linux-kernel - RE: [PATCH 0/6] Macrofying inline assembly for better compilation

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Date:   Fri, 18 May 2018 09:20:40 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Nadav Amit' <namit@...are.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "nadav.amit@...il.com" <nadav.amit@...il.com>,
        Alok Kataria <akataria@...are.com>,
        Christopher Li <sparse@...isli.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jan Beulich <JBeulich@...e.com>,
        Jonathan Corbet <corbet@....net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        "linux-sparse@...r.kernel.org" <linux-sparse@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Randy Dunlap" <rdunlap@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>
Subject: RE: [PATCH 0/6] Macrofying inline assembly for better compilation

From: Nadav Amit
> Sent: 17 May 2018 17:14
> This patch-set deals with an interesting yet stupid problem: kernel code
> that does not get inlined despite its simplicity. There are several
> causes for this behavior: "cold" attribute on __init, different function
> optimization levels; conditional constant computations based on
> __builtin_constant_p(); and finally large inline assembly blocks.
> 
> This patch-set deals with the inline assembly problem. I separated these
> patches from the others (that were sent in the RFC) for easier
> inclusion.
> 
> The problem with inline assembly is that inline assembly is often used
> by the kernel for things that are other than code - for example,
> assembly directives and data. GCC however is oblivious to the content of
> the blocks and assumes their cost in space and time is proportional to
> the number of the perceived assembly "instruction", according to the
> number of newlines and semicolons. Alternatives, paravirt and other
> mechanisms are affected, causing code not to be inlined, and degrading
> compilation quality in general.
> 
> The solution that this patch-set carries for this problem is to create
> an assembly macro, and then call it from the inline assembly block.  As
> a result, the compiler sees a single "instruction" and assigns the more
> appropriate cost to the code. In addition, this patch-set removes
> unneeded new-lines from common x86 inline asm's, which "confuse" GCC
> heuristics.

Can't you get the same effect by using always_inline ?

	David