[ObjCARC] Strength reduce objc_retainBlock -> objc_retain if the objc_retainBlock...

If an objc_retainBlock has the copy_on_escape metadata attached to it
AND if the block pointer argument only escapes down the stack, we are
allowed to strength reduce the objc_retainBlock to to an objc_retain and
thus optimize it.

Current there is logic in the ARC data flow analysis to handle
this case which is complicated and involved making distinctions in
between objc_retainBlock and objc_retain in certain places and
considering them the same in others.

This patch simplifies said code by:

1. Performing the strength reduction in the initial ARC peephole
analysis (ObjCARCOpts::OptimizeIndividualCalls).

2. Changes the ARC dataflow analysis (which runs after the peephole
analysis) to consider all objc_retainBlock calls to not be optimizable
(since if the call was optimizable, we would have strength reduced it
already).

This patch leaves in the infrastructure in the ARC dataflow analysis to
handle this case, which due to 2 will just be dead code. I am doing this
on purpose to separate the removal of the old code from the testing of
the new code.

<rdar://problem/13249661>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178284 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/ObjCARC/ObjCARCOpts.cpp b/lib/Transforms/ObjCARC/ObjCARCOpts.cpp
index a21d92e48f1744dd7dcdd28d57be1e2cd47e9f7f..8e076e05c4fd694e420f42b7524b125408d76914 100644 (file)
--- a/lib/Transforms/ObjCARC/ObjCARCOpts.cpp
+++ b/lib/Transforms/ObjCARC/ObjCARCOpts.cpp
@@ -986,6 +986,8 @@ namespace {
     bool OptimizeRetainRVCall(Function &F, Instruction *RetainRV);
     void OptimizeAutoreleaseRVCall(Function &F, Instruction *AutoreleaseRV,
                                    InstructionClass &Class);
+    bool OptimizeRetainBlockCall(Function &F, Instruction *RetainBlock,
+                                 InstructionClass &Class);
     void OptimizeIndividualCalls(Function &F);
 
     void CheckForCFGHazards(const BasicBlock *BB,
@@ -1319,6 +1321,35 @@ ObjCARCOpt::OptimizeAutoreleaseRVCall(Function &F, Instruction *AutoreleaseRV,
 
 }
 
+// \brief Attempt to strength reduce objc_retainBlock calls to objc_retain
+// calls.
+//
+// Specifically: If an objc_retainBlock call has the copy_on_escape metadata and
+// does not escape (following the rules of block escaping), strength reduce the
+// objc_retainBlock to an objc_retain.
+//
+// TODO: If an objc_retainBlock call is dominated period by a previous
+// objc_retainBlock call, strength reduce the objc_retainBlock to an
+// objc_retain.
+bool
+ObjCARCOpt::OptimizeRetainBlockCall(Function &F, Instruction *Inst,
+                                    InstructionClass &Class) {
+  assert(GetBasicInstructionClass(Inst) == Class);
+  assert(IC_RetainBlock == Class);
+  
+  // If we can not optimize Inst, return false.
+  if (!IsRetainBlockOptimizable(Inst))
+    return false;
+  
+  CallInst *RetainBlock = cast<CallInst>(Inst);
+  RetainBlock->setCalledFunction(getRetainCallee(F.getParent()));
+  // Remove copy_on_escape metadata.
+  RetainBlock->setMetadata(CopyOnEscapeMDKind, 0);
+  Class = IC_Retain;
+  
+  return true;
+}
+
 /// Visit each call, one at a time, and make simplifications without doing any
 /// additional analysis.
 void ObjCARCOpt::OptimizeIndividualCalls(Function &F) {
@@ -1402,6 +1433,12 @@ void ObjCARCOpt::OptimizeIndividualCalls(Function &F) {
       }
       break;
     }
+    case IC_RetainBlock:
+      // If we strength reduce an objc_retainBlock to amn objc_retain, continue
+      // onto the objc_retain peephole optimizations. Otherwise break.
+      if (!OptimizeRetainBlockCall(F, Inst, Class))
+        break;
+      // FALLTHROUGH
     case IC_Retain:
       OptimizeRetainCall(F, Inst);
       break;
@@ -1748,11 +1785,10 @@ ObjCARCOpt::VisitInstructionBottomUp(Instruction *Inst,
     break;
   }
   case IC_RetainBlock:
-    // An objc_retainBlock call with just a use may need to be kept,
-    // because it may be copying a block from the stack to the heap.
-    if (!IsRetainBlockOptimizable(Inst))
-      break;
-    // FALLTHROUGH
+    // In OptimizeIndividualCalls, we have strength reduced all optimizable
+    // objc_retainBlocks to objc_retains. Thus at this point any
+    // objc_retainBlocks that we see are not optimizable.
+    break;
   case IC_Retain:
   case IC_RetainRV: {
     Arg = GetObjCArg(Inst);
@@ -1961,11 +1997,10 @@ ObjCARCOpt::VisitInstructionTopDown(Instruction *Inst,
 
   switch (Class) {
   case IC_RetainBlock:
-    // An objc_retainBlock call with just a use may need to be kept,
-    // because it may be copying a block from the stack to the heap.
-    if (!IsRetainBlockOptimizable(Inst))
-      break;
-    // FALLTHROUGH
+    // In OptimizeIndividualCalls, we have strength reduced all optimizable
+    // objc_retainBlocks to objc_retains. Thus at this point any
+    // objc_retainBlocks that we see are not optimizable.
+    break;
   case IC_Retain:
   case IC_RetainRV: {
     Arg = GetObjCArg(Inst);

diff --git a/test/Transforms/ObjCARC/basic.ll b/test/Transforms/ObjCARC/basic.ll
index 4c24ebf8652b27a81fbf6f7b76fa18984ab29782..bf6ccc8a7fc2bec8c803fc97f3fe7e24693cad35 100644 (file)
--- a/test/Transforms/ObjCARC/basic.ll
+++ b/test/Transforms/ObjCARC/basic.ll
@@ -795,10 +795,10 @@ entry:
   ret void
 }
 
-; Don't optimize objc_retainBlock.
+; Don't optimize objc_retainBlock, but do strength reduce it.
 
 ; CHECK: define void @test23b
-; CHECK: @objc_retainBlock
+; CHECK: @objc_retain
 ; CHECK: @objc_release
 ; CHECK: }
 define void @test23b(i8* %p) {

diff --git a/test/Transforms/ObjCARC/no-objc-arc-exceptions.ll b/test/Transforms/ObjCARC/no-objc-arc-exceptions.ll
index 9728f6e0d94f74b3750efdb253ec9367a7f1bc50..58b5bbe9c7e944ff064c32318c51aa91f38836af 100644 (file)
--- a/test/Transforms/ObjCARC/no-objc-arc-exceptions.ll
+++ b/test/Transforms/ObjCARC/no-objc-arc-exceptions.ll
@@ -59,11 +59,12 @@ lpad:                                             ; preds = %entry
   resume { i8*, i32 } %t8
 }
 
-; There is no !clang.arc.no_objc_arc_exceptions
-; metadata here, so the optimizer shouldn't eliminate anything.
+; There is no !clang.arc.no_objc_arc_exceptions metadata here, so the optimizer
+; shouldn't eliminate anything, but *CAN* strength reduce the objc_retainBlock
+; to an objc_retain.
 
 ; CHECK: define void @test0_no_metadata(
-; CHECK: call i8* @objc_retainBlock(
+; CHECK: call i8* @objc_retain(
 ; CHECK: invoke
 ; CHECK: call void @objc_release(
 ; CHECK: }