Re: [PATCH v2 4/6] compiler-gcc.h: add asm_inline definition

[Nick Desaulniers]

On Fri, Sep 6, 2019 at 3:03 PM Segher Boessenkool
<segher@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, Sep 06, 2019 at 11:14:08AM -0700, Nick Desaulniers wrote:
> > Here's the case that I think is perfect:
> > https://developers.redhat.com/blog/2016/02/25/new-asm-flags-feature-for-x86-in-gcc-6/
> >
> > Specifically the feature test preprocessor define __GCC_ASM_FLAG_OUTPUTS__.
> >
> > See exactly how we handle it in the kernel:
> > - https://github.com/ClangBuiltLinux/linux/blob/0445971000375859008414f87e7c72fa0d809cf8/arch/x86/include/asm/asm.h#L112-L118
> > - https://github.com/ClangBuiltLinux/linux/blob/0445971000375859008414f87e7c72fa0d809cf8/arch/x86/include/asm/rmwcc.h#L14-L30
> >
> > Feature detection of the feature makes it trivial to detect when the
> > feature is supported, rather than brittle compiler version checks.
> > Had it been a GCC version check, it wouldn't work for clang out of the
> > box when clang added support for __GCC_ASM_FLAG_OUTPUTS__.  But since
> > we had the helpful __GCC_ASM_FLAG_OUTPUTS__, and wisely based our use
> > of the feature on that preprocessor define, the code ***just worked***
> > for compilers that didn't support the feature ***and*** compilers when
> > they did support the feature ***without changing any of the source
> > code*** being compiled.
>
> And if instead you tested whether the actual feature you need works as
> you need it to, it would even work fine if there was a bug we fixed that
> breaks things for the kernel.  Without needing a new compiler.

That assumes a feature is broken out of the gate and is putting the
cart before the horse.  If a feature is available, it should work.  If
you later find it to be unsatisfactory, sure go out of your way to add
ugly compiler-specific version checks or upgrade your minimally
supported toolchain; until then feature detection is much cleaner (see
again __GCC_ASM_FLAG_OUTPUTS__).

>
> Or as another example, if we added support for some other flags. (x86
> has only a few flags; many other archs have many more, and in some cases
> newer hardware actually has more flags than older).

I think compiler flags are orthogonal to GNU C extensions we're discussing here.

>
> With the "macro" scheme we would need to add new macros in all these
> cases.  And since those are target-specific macros, that quickly expands
> beyond reasonable bounds.

I don't think so.  Can you show me an example codebase that proves me wrong?

>
> If you want to know if you can do X in some environment, just try to do X.

That's a very autoconf centric viewpoint.  Why doesn't the kernel take
that approach for __GCC_ASM_FLAG_OUTPUTS__?  Why not repeatedly invoke
$CC to find if every compiler __attribute__ is supported?  Do you
think it's faster for the C preprocessor to check for a few #ifdefs,
or to repeatedly invoke $CC at build or compile time to detect new
features?
-- 
Thanks,
~Nick Desaulniers