ARM patch: optimize consecutive Thumb comparisons, PR 41514

[Bernd Schmidt <bernds at codesourcery dot com>]

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

Attachment: thumb-41514.diff
Description: Text document