Avoid reserving an ARM base pointer during register allocation.

Once register allocation has started the reserved registers are frozen.

Fix the ARM canRealignStack() hook to respect the frozen register state.
Now the hook returns false if register allocation was started with frame
pointer elimination enabled.

It also returns false if register allocation started without a reserved
base pointer, and stack realignment would require a base pointer.  This
bug was breaking oggenc on armv6.

No test case, an upcoming patch will use this functionality to realign
the stack for spill slots when possible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147578 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 17 insertions, 2 deletions

diff --git a/lib/Target/ARM/ARMBaseRegisterInfo.cpp b/lib/Target/ARM/ARMBaseRegisterInfo.cpp
index 9c91bcc307..9f46ce9a17 100644
--- a/lib/Target/ARM/ARMBaseRegisterInfo.cpp
+++ b/lib/Target/ARM/ARMBaseRegisterInfo.cpp
@@ -528,13 +528,28 @@ bool ARMBaseRegisterInfo::hasBasePointer(const MachineFunction &MF) const {
 
 bool ARMBaseRegisterInfo::canRealignStack(const MachineFunction &MF) const {
   const MachineFrameInfo *MFI = MF.getFrameInfo();
+  const MachineRegisterInfo *MRI = &MF.getRegInfo();
   const ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
   // We can't realign the stack if:
   // 1. Dynamic stack realignment is explicitly disabled,
   // 2. This is a Thumb1 function (it's not useful, so we don't bother), or
   // 3. There are VLAs in the function and the base pointer is disabled.
-  return (MF.getTarget().Options.RealignStack && !AFI->isThumb1OnlyFunction() &&
-          (!MFI->hasVarSizedObjects() || EnableBasePointer));
+  if (!MF.getTarget().Options.RealignStack)
+    return false;
+  if (AFI->isThumb1OnlyFunction())
+    return false;
+  // Stack realignment requires a frame pointer.  If we already started
+  // register allocation with frame pointer elimination, it is too late now.
+  if (!MRI->canReserveReg(FramePtr))
+    return false;
+  // We may also need a base pointer if there are dynamic allocas.
+  if (!MFI->hasVarSizedObjects())
+    return true;
+  if (!EnableBasePointer)
+    return false;
+  // A base pointer is required and allowed.  Check that it isn't too late to
+  // reserve it.
+  return MRI->canReserveReg(BasePtr);
 }
 
 bool ARMBaseRegisterInfo::