On Thu, Oct 08, 2020 at 11:04:01AM +0000, Wilco Dijkstra wrote:
> > Perhaps another way out of this would be document and enforce that
> > __builtin_c[lt]z{,l,ll} etc calls are undefined at zero, but C[TL]Z ifn
> > calls are defined there based on *_DEFINED_VALUE_AT_ZERO (*) == 2, and then
> > we would need to make sure that e.g. in simplify_count_trailing_zeroes
> > we emit a .CTZ or __builtin_ctz call depending on whether it is undefined
> > there or not; or give .C[TL]Z an additional argument (0/1) which would tell
> > if it is defined at zero or not.
>
> I think a better way forward would be to make the builtin_clz/ctz more
> defined.
> Having undefined values is a source of unnecessary bugs given practically all
> modern targets return the number of bits for the zero input - it is relatively
> easy to ensure this on the few targets that don't.
Well, e.g. i?86/x86_64 in most commonly used CPU flags is really undefined
(the register is unchanged). And -1 is also quite commonly used value,
e.g. powerpc, gcn, xtensa.
Jakub
