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?
And which gcc version first added the builtin version?
cheers
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