instead.

Trouble is, ObjCARCOpt::OptimizeRetainRVCall doesn't know about equivalent PHIs
so optimizes in a different way and we are left with an unoptimized llvm.objc.autoreleaseReturnValue.

This teaches ObjCARCOpt::OptimizeRetainRVCall to also understand PHI equivalence.

rdar://problem/47005143

Reviewed By: ahatanak

Differential Revision: https://reviews.llvm.org/D56235

llvm-svn: 350284

1 files changed, 12 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp b/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp
index 6ffaadc2b5f..9a02174556f 100644
--- a/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp
+++ b/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp
@@ -600,6 +600,17 @@ ObjCARCOpt::OptimizeRetainRVCall(Function &F, Instruction *RetainRV) {
     }
   }
 
+  // Track PHIs which are equivalent to our Arg.
+  SmallDenseSet<const Value*, 2> EquivalentArgs;
+  EquivalentArgs.insert(Arg);
+
+  // Add PHIs that are equivalent to Arg to ArgUsers.
+  if (const PHINode *PN = dyn_cast<PHINode>(Arg)) {
+    SmallVector<const Value *, 2> ArgUsers;
+    getEquivalentPHIs(*PN, ArgUsers);
+    EquivalentArgs.insert(ArgUsers.begin(), ArgUsers.end());
+  }
+
   // Check for being preceded by an objc_autoreleaseReturnValue on the same
   // pointer. In this case, we can delete the pair.
   BasicBlock::iterator I = RetainRV->getIterator(),
@@ -609,7 +620,7 @@ ObjCARCOpt::OptimizeRetainRVCall(Function &F, Instruction *RetainRV) {
       --I;
     while (I != Begin && IsNoopInstruction(&*I));
     if (GetBasicARCInstKind(&*I) == ARCInstKind::AutoreleaseRV &&
-        GetArgRCIdentityRoot(&*I) == Arg) {
+        EquivalentArgs.count(GetArgRCIdentityRoot(&*I))) {
       Changed = true;
       ++NumPeeps;