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

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Date:   Tue, 15 May 2018 14:53:52 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     linux-kernel@...r.kernel.org, Christopher Li <sparse@...isli.org>,
        linux-sparse@...r.kernel.org
Subject: Re: [RFC 1/8] x86: objtool: use asm macro for better compiler
 decisions

Josh Triplett <josh@...htriplett.org> wrote:

> 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?

I use my custom config and to include the drivers in the image. Other
configs were not too appropriate: defconfig does not enable paravirt and
allyesconfig is too heavy.

There is no effect with allnoconfig on size:

   text	   data	    bss	    dec	    hex	filename
1018441	 230148	1215604	2464193	 2599c1	./vmlinux
1018441	 230148	1215604	2464193	 2599c1	./vmlinux

I think it is expected since CONFIG_STACK_VALIDATION is off. No?

Thanks,
Nadav