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

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Message-ID: <8E8FD6A0-DA57-4523-A16F-C68167C2B5ED@vmware.com>
Date:   Tue, 1 May 2018 16:46:25 +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

Nadav Amit <namit@...are.com> wrote:

> 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?

My bad. It’s not the new-line. Let me do some more digging.

Nadav