ARM: fold prologue/epilogue sp updates into push/pop for code size

ARM prologues usually look like:
    push {r7, lr}
    sub sp, sp, #4

If code size is extremely important, this can be optimised to the single
instruction:
    push {r6, r7, lr}

where we don't actually care about the contents of r6, but pushing it subtracts
4 from sp as a side effect.

This should implement such a conversion, predicated on the "minsize" function
attribute (-Oz) since I've yet to find any code it actually makes faster.

llvm-svn: 194264

1 files changed, 11 insertions, 4 deletions

diff --git a/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp b/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp
index cfaa792fc9b..d921c82cfb0 100644
--- a/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp
+++ b/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp
@@ -164,11 +164,17 @@ void Thumb1FrameLowering::emitPrologue(MachineFunction &MF) const {
   AFI->setDPRCalleeSavedAreaOffset(DPRCSOffset);
   NumBytes = DPRCSOffset;
 
+  int FramePtrOffsetInBlock = 0;
+  if (tryFoldSPUpdateIntoPushPop(MF, prior(MBBI), NumBytes)) {
+    FramePtrOffsetInBlock = NumBytes;
+    NumBytes = 0;
+  }
+
   // Adjust FP so it point to the stack slot that contains the previous FP.
   if (HasFP) {
-    int FramePtrOffset = MFI->getObjectOffset(FramePtrSpillFI) + GPRCS1Size;
+    FramePtrOffsetInBlock += MFI->getObjectOffset(FramePtrSpillFI) + GPRCS1Size;
     AddDefaultPred(BuildMI(MBB, MBBI, dl, TII.get(ARM::tADDrSPi), FramePtr)
-      .addReg(ARM::SP).addImm(FramePtrOffset / 4)
+      .addReg(ARM::SP).addImm(FramePtrOffsetInBlock / 4)
       .setMIFlags(MachineInstr::FrameSetup));
     if (NumBytes > 508)
       // If offset is > 508 then sp cannot be adjusted in a single instruction,
@@ -292,8 +298,9 @@ void Thumb1FrameLowering::emitEpilogue(MachineFunction &MF,
           &MBB.front() != MBBI &&
           prior(MBBI)->getOpcode() == ARM::tPOP) {
         MachineBasicBlock::iterator PMBBI = prior(MBBI);
-        emitSPUpdate(MBB, PMBBI, TII, dl, *RegInfo, NumBytes);
-      } else
+        if (!tryFoldSPUpdateIntoPushPop(MF, PMBBI, NumBytes))
+          emitSPUpdate(MBB, PMBBI, TII, dl, *RegInfo, NumBytes);
+      } else if (!tryFoldSPUpdateIntoPushPop(MF, MBBI, NumBytes))
         emitSPUpdate(MBB, MBBI, TII, dl, *RegInfo, NumBytes);
     }
   }