linux-kernel - Re: clang asm-goto support (Was Re: [PATCH v2] x86/retpoline: Add clang support)

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Message-ID: <CA+55aFy+OxjvzxE3AXDwOwWJZLRAAR32OSsT9f5qy4r8qA+KCw@mail.gmail.com>
Date:   Wed, 14 Feb 2018 09:43:25 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     James Y Knight <jyknight@...gle.com>
Cc:     "Yatsina, Marina" <marina.yatsina@...el.com>,
        Kees Cook <keescook@...gle.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Chandler Carruth <chandlerc@...gle.com>,
        "Kreitzer, David L" <david.l.kreitzer@...el.com>,
        "Grischenko, Andrei L" <andrei.l.grischenko@...el.com>,
        "rnk@...gle.com" <rnk@...gle.com>,
        LLVM Developers <llvm-dev@...ts.llvm.org>,
        "ehsan@...illa.com" <ehsan@...illa.com>,
        "Tayree, Coby" <coby.tayree@...el.com>,
        Matthias Braun <matze@...unis.de>,
        Dean Michael Berris <dean.berris@...il.com>,
        Guenter Roeck <linux@...ck-us.net>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Rik van Riel <riel@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Kosina <jikos@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Paul Turner <pjt@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: clang asm-goto support (Was Re: [PATCH v2] x86/retpoline: Add
 clang support)

On Wed, Feb 14, 2018 at 7:31 AM, James Y Knight <jyknight@...gle.com> wrote:
>
> IMO, inline asm is, generally, a valuable feature to provide in the
> compiler as an escape hatch, and asm goto is a relatively sane
> extension of it.

Side note: one thing that limits "asm goto" in gcc is the fact that
you can't have outputs.

If clang people are looking at "asm goto", the _syntax_ is actually
very straightforward, and there's really absolutely no point in trying
to make up some other interface.

But extending on what gcc does, and allowing outputs (possibly valid
in the fall-through case only, not in the cases where it jumps away to
a label) would be a big improvement on what gcc does.

At that point, we'd start to pester the gcc people to do the clang
thing, rather than the other way around. Because that's only fair ;)

                  Linus