linux-kernel - RE: PROPOSAL: Extend inline asm syntax with size spec

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Date:   Mon, 8 Oct 2018 16:24:12 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Michael Matz' <matz@...e.de>,
        Segher Boessenkool <segher@...nel.crashing.org>
CC:     Borislav Petkov <bp@...en8.de>,
        "gcc@....gnu.org" <gcc@....gnu.org>,
        Richard Biener <rguenther@...e.de>,
        Nadav Amit <namit@...are.com>,
        "Ingo Molnar" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Masahiro Yamada" <yamada.masahiro@...ionext.com>,
        Sam Ravnborg <sam@...nborg.org>,
        "Alok Kataria" <akataria@...are.com>,
        Christopher Li <sparse@...isli.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "H. Peter Anvin" <hpa@...or.com>, Jan Beulich <JBeulich@...e.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        "linux-sparse@...r.kernel.org" <linux-sparse@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Chris Zankel <chris@...kel.net>,
        "Max Filippov" <jcmvbkbc@...il.com>,
        "linux-xtensa@...ux-xtensa.org" <linux-xtensa@...ux-xtensa.org>
Subject: RE: PROPOSAL: Extend inline asm syntax with size spec

From: Michael Matz
> Sent: 07 October 2018 16:53
...
> I think the examples I saw from Boris were all indirect inlines:
> 
>   static inline void foo() { asm("large-looking-but-small-asm"); }
>   static void bar1() { ... foo() ... }
>   static void bar2() { ... foo() ... }
>   void goo (void) { bar1(); }  // bar1 should have been inlined
> 
> So, while the immediate asm user was marked as always inline that in turn
> caused users of it to become non-inlined.  I'm assuming the kernel guys
> did proper measurements that they _really_ get some non-trivial speed
> benefit by inlining bar1/bar2, but for some reasons (I didn't inquire)
> didn't want to mark them all as inline as well.

Could you add a 'size' attribute to the 'always inlined' foo() above
rather than trying to add one to the asm() statement itself.
Then add a warning in the documentation that small size attributes
might make the assembly fail due to limited branch offsets (etc).

Size '1' probably ought to be reserved for things that definitely
fit in a delay slot.

	David

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