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

[pete]

Summary

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

[ahatanak]

LGTM

lib/Transforms/ObjCARC/ObjCARCOpts.cpp

613

If you'd like, you can use the insert method that takes the begin and end iterators instead of writing a loop.

625

You can reduce indentation if you use && instead of introducing an if statement here.

[pete]

Thanks for the review. Submitted as r350284.

lib/Transforms/ObjCARC/ObjCARCOpts.cpp

625

Good point. I don't know why I changed that in the first place. Much smaller patch now.

lib/Transforms/ObjCARC/ObjCARCOpts.cpp

if (Call->getParent() == RetainRV->getParent()) {
         while (IsNoopInstruction(&*I))
           ++I;
         if (&*I == RetainRV)
           return false;
       }
     }
   }
 
+  // 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);
+
+    for (const Value *ArgUser : ArgUsers)
+      EquivalentArgs.insert(ArgUser);
+  }
+
   // 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(),
                        Begin = RetainRV->getParent()->begin();
   if (I != Begin) {
     do
       --I;
     while (I != Begin && IsNoopInstruction(&*I));
-    if (GetBasicARCInstKind(&*I) == ARCInstKind::AutoreleaseRV &&
-        GetArgRCIdentityRoot(&*I) == Arg) {
+    if (GetBasicARCInstKind(&*I) == ARCInstKind::AutoreleaseRV) {
+      if (EquivalentArgs.count(GetArgRCIdentityRoot(&*I))) {
         Changed = true;
         ++NumPeeps;
 
-      LLVM_DEBUG(dbgs() << "Erasing autoreleaseRV,retainRV pair: " << *I << "\n"
+        LLVM_DEBUG(dbgs() << "Erasing autoreleaseRV,retainRV pair: "
+                          << *I << "\n"
                           << "Erasing " << *RetainRV << "\n");
 
         EraseInstruction(&*I);
         EraseInstruction(RetainRV);
         return true;
       }
     }
+  }
 
   // Turn it to a plain objc_retain.
   Changed = true;
   ++NumPeeps;
 
   LLVM_DEBUG(dbgs() << "Transforming objc_retainAutoreleasedReturnValue => "
                        "objc_retain since the operand is not a return value.\n"
                        "Old = "

test/Transforms/ObjCARC/rv.ll

 ; CHECK: define i8* @test19(i8* %p) {
 ; CHECK-NEXT: ret i8* %p
 define i8* @test19(i8* %p) {
   call i8* @llvm.objc.autoreleaseReturnValue(i8* %p)
   call i8* @llvm.objc.retainAutoreleasedReturnValue(i8* %p)
   ret i8* %p
 }
 
+; Delete autoreleaseRV+retainRV pairs when they have equivalent PHIs as inputs
+
+; CHECK: define i8* @test19phi(i8* %p) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br label %test19bb
+; CHECK: test19bb:
+; CHECK-NEXT: ret i8* %p
+define i8* @test19phi(i8* %p) {
+entry:
+  br label %test19bb
+test19bb:
+  %phi1 = phi i8* [ %p, %entry ]
+  %phi2 = phi i8* [ %p, %entry ]
+  call i8* @llvm.objc.autoreleaseReturnValue(i8* %phi1)
+  call i8* @llvm.objc.retainAutoreleasedReturnValue(i8* %phi2)
+  ret i8* %p
+}
+
 ; Like test19 but with plain autorelease.
 
 ; CHECK: define i8* @test20(i8* %p) {
 ; CHECK-NEXT: call i8* @llvm.objc.autorelease(i8* %p)
 ; CHECK-NEXT: call i8* @llvm.objc.retain(i8* %p)
 ; CHECK-NEXT: ret i8* %p
 define i8* @test20(i8* %p) {
   call i8* @llvm.objc.autorelease(i8* %p)