http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55181
Steven Bosscher <steven at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target|avr |
CC| |steven at gcc dot gnu.org
--- Comment #5 from Steven Bosscher <steven at gcc dot gnu.org> 2013-03-05
23:45:38 UTC ---
At trunk r196410, the .164t.optimized dump looks like this:
lfsr (long unsigned int number)
{
unsigned char b;
long unsigned int _4;
long unsigned int _5;
_Bool _8;
<bb 2>:
_4 = number_3(D) & 536870912;
_8 = _4 != 0;
b_10 = (unsigned char) _8;
_5 = number_3(D) & 8192;
if (_5 != 0)
goto <bb 3>;
else
goto <bb 4>;
<bb 3>:
b_6 = b_10 + 1;
<bb 4>:
# b_2 = PHI <b_10(2), b_6(3)>
return b_2;
}
and exact_log2 could be used to identify the and-instruction as
a bit test instruction:
(gdb) p exact_log2(536870912)
$1 = 29
(gdb)
AFAIK there are no named patterns for bit tests, perhaps there should be.
(http://gcc.gnu.org/onlinedocs/gccint/Standard-Names.html#Standard-Names)
So at least on x86_64 the test gets expanded as a shift:
6: {r65:DI=r64:DI 0>>0x1d;clobber flags:CC;}
7: {r59:QI=r65:DI#0&0x1;clobber flags:CC;}
8: {r66:DI=r64:DI&0x2000;clobber flags:CC;}
9: flags:CCZ=cmp(r66:DI,0)
i.e. also far from optimal.
