[PATCH] Allow the RegisterCoalescer to remove copies to reserved registers

[hfinkel at anl.gov]

Hi qcolombet, atrick, MatzeB,

Hi everyone,

Please review this patch that allows the RegisterCoalescer to join "non-flipped" range pairs with a physical destination register. This allows the RegisterCoalescer to remove copies like this:

<vreg> = something (maybe a load, for example)
 ... (things that don't use PHYSREG)
PHYSREG = COPY <vreg>

(with all of the restrictions normally applied by the RegisterCoalescer: having compatible register classes, etc. )

Currently, RegisterCoalescer handles only the opposite case (copying *from* a physical register). I don't handle the problem fully here, but try to get the common case where there is only one use of <vreg> (the COPY).

One thing that is not clear to me: Do I need to do anything special to update the 'dead def' live range corresponding to the updated PHYSREG?

Once this functionality is committed, I'll commit a change to the PowerPC backend to make this pattern *very* common, and so it is important that the RegisterCoalescer can eliminate it when that's profitable.

Thanks again!

http://reviews.llvm.org/D6978

Files:
  lib/CodeGen/RegisterCoalescer.cpp
  test/CodeGen/ARM/dyn-stackalloc.ll
  test/CodeGen/X86/2009-02-12-DebugInfoVLA.ll

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