Re: clang asm-goto support (Was Re: [PATCH v2] x86/retpoline: Add clang support)

[Andy Lutomirski]

On Wed, Feb 14, 2018 at 3:31 PM, James Y Knight <jyknight@xxxxxxxxxx> wrote:
> I'd be definitely in favor having clang support asm goto. I wouldn't
> want to exclude having other conversations about how to more directly
> provide compiler features that the linux kernel could use, ALSO, but I
> do not think that conversation should block implementing asm-goto.
>
> IMO, inline asm is, generally, a valuable feature to provide in the
> compiler as an escape hatch, and asm goto is a relatively sane
> extension of it. Supporting outgoing edges from an inline asm block is
> a reasonable thing for users to desire, and as far as anyone's said so
> far, seems like it ought to be fairly easily implementable, without
> causing bad side-effects in the compiler.
>
> Of course, we generally do want to minimize the need for users to use
> inline asm, by providing appropriate compiler support for the features
> people would otherwise be forced to implement using asm. But I don't
> see that as really any more important for asm-goto than any other
> inline asm. There will always be a desire for escape hatches, to do
> weird and unique things which aren't supported directly in the
> compiler. (Also, the kernel is a pretty special case in terms of its
> requirements, it seems exceedingly unlikely that we could ever provide
> enough compiler support to let it eliminate inline asm.).
>

If clang implements asm goto, could you also pretty please make it
better than GCC and support output constraints in the same asm
statements?  This would give us a very clean, short, and fast
implementation for __get_user() and __put_user(), and it would also
help put pressure on GCC to implement the same feature.