Le 13/05/2016 à 08:16, Michael Ellerman a écrit :
On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote:
With the ffs() function as defined in arch/powerpc/include/asm/bitops.h
GCC will not optimise the code in case of constant parameter, as shown
by the small exemple below.
int ffs_test(void)
{
return 4 << ffs(31);
}
c0012334 <ffs_test>:
c0012334: 39 20 00 01 li r9,1
c0012338: 38 60 00 04 li r3,4
c001233c: 7d 29 00 34 cntlzw r9,r9
c0012340: 21 29 00 20 subfic r9,r9,32
c0012344: 7c 63 48 30 slw r3,r3,r9
c0012348: 4e 80 00 20 blr
With this patch, the same function will compile as follows:
c0012334 <ffs_test>:
c0012334: 38 60 00 08 li r3,8
c0012338: 4e 80 00 20 blr
But what code does it generate when it's not a constant?
The generated code is the same with and without the patch when not a
constant:
int ffs_test2(int x)
{
return ffs(x);
}
c001233c <ffs_test2>:
c001233c: 7d 23 00 d0 neg r9,r3
c0012340: 7d 23 18 38 and r3,r9,r3
c0012344: 7c 63 00 34 cntlzw r3,r3
c0012348: 20 63 00 20 subfic r3,r3,32
c001234c: 4e 80 00 20 blr
And which gcc version first added the builtin version?
Don't know, but __builtin_ffs() is already used in
arch/powerpc/include/asm/page_32.h
Christophe
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