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Date:   Fri, 6 Sep 2019 17:56:06 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Jakub Jelinek <jakub@redhat.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH v2 4/6] compiler-gcc.h: add asm_inline definition
Message-ID: <20190906225606.GF9749@gate.crashing.org>
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On Fri, Sep 06, 2019 at 03:35:02PM -0700, Nick Desaulniers wrote:
> On Fri, Sep 6, 2019 at 3:03 PM Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> > 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.

GCC currently has 91696 bug reports.

> > 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.

No, I am talking exactly about what you brought up.  The flags output
for inline assembler, using the =@ syntax.  If I had implemented that
for Power when it first came up, I would by now have had to add support
for another flag (the CA32 flag).  Oh, and I would not have implemented
support for OV or SO at all originally, but perhaps they are useful, so
let's add that as well.  And there is OV32 now as well.

> > 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?

No, of course not, because we aren't silly enough to implement something
like that.

> > 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__?

Ask them, not me.


Segher