linux-kernel - Re: Suboptimal inline heuristics due to non-code sections

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Date:   Tue, 1 May 2018 16:39:09 +0000
From:   Nadav Amit <namit@...are.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Suboptimal inline heuristics due to non-code sections

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, May 1, 2018 at 6:40 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
>> But if I remove the section completely by removing the
>> pushsection/popsection, then copy_overflow() gets inlined.
> 
>> So GCC's inlining decisions are somehow influenced by the existence of
>> some random empty section.  This definitely seems like a GCC bug to me.
> 
> I think gcc uses the size of the string to approximate the size of an
> inline asm.
> 
> So I don't think it's the "empty section" that makes gcc do this, I think
> it's literally "our inline asms _look_ big”.

I didn’t think about that.

Playing with the code a bit more, it seems that it is actually related to
the number of “new-lines” in the inline assembly. Removing 4 new-lines from
_BUG_FLAGS (those that can be removed without breaking assembly) eliminated
most of the non-inlined versions of copy_overflow().

Would it be reasonable to remove new-lines in such cases?

Regards,
Nadav