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SubjectRe: clang asm-goto support (Was Re: [PATCH v2] x86/retpoline: Add clang support)
On Wed, Feb 14, 2018 at 3:31 PM, James Y Knight <> 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.

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