CodeGen peephole: fold redundant phys reg copies

CodeGen peephole: fold redundant phys reg copies

Code generation often exposes redundant physical register copies through
virtual registers such as:

  %vreg = COPY %PHYSREG
  ...
  %PHYSREG = COPY %vreg

There are cases where no intervening clobber of %PHYSREG occurs, and the
later copy could therefore be removed. In some cases this further allows
us to remove the initial copy.

This patch contains a motivating example which comes from the x86 build
of Chrome, specifically cc::ResourceProvider::UnlockForRead uses
libstdc++'s implementation of hash_map. That example has two tests live
at the same time, and after machine sinking LLVM has confused itself
enough and things spilling EFLAGS is a great idea even though it's
never restored and the comparison results are both live.

Before this patch we have:
  DEC32m %RIP, 1, %noreg, <ga:@L>, %noreg, %EFLAGS<imp-def>
  %vreg1<def> = COPY %EFLAGS; GR64:%vreg1
  %EFLAGS<def> = COPY %vreg1; GR64:%vreg1
  JNE_1 <BB#1>, %EFLAGS<imp-use>

Both copies are useless. This patch tries to eliminate the later copy in
a generic manner.

dec is especially confusing to LLVM when compared with sub.

I wrote this patch to treat all physical registers generically, but only
remove redundant copies of non-allocatable physical registers because
the allocatable ones caused issues (e.g. when calling conventions weren't
properly modeled) and should be handled later by the register allocator
anyways.

The following tests used to failed when the patch also replaced allocatable
registers:
  CodeGen/X86/StackColoring.ll
  CodeGen/X86/avx512-calling-conv.ll
  CodeGen/X86/copy-propagation.ll
  CodeGen/X86/inline-asm-fpstack.ll
  CodeGen/X86/musttail-varargs.ll
  CodeGen/X86/pop-stack-cleanup.ll
  CodeGen/X86/preserve_mostcc64.ll
  CodeGen/X86/tailcallstack64.ll
  CodeGen/X86/this-return-64.ll
This happens because COPY has other special meaning for e.g. dependency
breakage and x87 FP stack.

Note that all other backends' tests pass.

Reviewers: qcolombet
Subscribers: llvm-commits
Differential Revision: http://reviews.llvm.org/D15157

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254665 91177308-0d34-0410-b5e6-96231b3b80d8