https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97445
--- Comment #47 from Christophe Leroy <christophe.leroy at csgroup dot eu> ---
(In reply to Segher Boessenkool from comment #46)
> (In reply to Christophe Leroy from comment #43)
> > int g(int x)
> > {
> > return __builtin_clz(0);
> > }
> >
> > Gives
> >
> > 00000018 <g>:
> > 18: 38 60 00 20 li r3,32
> > 1c: 4e 80 00 20 blr
>
> That is because rs6000 has
>
> /* The cntlzw and cntlzd instructions return 32 and 64 for input of zero. */
> #define CLZ_DEFINED_VALUE_AT_ZERO(MODE, VALUE) \
> ((VALUE) = GET_MODE_BITSIZE (MODE), 2)
>
> This says that at RTL level and in the optabs, clz of 0 *is* defined,
> for rs6000. But the builtin is not valid with an arg of 0!
I opened bug #97503 for that
