[StatepointLowering] Change AllocatedStackSlots to use SmallBitVector

NFCI.  They key motivation here is that I'd like to use
SmallBitVector::all() in a later change.  Also, using a bit vector here
seemed better in general.

The only interesting change here is that in the failure case of
allocateStackSlot, we no longer (the equivalent of) push_back(true) to
AllocatedStackSlots.  As far as I can tell, this is fine, since we'd
never re-use those slots in the same StatepointLoweringState instance.

Technically there was no need to change the operator[] type accesses to
set() and test(), but I thought it'd be nice to make it obvious that
we're using something other than a std::vector like thing.

llvm-svn: 261337

1 files changed, 10 insertions, 9 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp
index 0a1f135e2e0..0116fdddd95 100644
--- a/llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp
@@ -53,13 +53,10 @@ void StatepointLoweringState::startNewStatepoint(SelectionDAGBuilder &Builder) {
          "Trying to visit statepoint before finished processing previous one");
   Locations.clear();
   NextSlotToAllocate = 0;
-  // Need to resize this on each safepoint - we need the two to stay in
-  // sync and the clear patterns of a SelectionDAGBuilder have no relation
-  // to FunctionLoweringInfo.
+  // Need to resize this on each safepoint - we need the two to stay in sync and
+  // the clear patterns of a SelectionDAGBuilder have no relation to
+  // FunctionLoweringInfo.  SmallBitVector::reset initializes all bits to false.
   AllocatedStackSlots.resize(Builder.FuncInfo.StatepointStackSlots.size());
-  for (size_t i = 0; i < AllocatedStackSlots.size(); i++) {
-    AllocatedStackSlots[i] = false;
-  }
 }
 
 void StatepointLoweringState::clear() {
@@ -85,11 +82,16 @@ StatepointLoweringState::allocateStackSlot(EVT ValueType,
   const size_t NumSlots = AllocatedStackSlots.size();
   assert(NextSlotToAllocate <= NumSlots && "Broken invariant");
 
+  // The stack slots in StatepointStackSlots beyond the first NumSlots were
+  // added in this instance of StatepointLoweringState, and cannot be re-used.
+  assert(NumSlots <= Builder.FuncInfo.StatepointStackSlots.size() &&
+         "Broken invariant");
+
   for (; NextSlotToAllocate < NumSlots; NextSlotToAllocate++) {
-    if (!AllocatedStackSlots[NextSlotToAllocate] &&
+    if (!AllocatedStackSlots.test(NextSlotToAllocate) &&
         MFI->getObjectSize(NextSlotToAllocate) == SpillSize) {
       const int FI = Builder.FuncInfo.StatepointStackSlots[NextSlotToAllocate];
-      AllocatedStackSlots[NextSlotToAllocate] = true;
+      AllocatedStackSlots.set(NextSlotToAllocate);
       return Builder.DAG.getFrameIndex(FI, ValueType);
     }
   }
@@ -104,7 +106,6 @@ StatepointLoweringState::allocateStackSlot(EVT ValueType,
   MFI->markAsStatepointSpillSlotObjectIndex(FI);
 
   Builder.FuncInfo.StatepointStackSlots.push_back(FI);
-  AllocatedStackSlots.push_back(true);
   return SpillSlot;
 }