Re: [PATCH] x86: support i386 with Clang

[Nick Desaulniers]

On Mon, May 11, 2020 at 11:24 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, May 11, 2020 at 11:12 AM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Would using "__builtin_choose_expr()" be able to avoid this whole issue?
>
> We actually have a fair amount of "pick expression based on size", so
> with a few helper macros we could make the code look better than the
> case statements too.
>
> Something (ENTIRELY UNTESTED!) like the attached patch, perhaps?
>
> NOTE! I only converted one single use to that "pick_size_xyz()" model.
> If this actually works for clang too, we could do the others.
>
> I guess I should just test it, since I have that clang tree.

Interesting approach.  Researching __builtin_choose_expr, it looks
like it was cited as prior art for C11's _Generic keyword.
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1404.htm

I'm kind of tempted now to play with _Generic and see if that makes a
difference, though I'm not hopeful.  Without checking, I don't know if
that will produce warnings with `-std=gnu89`.

...

I'm playing around with _Generic now to see if that's a possible
solution, but it looks like it can't be used for selecting inline asm
constraint strings.  But maybe there's another way to use _Generic
here.  TODO: more testing.

I don't quite understand the use of GNU C statement expressions in
pick_type_statement, though I'm guessing the return value is important
somewhere.  Maybe just looking at the preprocessed source would make
it clearer.

> "why do we cast to unsigned long and then to char,
> when we just checked that the size of the type is 1"?

Deja vu.  I don't remember who I discussed that with, maybe Arnd, but
that was something else I had asked at some point.  I must have
forgotten to look into it more before sending the patch.  Can likely
drop that at least.

--
Thanks,
~Nick Desaulniers