[PATCH] D56235: Teach ObjCARC optimizer about equivalent PHIs when eliminating autoreleaseRV/retainRV pairs

[Pete Cooper via Phabricator via llvm-commits]

pete created this revision.
pete added a reviewer: ahatanak.
Herald added a subscriber: llvm-commits.

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.


Repository:
  rL LLVM

https://reviews.llvm.org/D56235

Files:
  lib/Transforms/ObjCARC/ObjCARCOpts.cpp
  test/Transforms/ObjCARC/rv.ll


Index: test/Transforms/ObjCARC/rv.ll
===================================================================
--- test/Transforms/ObjCARC/rv.ll
+++ test/Transforms/ObjCARC/rv.ll
@@ -239,6 +239,24 @@
   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) {
Index: lib/Transforms/ObjCARC/ObjCARCOpts.cpp
===================================================================
--- lib/Transforms/ObjCARC/ObjCARCOpts.cpp
+++ lib/Transforms/ObjCARC/ObjCARCOpts.cpp
@@ -600,6 +600,19 @@
     }
   }
 
+  // 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(),
@@ -608,17 +621,19 @@
     do
       --I;
     while (I != Begin && IsNoopInstruction(&*I));
-    if (GetBasicARCInstKind(&*I) == ARCInstKind::AutoreleaseRV &&
-        GetArgRCIdentityRoot(&*I) == Arg) {
-      Changed = true;
-      ++NumPeeps;
+    if (GetBasicARCInstKind(&*I) == ARCInstKind::AutoreleaseRV) {
+      if (EquivalentArgs.count(GetArgRCIdentityRoot(&*I))) {
+        Changed = true;
+        ++NumPeeps;
 
-      LLVM_DEBUG(dbgs() << "Erasing autoreleaseRV,retainRV pair: " << *I << "\n"
-                        << "Erasing " << *RetainRV << "\n");
+        LLVM_DEBUG(dbgs() << "Erasing autoreleaseRV,retainRV pair: "
+                          << *I << "\n"
+                          << "Erasing " << *RetainRV << "\n");
 
-      EraseInstruction(&*I);
-      EraseInstruction(RetainRV);
-      return true;
+        EraseInstruction(&*I);
+        EraseInstruction(RetainRV);
+        return true;
+      }
     }
   }
 


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