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:

quoted

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?