For ARM stack frames that utilize variable sized objects and have either

large local stack areas or require dynamic stack realignment, allocate a
base register via which to access the local frame. This allows efficient
access to frame indices not accessible via the FP (either due to being out
of range or due to dynamic realignment) or the SP (due to variable sized
object allocation). In particular, this greatly improves efficiency of access
to spill slots in Thumb functions which contain VLAs.

rdar://7352504
rdar://8374540
rdar://8355680

llvm-svn: 112883

1 files changed, 13 insertions, 3 deletions

diff --git a/llvm/lib/Target/ARM/Thumb1RegisterInfo.cpp b/llvm/lib/Target/ARM/Thumb1RegisterInfo.cpp
index c562c371469..a21a3da10bd 100644
--- a/llvm/lib/Target/ARM/Thumb1RegisterInfo.cpp
+++ b/llvm/lib/Target/ARM/Thumb1RegisterInfo.cpp
@@ -604,9 +604,12 @@ Thumb1RegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
   else if (MF.getFrameInfo()->hasVarSizedObjects()) {
     assert(SPAdj == 0 && hasFP(MF) && "Unexpected");
     // There are alloca()'s in this function, must reference off the frame
-    // pointer instead.
-    FrameReg = getFrameRegister(MF);
-    Offset -= AFI->getFramePtrSpillOffset();
+    // pointer or base pointer instead.
+    if (!hasBasePointer(MF)) {
+      FrameReg = getFrameRegister(MF);
+      Offset -= AFI->getFramePtrSpillOffset();
+    } else
+      FrameReg = BasePtr;
   }
 
   // Special handling of dbg_value instructions.
@@ -787,6 +790,13 @@ void Thumb1RegisterInfo::emitPrologue(MachineFunction &MF) const {
   AFI->setGPRCalleeSavedArea1Size(GPRCS1Size);
   AFI->setGPRCalleeSavedArea2Size(GPRCS2Size);
   AFI->setDPRCalleeSavedAreaSize(DPRCSSize);
+
+  // If we need a base pointer, set it up here. It's whatever the value
+  // of the stack pointer is at this point. Any variable size objects
+  // will be allocated after this, so we can still use the base pointer
+  // to reference locals.
+  if (hasBasePointer(MF))
+    BuildMI(MBB, MBBI, dl, TII.get(ARM::tMOVgpr2gpr), BasePtr).addReg(ARM::SP);
 }
 
 static bool isCalleeSavedRegister(unsigned Reg, const unsigned *CSRegs) {