[PATCH] ARM: fold stack update into push/pop

[Tim Northover]

Hi all,

The attached patch is designed to help embedded work, and particularly situations where code-size is paramount. It converts prologue sequences like:
    push {r7, lr}
    sub sp, sp, #16

into:
    push {r3, r4, r5, r6, r7, lr}

(where we don't actually care about r3-r6, but they take up space on the stack).

Analogous epilogue sequences are also converted where possible, saving up to 4 bytes per function.

As far as I can tell from some simple tests this almost always slows code down though, so it's gated on the MinSize function attribute.

Could someone take a look, please? Frame lowering code is always rather hairy.

http://llvm-reviews.chandlerc.com/D2105

Files:
  lib/Target/ARM/ARMBaseInstrInfo.cpp
  lib/Target/ARM/ARMBaseInstrInfo.h
  lib/Target/ARM/ARMFrameLowering.cpp
  lib/Target/ARM/Thumb1FrameLowering.cpp
  test/CodeGen/ARM/fold-stack-adjust.ll
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