[PATCH] D11197: [PeepholeOptimizer] Look through PHIs to find additional register sources

[Bruno Cardoso Lopes]

bruno created this revision.
bruno added reviewers: qcolombet, MatzeB.
bruno added a subscriber: llvm-commits.
bruno set the repository for this revision to rL LLVM.

The PeepholeOptimizer misses some rewrite opportunities since it can't look through PHI instructions.
This patch implements looking through PHIs while trying to rewrite Uncoalescable copies.

It depends on the refactoring posted in: http://reviews.llvm.org/D11195

Changes include:

- Improve the ValueTracker in the PeepholeOptimizer to look through PHI instructions.
- Teach findNextSource to lookup into multiple sources returned by the ValueTracker and generate a rewrite map for clients to consume.
- Although all support is there, don't enable this for coalescable copies until we get the motivating testcases.
- Limit the length of nested PHI chains to lookup to avoid potential compile time issues. This can further improved in the presence of compelling testcases.
    
With these changes we can find more register sources and rewrite more copies to allow coaslescing of bitcast instructions.
Hence, we eliminate unnecessary VR64 <-> GR64 copies in x86, but it could be extended to other archs by marking "isBitcast"
on target specific instructions.

A x86 MMX example follows:
    
    A:
      psllq %mm1, %mm0
      movd  %mm0, %r9
      jmp C
    
    B:
      por %mm1, %mm0
      movd  %mm0, %r9
      jmp C
    
    C:
      movd  %r9, %mm0
      pshufw  $238, %mm0, %mm0
    
    Becomes:
    
    A:
      psllq %mm1, %mm0
      jmp C
    
    B:
      por %mm1, %mm0
      jmp C
    
    C:
      pshufw  $238, %mm0, %mm0


Repository:
  rL LLVM

http://reviews.llvm.org/D11197

Files:
  include/llvm/Target/TargetInstrInfo.h
  lib/CodeGen/PeepholeOptimizer.cpp
  lib/Target/X86/X86InstrMMX.td
  test/CodeGen/X86/mmx-coalescing.ll

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