[PEI] Don't re-allocate a pre-allocated stack protector slot

The LocalStackSlotPass pre-allocates a stack protector and makes sure
that it comes before the local variables on the stack.

We need to make sure that later during PEI we don't re-allocate a new
stack protector slot. If that happens, the new stack protector slot will
end up being **after** the local variables that it should be protecting.

Therefore, we would have two slots assigned for two different stack
protectors, one at the top of the stack, and one at the bottom. Since
PEI will overwrite the assigned slot for the stack protector, the load
that is used to compare the value of the stack protector will use the
slot assigned by PEI, which is wrong.

For this, we need to check if the object is pre-allocated, and re-use
that pre-allocated slot.

Differential Revision: https://reviews.llvm.org/D64757

llvm-svn: 366371

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/LocalStackSlotAllocation.cpp b/llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
index aa8f824c6b9..b14d76a585f 100644
--- a/llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
+++ b/llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
@@ -201,6 +201,14 @@ void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) {
   SmallSet<int, 16> ProtectedObjs;
   if (MFI.hasStackProtectorIndex()) {
     int StackProtectorFI = MFI.getStackProtectorIndex();
+
+    // We need to make sure we didn't pre-allocate the stack protector when
+    // doing this.
+    // If we already have a stack protector, this will re-assign it to a slot
+    // that is **not** covering the protected objects.
+    assert(!MFI.isObjectPreAllocated(StackProtectorFI) &&
+           "Stack protector pre-allocated in LocalStackSlotAllocation");
+
     StackObjSet LargeArrayObjs;
     StackObjSet SmallArrayObjs;
     StackObjSet AddrOfObjs;