linux-kernel - Re: [Q] ld: Does LTO reorder ro variables in two files?

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Message-ID: <15d40d09-3258-9b78-7120-bc708a429855@virtuozzo.com>
Date:   Thu, 19 Dec 2019 18:30:56 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     law@...hat.com, Peter Zijlstra <peterz@...radead.org>,
        gcc-help@....gnu.org
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        linux-kernel@...r.kernel.org
Subject: Re: [Q] ld: Does LTO reorder ro variables in two files?

On 19.12.2019 18:21, Jeff Law wrote:
> On Thu, 2019-12-19 at 17:04 +0300, Kirill Tkhai wrote:
>> CC: gcc-help@....gnu.org
>>
>> Hi, gcc guys,
>>
>> this thread starts here: https://lkml.org/lkml/2019/12/19/403
>>
>> There are two const variables:
>>
>>    struct sched_class idle_sched_class
>> and
>>    struct sched_class fair_sched_class,
>>
>> which are declared in two files idle.c and fair.c.
>>
>> 1)In Makefile the order is: idle.o fair.o
>> 2)the variables go to the same ro section
>> 3)there is no SORT(.*) keyword in linker script.
>>
>> Is it always true, that after linkage &idle_sched_class < &fair_sched_class?
> I certainly wouldn't depend on it.   The first and most obvious problem
> is symbol sorting by the linker.  Longer term I'd be worried about LTO
> reordering things.
> 
> In the end I'm pretty sure it'd be well outside what I'd be comfortable
> depending on.

Ok, I'd be comfortable too :) Thanks for the clarification, Jeff.