http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55306
Bug #: 55306
Summary: Make combine emit converted logical right shifts
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: rtl-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: olege...@gcc.gnu.org
As of rev 193423 the combine pass can successfully convert arithmetic right
shifts into logical right shifts, but will not split / emit them as individual
insns if ...
* the target has arith. right shift patterns defined that match the shift
* the target does not have an insn that combine came up with, which inlcudes
the logical shift.
There are some targets (SH, ARC, see PR 54089) where logical right shifts are
cheaper than arithmetic right shifts.
Since combine already has shift conversion integrated, it would probably make
sense to extend it, instead of replicating it in some separate pass or doing
fancy stuff in the target code.
Ideally, whenever combine converts a arith shift into a logical shift it should
look at the costs of the shifts and always split out the cheaper shift insn (if
the target has a pattern for the logical shift).
