[X86] Make sure it is safe to clobber EFLAGS, if need be, when choosing

the prologue.

Do not use basic blocks that have EFLAGS live-in as prologue if we need
to realign the stack. Realigning the stack uses AND instruction and this
clobbers EFLAGS.

An other alternative would have been to save and restore EFLAGS around
the stack realignment code, but this is likely inefficient.

Fixes PR27531.

llvm-svn: 267634

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86FrameLowering.cpp b/llvm/lib/Target/X86/X86FrameLowering.cpp
index 8efe7038daf..155cb3c02b2 100644
--- a/llvm/lib/Target/X86/X86FrameLowering.cpp
+++ b/llvm/lib/Target/X86/X86FrameLowering.cpp
@@ -2609,6 +2609,12 @@ eliminateCallFramePseudoInstr(MachineFunction &MF, MachineBasicBlock &MBB,
   return I;
 }
 
+bool X86FrameLowering::canUseAsPrologue(const MachineBasicBlock &MBB) const {
+  assert(MBB.getParent() && "Block is not attached to a function!");
+  const MachineFunction &MF = *MBB.getParent();
+  return !TRI->needsStackRealignment(MF) || !MBB.isLiveIn(X86::EFLAGS);
+}
+
 bool X86FrameLowering::canUseAsEpilogue(const MachineBasicBlock &MBB) const {
   assert(MBB.getParent() && "Block is not attached to a function!");