ARM patch: optimize consecutive Thumb comparisons, PR 41514

[Richard Earnshaw]

On Tue, 2010-03-30 at 13:18 +0100, Bernd Schmidt wrote:
> Since Thumb mode conditional branches are represented as cbranchsi
> patterns, none of the mechanisms for optimizing multiple comparisons of
> the same arguments work for the Thumb.  Attached is a simple patch that
> in the cbranchsi4_insn output code looks at the previous insn and
> determines if it's also a cbranch with the same comparison.  If so, we
> can omit the compare.
> 
> I wouldn't necessarily disagree with anyone who thought this was a bit
> crude; for one thing we overestimate the size of optimized cbranch
> insns.  The alternative I see is to add another cbranch instruction
> pattern that does not output the compare and look for opportunities to
> use it in arm_reorg.
> 
> Will test on arm-eabi{,mthumb} overnight (it takes forever).  Ok for
> 4.6, or should I use a different approach?
> 
> 
> Bernd

I can't immediately think of a better way (and indeed, this is
essentially what is done generically for CC1 targets, where there is a
similar loss of accounting).  It wouldn't be the only place where we
estimate more bytes than are finally needed.

OK
R.