Make isAliased property for fixed-offset stack objects adjustable

We used to assume that any fixed-offset stack object was not aliased. This
meant that no IR value could point to the memory contained in such an object.
This is a reasonable default, but is not a universally-correct
target-independent fact. For example, on PowerPC (both Darwin and non-Darwin),
some byval arguments are allocated at fixed offsets by the ABI. These, however,
certainly can be pointed to by IR values. This change moves the 'isAliased'
logic out of FixedStackPseudoSourceValue and into MFI, and allows the isAliased
property to be overridden for fixed-offset objects.

This will be used by an upcoming commit to the PowerPC backend to fix PR20280.

No functionality change intended (the behavior of
FixedStackPseudoSourceValue::isAliased has been made more conservative for
callers that don't pass an MFI object, but I don't see any in-tree callers that
do that).

llvm-svn: 215794

2 files changed, 9 insertions, 11 deletions

diff --git a/llvm/lib/CodeGen/MachineFunction.cpp b/llvm/lib/CodeGen/MachineFunction.cpp
index 7b73df0466a..13dcab03906 100644
--- a/llvm/lib/CodeGen/MachineFunction.cpp
+++ b/llvm/lib/CodeGen/MachineFunction.cpp
@@ -521,7 +521,8 @@ int MachineFrameInfo::CreateStackObject(uint64_t Size, unsigned Alignment,
     clampStackAlignment(!getFrameLowering()->isStackRealignable() ||
                           !RealignOption,
                         Alignment, getFrameLowering()->getStackAlignment());
-  Objects.push_back(StackObject(Size, Alignment, 0, false, isSS, Alloca));
+  Objects.push_back(StackObject(Size, Alignment, 0, false, isSS, Alloca,
+                                !isSS));
   int Index = (int)Objects.size() - NumFixedObjects - 1;
   assert(Index >= 0 && "Bad frame index!");
   ensureMaxAlignment(Alignment);
@@ -554,7 +555,7 @@ int MachineFrameInfo::CreateVariableSizedObject(unsigned Alignment,
   Alignment = clampStackAlignment(
       !getFrameLowering()->isStackRealignable() || !RealignOption, Alignment,
       getFrameLowering()->getStackAlignment());
-  Objects.push_back(StackObject(0, Alignment, 0, false, false, Alloca));
+  Objects.push_back(StackObject(0, Alignment, 0, false, false, Alloca, true));
   ensureMaxAlignment(Alignment);
   return (int)Objects.size()-NumFixedObjects-1;
 }
@@ -565,7 +566,7 @@ int MachineFrameInfo::CreateVariableSizedObject(unsigned Alignment,
 /// index with a negative value.
 ///
 int MachineFrameInfo::CreateFixedObject(uint64_t Size, int64_t SPOffset,
-                                        bool Immutable) {
+                                        bool Immutable, bool isAliased) {
   assert(Size != 0 && "Cannot allocate zero size fixed stack objects!");
   // The alignment of the frame index can be determined from its offset from
   // the incoming frame position.  If the frame object is at offset 32 and
@@ -578,7 +579,7 @@ int MachineFrameInfo::CreateFixedObject(uint64_t Size, int64_t SPOffset,
                               Align, getFrameLowering()->getStackAlignment());
   Objects.insert(Objects.begin(), StackObject(Size, Align, SPOffset, Immutable,
                                               /*isSS*/   false,
-                                              /*Alloca*/ nullptr));
+                                              /*Alloca*/ nullptr, isAliased));
   return -++NumFixedObjects;
 }
 
@@ -594,7 +595,8 @@ int MachineFrameInfo::CreateFixedSpillStackObject(uint64_t Size,
   Objects.insert(Objects.begin(), StackObject(Size, Align, SPOffset,
                                               /*Immutable*/ true,
                                               /*isSS*/ true,
-                                              /*Alloca*/ nullptr));
+                                              /*Alloca*/ nullptr,
+                                              /*isAliased*/ false));
   return -++NumFixedObjects;
 }
 
diff --git a/llvm/lib/CodeGen/PseudoSourceValue.cpp b/llvm/lib/CodeGen/PseudoSourceValue.cpp
index 12b2c902f8d..b1c341d3a68 100644
--- a/llvm/lib/CodeGen/PseudoSourceValue.cpp
+++ b/llvm/lib/CodeGen/PseudoSourceValue.cpp
@@ -107,13 +107,9 @@ bool FixedStackPseudoSourceValue::isConstant(const MachineFrameInfo *MFI) const{
 }
 
 bool FixedStackPseudoSourceValue::isAliased(const MachineFrameInfo *MFI) const {
-  // Negative frame indices are used for special things that don't
-  // appear in LLVM IR. Non-negative indices may be used for things
-  // like static allocas.
   if (!MFI)
-    return FI >= 0;
-  // Spill slots should not alias others.
-  return !MFI->isFixedObjectIndex(FI) && !MFI->isSpillSlotObjectIndex(FI);
+    return true;
+  return MFI->isAliasedObjectIndex(FI);
 }
 
 bool FixedStackPseudoSourceValue::mayAlias(const MachineFrameInfo *MFI) const {