From: Segher Boessenkool <segher@kernel.crashing.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Jakub Jelinek <jakub@redhat.com>,
Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>,
"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 17:03:47 -0500 [thread overview]
Message-ID: <20190906220347.GD9749@gate.crashing.org> (raw)
In-Reply-To: <CAKwvOd=MT_=U250tR+t0jTtj7SxKJjnEZ1FmR3ir_PHjcXFLVw@mail.gmail.com>
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.
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).
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.
If you want to know if you can do X in some environment, just try to do X.
Segher
next prev parent reply other threads:[~2019-09-06 22:04 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 [this message]
2019-09-06 22:35 ` Nick Desaulniers
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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