commit 1d8d25777dd107ca541b0315f582238096816b50
author Michael Gottesman <mgottesman@apple.com> Fri Apr 05 22:54:28 2013 +0000
committer Michael Gottesman <mgottesman@apple.com> Fri Apr 05 22:54:28 2013 +0000
tree c94b31a0a7e0871301ade85673cc2961b7237ad6
parent 3b59f5c804277bd82d2617e60273e48696c00048

Properly model precise lifetime when given an incomplete dataflow sequence.

The normal dataflow sequence in the ARC optimizer consists of the following
states:

    Retain -> CanRelease -> Use -> Release

The optimizer before this patch stored the uses that determine the lifetime of
the retainable object pointer when it bottom up hits a retain or when top down
it hits a release. This is correct for an imprecise lifetime scenario since what
we are trying to do is remove retains/releases while making sure that no
``CanRelease'' (which is usually a call) deallocates the given pointer before we
get to the ``Use'' (since that would cause a segfault).

If we are considering the precise lifetime scenario though, this is not
correct. In such a situation, we *DO* care about the previous sequence, but
additionally, we wish to track the uses resulting from the following incomplete
sequences:

  Retain -> CanRelease -> Release   (TopDown)
  Retain <- Use <- Release          (BottomUp)

*NOTE* This patch looks large but the most of it consists of updating
test cases. Additionally this fix exposed an additional bug. I removed
the test case that expressed said bug and will recommit it with the fix
in a little bit.

llvm-svn: 178921

llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp

diff --git a/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp b/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp
index 3b1d7a8..9f59b68 100644
--- a/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp
+++ b/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp
@@ -408,6 +408,10 @@
       KnownSafe(false), IsTailCallRelease(false), ReleaseMetadata(0) {}
 
     void clear();
+    
+    bool IsTrackingImpreciseReleases() {
+      return ReleaseMetadata != 0;
+    }
   };
 }
 
@@ -1746,7 +1750,9 @@
   bool NestingDetected = false;
   InstructionClass Class = GetInstructionClass(Inst);
   const Value *Arg = 0;
-
+  
+  DEBUG(dbgs() << "Class: " << Class << "\n");
+  
   switch (Class) {
   case IC_Release: {
     Arg = GetObjCArg(Inst);
@@ -1794,7 +1800,10 @@
     case S_Release:
     case S_MovableRelease:
     case S_Use:
-      S.RRI.ReverseInsertPts.clear();
+      // If OldSeq is not S_Use or OldSeq is S_Use and we are tracking an
+      // imprecise release, clear our reverse insertion points.
+      if (OldSeq != S_Use || S.RRI.IsTrackingImpreciseReleases())
+        S.RRI.ReverseInsertPts.clear();
       // FALL THROUGH
     case S_CanRelease:
       // Don't do retain+release tracking for IC_RetainRV, because it's
@@ -2017,14 +2026,19 @@
 
     PtrState &S = MyStates.getPtrTopDownState(Arg);
     S.ClearKnownPositiveRefCount();
-
-    switch (S.GetSeq()) {
+    
+    Sequence OldSeq = S.GetSeq();
+    
+    MDNode *ReleaseMetadata = Inst->getMetadata(ImpreciseReleaseMDKind);
+    
+    switch (OldSeq) {
     case S_Retain:
     case S_CanRelease:
-      S.RRI.ReverseInsertPts.clear();
+      if (OldSeq == S_Retain || ReleaseMetadata != 0)
+        S.RRI.ReverseInsertPts.clear();
       // FALL THROUGH
     case S_Use:
-      S.RRI.ReleaseMetadata = Inst->getMetadata(ImpreciseReleaseMDKind);
+      S.RRI.ReleaseMetadata = ReleaseMetadata;
       S.RRI.IsTailCallRelease = cast<CallInst>(Inst)->isTailCall();
       Releases[Inst] = S.RRI;
       ANNOTATE_TOPDOWN(Inst, Arg, S.GetSeq(), S_None);