------- Comment #2 from ubizjak at gmail dot com 2007-06-06 08:15 -------
(In reply to comment #0)
> After the addition, which sets the ALU flags, the compiler issues two compare
> instructions and conditional branches. This sequence could be replaced by a
> conditional branch following the addl, testing one of the flags (overflow?
> carry? I forget which) set by it.
in config/i386/i386-modes.def, documentation says:
Add CCGOC to indicate comparisons against zero that allows
unspecified garbage in the Carry and Overflow flag. This
mode is used to simulate comparisons of (a-b) and (a+b)
against zero using sub/cmp/add operations.
addl operates in CCGOCmode, but you are requesting GTU conditional jump that
requires Carry flag=0 and Zero flag=0. GTU is incompatible with CCGOCmode due
to Carry flag handling.
Change one of conditions to "if (sum == 0) abort();" and you will see insn
elimination in action.
And BTW - wrapping is undefined operation.
--
ubizjak at gmail dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |INVALID
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30315
