[PATCH][PowerPC] Preparatory refactoring for making prologue & epilogue safe on PPC32 SVR4 ABI

[Mark Minich]

There are FIXME's in PowerPC/PPCFrameLowering.cpp, method PPCFrameLowering::emitPrologue() related to "negative offsets of R1" on PPC32 SVR4. They're true, but the real issue is that on PPC32 SVR4 (and any ABI without a Red Zone), no spills may be made until after the stackframe is claimed, which also includes the LR spill which is at a positive offset. The same problem exists in emitEpilogue(), though there's no FIXME for it. I intend to fix this issue, making LLVM-compiled code finally safe for use on SVR4/EABI/e500 32-bit platforms (including in particular, OS-free embedded systems & kernel code, where interrupts may share the same stack as user code).

In preparation for making these changes, to make the diffs for the functional changes less cluttered, I am providing the non-functional refactorings in two stages:

Stage 1 does some minor fluffy refactorings to pull multiple method calls up into a single bool, creating named bools for repeated uses of obscure logic, moving some code up earlier because either stage 2 or my final version will require it earlier, and rewording/adding some comments. My stage 1 changes can be characterized as primarily fluffy cleanup, the purpose of which may be unclear until the stage 2 or final changes are made.

My stage 2 refactorings combine the separate PPC32 & PPC64 logic, which is currently performed by largely duplicate code, into a single flow, with the differences handled by a group of constants initialized early in the methods.

This submission is for my stage 1 changes. There should be no functional changes whatsoever; this is a pure refactoring.

http://llvm-reviews.chandlerc.com/D1343



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