[PATCH] D23516: [ARM] Generate consistent frame records for Thumb2

[Oliver Stannard via llvm-commits]

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There is not an official documented ABI for frame pointers in Thumb2,
but we should try to emit something which is useful.

We use r7 as the frame pointer for Thumb code, which currently means
that if a function needs to save a high register (r8-r11), it will get
pushed to the stack between the frame pointer (r7) and link register
(r14). This means that while a stack unwinder can follow the chain of
frame pointers up the stack, it cannot know the offset to lr, so does
not know which functions correspond to the stack frames.

To fix this, we need to push the callee-saved registers in two batches,
with the first push saving the low registers, fp and lr, and the second
push saving the high registers. This is already implemented, but
previously only used for iOS. This patch turns it on for all Thumb2
targets when frame pointers are required by the ABI, and the frame
pointer is r7 (Windows uses r11, so this isn't a problem there). If
frame pointer elimination is enabled we still emit a single push/pop
even if we need a frame pointer for other reasons, to avoid increasing
code size.

We must also ensure that lr is pushed to the stack when using a frame
pointer, so that we end up with a complete frame record. Situations that
could cause this were rare, because we already push lr in most
situations so that we can return using the pop instruction.

@Tim, or anyone who knows about the Apple ARM calling convention:
There is existing code to force a frame pointer and enable split push/pop when
using the Apple ABI (documented at
https://developer.apple.com/library/ios/documentation/Xcode/Conceptual/iPhoneOSABIReference/Articles/ARMv6FunctionCallingConventions.html).
However, the code to do this is spread across several functions, and the exact
conditions under which is enabled are different. I've found these different
conditions:
* isTargetIOS() || isTargetWatchOS() // In ARMFrameLowering::hasFP
* isTargetMachO() // In ARMSubtarget::splitFramePushPop
* isTargetDarwin() // In ARMBaseRegisterInfo::getCalleeSavedRegs

Which of these conditions is correct? I've left these as they are for now, but
we should probably clear this up. I found one test, ifcvt-iter-indbr.ll, which
used a thumbv7s-apple-darwin triple, which currently (before and after this
patch) causes the split push/pop to be used, but does still allow the frame
pointer to be optimised out.

Repository:
  rL LLVM

https://reviews.llvm.org/D23516

Files:
  lib/Target/ARM/ARMBaseRegisterInfo.cpp
  lib/Target/ARM/ARMFrameLowering.cpp
  lib/Target/ARM/ARMFrameLowering.h
  lib/Target/ARM/ARMSubtarget.h
  lib/Target/ARM/Thumb1FrameLowering.cpp
  test/CodeGen/ARM/debug-frame-large-stack.ll
  test/CodeGen/ARM/dwarf-unwind.ll
  test/CodeGen/ARM/fast-isel-frameaddr.ll
  test/CodeGen/Thumb/push.ll
  test/CodeGen/Thumb2/frame-pointer.ll

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