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

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Date:   Thu, 19 Dec 2019 19:08:50 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Alexander Monakov <amonakov@...ras.ru>
Cc:     Peter Zijlstra <peterz@...radead.org>, gcc-help@....gnu.org,
        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, Jan Hubicka <hubicka@....cz>
Subject: Re: [Q] ld: Does LTO reorder ro variables in two files?

On 19.12.2019 18:45, Alexander Monakov wrote:
> [adding Jan Hubicka, GCC LTO maintainer]
> 
> On Thu, 19 Dec 2019, 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?
> 
> No, with LTO you don't have that guarantee. For functions it's more obvious,
> GCC wants to analyze functions in reverse topological order so callees are
> generally optimized before callers, and it will emit assembly as it goes, so
> function ordering with LTO does not give much care to translation unit
> boundaries. For variables it's a bit more subtle, GCC partitions all variables
> and functions so it can hand them off to multiple compiler processes while doing
> LTO. There's no guarantees about order of variables that end up in different
> partitions.
> 
> There's __attribute__((no_reorder)) that is intended to enforce ordering even
> with LTO (it's documented under "Common function attributes" but works for
> global variables as well).

Thanks, Alexander!

Kirill