Fix coordination for using R4 in Thumb1 as a scratch for SP restore.

The logic for reserving R4 for use as a scratch needs to match that for
actually using it. Also, it's not necessary for immediate <=508, so adjust
the value checked.

llvm-svn: 132934

1 files changed, 7 insertions, 2 deletions

diff --git a/llvm/lib/Target/ARM/ARMFrameLowering.cpp b/llvm/lib/Target/ARM/ARMFrameLowering.cpp
index e2e95d47b37..516e34ae04e 100644
--- a/llvm/lib/Target/ARM/ARMFrameLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMFrameLowering.cpp
@@ -841,9 +841,14 @@ ARMFrameLowering::processFunctionBeforeCalleeSavedScan(MachineFunction &MF,
     if (AFI->getVarArgsRegSaveSize() > 0)
       MF.getRegInfo().setPhysRegUsed(ARM::LR);
 
-    // Spill R4 if Thumb1 epilogue has to restore SP from FP since 
+    // Spill R4 if Thumb1 epilogue has to restore SP from FP. We don't know
+    // for sure what the stack size will be, but for this, an estimate is good
+    // enough. If there anything changes it, it'll be a spill, which implies
+    // we've used all the registers and so R4 is already used, so not marking
+    // it here will be OK.
     // FIXME: It will be better just to find spare register here.
-    if (MFI->hasVarSizedObjects())
+    unsigned StackSize = estimateStackSize(MF);
+    if (MFI->hasVarSizedObjects() || StackSize > 508)
       MF.getRegInfo().setPhysRegUsed(ARM::R4);
   }