[llvm] r200093 - Disable the use of TBAA when using AA in CodeGen

[Hal Finkel]

Author: hfinkel
Date: Sat Jan 25 13:24:54 2014
New Revision: 200093

URL: http://llvm.org/viewvc/llvm-project?rev=200093&view=rev
Log:
Disable the use of TBAA when using AA in CodeGen

There are currently two issues, of which I currently know, that prevent TBAA
from being correctly usable in CodeGen:

  1. Stack coloring does not update TBAA when merging allocas. This is easy
     enough to fix, but is not the largest problem.

  2. CGP inserts ptrtoint/inttoptr pairs when sinking address computations.
     Because BasicAA does not handle inttoptr, we'll often miss basic type punning
     idioms that we need to catch so we don't miscompile real-world code (like LLVM).

I don't yet have a small test case for this, but this fixes self hosting a
non-asserts build of LLVM on PPC64 when using -enable-aa-sched-mi and -misched=shuffle.

Modified:
    llvm/trunk/lib/CodeGen/ScheduleDAGInstrs.cpp
    llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    llvm/trunk/lib/CodeGen/StackColoring.cpp

Modified: llvm/trunk/lib/CodeGen/ScheduleDAGInstrs.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/ScheduleDAGInstrs.cpp?rev=200093&r1=200092&r2=200093&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/ScheduleDAGInstrs.cpp (original)
+++ llvm/trunk/lib/CodeGen/ScheduleDAGInstrs.cpp Sat Jan 25 13:24:54 2014
@@ -45,6 +45,15 @@ static cl::opt<bool> EnableAASchedMI("en
     cl::ZeroOrMore, cl::init(false),
     cl::desc("Enable use of AA during MI GAD construction"));
 
+// FIXME: Enable the use of TBAA. There are two known issues preventing this:
+//   1. Stack coloring does not update TBAA when merging allocas
+//   2. CGP inserts ptrtoint/inttoptr pairs when sinking address computations.
+//      Because BasicAA does not handle inttoptr, we'll often miss basic type
+//      punning idioms that we need to catch so we don't miscompile real-world
+//      code.
+static cl::opt<bool> UseTBAA("use-tbaa-in-sched-mi", cl::Hidden,
+    cl::init(false), cl::desc("Enable use of TBAA during MI GAD construction"));
+
 ScheduleDAGInstrs::ScheduleDAGInstrs(MachineFunction &mf,
                                      const MachineLoopInfo &mli,
                                      const MachineDominatorTree &mdt,
@@ -556,9 +565,9 @@ static bool MIsNeedChainEdge(AliasAnalys
 
   AliasAnalysis::AliasResult AAResult = AA->alias(
   AliasAnalysis::Location(MMOa->getValue(), Overlapa,
-                          MMOa->getTBAAInfo()),
+                          UseTBAA ? MMOa->getTBAAInfo() : 0),
   AliasAnalysis::Location(MMOb->getValue(), Overlapb,
-                          MMOb->getTBAAInfo()));
+                          UseTBAA ? MMOb->getTBAAInfo() : 0));
 
   return (AAResult != AliasAnalysis::NoAlias);
 }

Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=200093&r1=200092&r2=200093&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Sat Jan 25 13:24:54 2014
@@ -56,6 +56,16 @@ namespace {
     CombinerGlobalAA("combiner-global-alias-analysis", cl::Hidden,
                cl::desc("Enable DAG combiner's use of IR alias analysis"));
 
+// FIXME: Enable the use of TBAA. There are two known issues preventing this:
+//   1. Stack coloring does not update TBAA when merging allocas
+//   2. CGP inserts ptrtoint/inttoptr pairs when sinking address computations.
+//      Because BasicAA does not handle inttoptr, we'll often miss basic type
+//      punning idioms that we need to catch so we don't miscompile real-world
+//      code.
+  static cl::opt<bool>
+    UseTBAA("combiner-use-tbaa", cl::Hidden, cl::init(false),
+               cl::desc("Enable DAG combiner's use of TBAA"));
+
 #ifndef NDEBUG
   static cl::opt<std::string>
     CombinerAAOnlyFunc("combiner-aa-only-func", cl::Hidden,
@@ -11079,8 +11089,10 @@ bool DAGCombiner::isAlias(SDValue Ptr1,
     int64_t Overlap1 = Size1 + SrcValueOffset1 - MinOffset;
     int64_t Overlap2 = Size2 + SrcValueOffset2 - MinOffset;
     AliasAnalysis::AliasResult AAResult =
-      AA.alias(AliasAnalysis::Location(SrcValue1, Overlap1, TBAAInfo1),
-               AliasAnalysis::Location(SrcValue2, Overlap2, TBAAInfo2));
+      AA.alias(AliasAnalysis::Location(SrcValue1, Overlap1,
+                                       UseTBAA ? TBAAInfo1 : 0),
+               AliasAnalysis::Location(SrcValue2, Overlap2,
+                                       UseTBAA ? TBAAInfo2 : 0));
     if (AAResult == AliasAnalysis::NoAlias)
       return false;
   }

Modified: llvm/trunk/lib/CodeGen/StackColoring.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/StackColoring.cpp?rev=200093&r1=200092&r2=200093&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/StackColoring.cpp (original)
+++ llvm/trunk/lib/CodeGen/StackColoring.cpp Sat Jan 25 13:24:54 2014
@@ -553,6 +553,13 @@ void StackColoring::remapInstructions(De
         if (!V)
           continue;
 
+        // FIXME: In order to enable the use of TBAA when using AA in CodeGen,
+        // we'll also need to update the TBAA nodes in MMOs with values
+        // derived from the merged allocas. When doing this, we'll need to use
+        // the same variant of GetUnderlyingObjects that is used by the
+        // instruction scheduler (that can look through ptrtoint/inttoptr
+        // pairs).
+
         // We've replaced IR-level uses of the remapped allocas, so we only
         // need to replace direct uses here.
         if (!isa<AllocaInst>(V))