Re: [RFC] LKMM: Add volatile_if()

Peter Zijlstra
2021-06-07T10:52:26Z

On Sun, Jun 06, 2021 at 03:26:16PM -0500, Segher Boessenkool wrote:
> On Sun, Jun 06, 2021 at 01:11:53PM -0700, Linus Torvalds wrote:

> > We are very used to just making the compiler generate the code we
> > need. That is, fundamentally, what any use of inline asm is all about.
> > We want the compiler to generate all the common cases and all the
> > regular instructions.
> >
> > The conditional branch itself - and the instructions leading up to it
> > - are exactly those "common regular instructions" that we'd want the
> > compiler to generate. That is in fact more true here than for most
> > inline asm, exactly because there are so many different possible
> > combinations of conditional branches (equal, not equal, less than,..)
> > and so many ways to generate the code that generates the condition.
> >
> > So we are much better off letting the compiler do all that for us -
> > it's very much what the compiler is good at.
>
> Yes, exactly.
>
> I am saying that if you depend on that some C code you write will result
> in some particular machine code, without actually *forcing* the compiler
> to output that exact machine code, then you will be disappointed. Maybe
> not today, and maybe it will take years, if you are lucky.
>
> (s/forcing/instructing/ of course, compilers have feelings too!)

And hence the request for a language extension. Both compilers have a
vast array of language extensions that are outside of the C spec (thank
you!), so can we please get one more?