From: Nick Desaulniers <ndesaulniers@google.com>
To: Segher Boessenkool <segher@kernel.crashing.org>,
Jakub Jelinek <jakub@redhat.com>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: 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
Date: Fri, 6 Sep 2019 15:35:02 -0700 [thread overview]
Message-ID: <CAKwvOdnWBV35SCRHwMwXf+nrFc+D1E7BfRddb20zoyVJSdecCA@mail.gmail.com> (raw)
In-Reply-To: <20190906220347.GD9749@gate.crashing.org>
On Fri, Sep 6, 2019 at 3:03 PM Segher Boessenkool
<segher@kernel.crashing.org> 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
next prev parent reply other threads:[~2019-09-06 22:35 UTC|newest]
Thread overview: 66+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-29 8:32 [RFC PATCH 0/5] make use of gcc 9's "asm inline()" Rasmus Villemoes
2019-08-29 8:32 ` [RFC PATCH 1/5] treewide: replace __inline__ by inline Rasmus Villemoes
2019-08-29 16:29 ` Joe Perches
2019-08-29 8:32 ` [RFC PATCH 2/5] compiler_types.h: don't #define __inline__ Rasmus Villemoes
2019-08-29 8:32 ` [RFC PATCH 3/5] compiler-gcc.h: add asm_inline definition Rasmus Villemoes
2019-08-29 8:32 ` [RFC PATCH 4/5] x86: alternative.h: use asm_inline for all alternative variants Rasmus Villemoes
2019-08-29 8:32 ` [RFC PATCH 5/5] x86: bug.h: use asm_inline in _BUG_FLAGS definitions Rasmus Villemoes
2019-08-29 16:05 ` [RFC PATCH 0/5] make use of gcc 9's "asm inline()" Linus Torvalds
2019-08-30 7:45 ` Rasmus Villemoes
2019-08-29 17:36 ` Nick Desaulniers
2019-08-29 18:15 ` Linus Torvalds
2019-08-29 18:26 ` Nadav Amit
2019-08-29 18:42 ` Borislav Petkov
2019-08-29 19:41 ` Masahiro Yamada
2019-08-30 23:15 ` [PATCH v2 0/6] " Rasmus Villemoes
2019-08-30 23:15 ` [PATCH v2 1/6] staging: rtl8723bs: replace __inline by inline Rasmus Villemoes
2019-09-04 23:54 ` Nick Desaulniers
2019-08-30 23:15 ` [PATCH v2 2/6] lib/zstd/mem.h: " Rasmus Villemoes
2019-09-04 23:59 ` Nick Desaulniers
2019-09-05 0:07 ` Miguel Ojeda
2019-09-05 9:28 ` Rasmus Villemoes
2019-08-30 23:15 ` [PATCH v2 3/6] compiler_types.h: don't #define __inline Rasmus Villemoes
2019-09-05 0:13 ` Nick Desaulniers
2019-09-05 9:45 ` Rasmus Villemoes
2019-08-30 23:15 ` [PATCH v2 4/6] compiler-gcc.h: add asm_inline definition Rasmus Villemoes
2019-09-05 0:18 ` Nick Desaulniers
2019-09-05 5:43 ` Nadav Amit
2019-09-05 11:07 ` Rasmus Villemoes
2019-09-05 13:45 ` Segher Boessenkool
2019-09-05 14:23 ` Rasmus Villemoes
2019-09-05 14:47 ` Segher Boessenkool
2019-09-05 15:52 ` Miguel Ojeda
2019-09-05 16:13 ` Miguel Ojeda
2019-09-06 12:23 ` Segher Boessenkool
2019-09-06 15:13 ` Miguel Ojeda
2019-09-06 16:30 ` Segher Boessenkool
2019-09-06 16:39 ` Jakub Jelinek
2019-09-06 18:14 ` Nick Desaulniers
2019-09-06 22:03 ` Segher Boessenkool
2019-09-06 22:35 ` Nick Desaulniers [this message]
2019-09-06 22:56 ` Segher Boessenkool
2019-09-06 23:42 ` Nick Desaulniers
2019-09-07 0:14 ` Segher Boessenkool
2019-09-07 1:04 ` Nick Desaulniers
2019-09-07 13:11 ` Segher Boessenkool
2019-09-08 13:55 ` Miguel Ojeda
2019-09-12 21:54 ` Nick Desaulniers
2019-09-12 22:12 ` Rasmus Villemoes
2019-09-20 0:50 ` Segher Boessenkool
2019-09-06 16:47 ` Miguel Ojeda
2019-08-30 23:15 ` [PATCH v2 5/6] x86: alternative.h: use asm_inline for all alternative variants Rasmus Villemoes
2019-08-30 23:15 ` [PATCH v2 6/6] x86: bug.h: use asm_inline in _BUG_FLAGS definitions Rasmus Villemoes
2019-09-12 22:19 ` [PATCH v3 0/6] make use of gcc 9's "asm inline()" Rasmus Villemoes
2019-09-12 22:19 ` [PATCH v3 1/6] staging: rtl8723bs: replace __inline by inline Rasmus Villemoes
2019-09-29 10:40 ` Greg Kroah-Hartman
2019-09-12 22:19 ` [PATCH v3 2/6] lib/zstd/mem.h: " Rasmus Villemoes
2019-09-12 22:19 ` [PATCH v3 3/6] compiler_types.h: don't #define __inline Rasmus Villemoes
2019-09-12 22:19 ` [PATCH v3 4/6] compiler-types.h: add asm_inline definition Rasmus Villemoes
2019-09-12 22:19 ` [PATCH v3 5/6] x86: alternative.h: use asm_inline for all alternative variants Rasmus Villemoes
2019-09-13 5:41 ` Ingo Molnar
2019-09-12 22:19 ` [PATCH v3 6/6] x86: bug.h: use asm_inline in _BUG_FLAGS definitions Rasmus Villemoes
2019-09-13 5:42 ` Ingo Molnar
2019-09-12 22:30 ` [PATCH v3 0/6] make use of gcc 9's "asm inline()" Miguel Ojeda
2019-09-13 6:11 ` Rasmus Villemoes
2019-09-13 15:21 ` Greg Kroah-Hartman
2019-09-15 18:20 ` Miguel Ojeda
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