linux-kernel - Re: [RFC 1/8] x86: objtool: use asm macro for better compiler decisions

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Message-ID: <20180515213713.GA7298@localhost>
Date:   Tue, 15 May 2018 14:37:13 -0700
From:   Josh Triplett <josh@...htriplett.org>
To:     Nadav Amit <namit@...are.com>
Cc:     linux-kernel@...r.kernel.org, nadav.amit@...il.com,
        Christopher Li <sparse@...isli.org>,
        linux-sparse@...r.kernel.org
Subject: Re: [RFC 1/8] x86: objtool: use asm macro for better compiler
 decisions

On Tue, May 15, 2018 at 07:11:08AM -0700, Nadav Amit wrote:
> GCC considers the number of statements in inlined assembly blocks,
> according to new-lines and semicolons, as an indication to the cost of
> the block in time and space. This data is distorted by the kernel code,
> which puts information in alternative sections. As a result, the
> compiler may perform incorrect inlining and branch optimizations.
> 
> In the case of objtool, this distortion is extreme, since anyhow the
> annotations of objtool are discarded during linkage.
> 
> The solution is to set an assembly macro and call it from the inlinedv
> assembly block. As a result GCC considers the inline assembly block as
> a single instruction.
> 
> This patch slightly increases the kernel size.
> 
>    text    data     bss     dec     hex filename
> 18126699 10066728 2936832 31130259 1db0293 ./vmlinux before
> 18126824 10067268 2936832 31130924 1db052c ./vmlinux after (+665)

With what kernel config? What's the text size change for an allnoconfig?