linux-kernel - Re: [PATCH] WARN_ONCE(): use bool for boolean flag

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Message-ID: <4ABFA583.6030908@cesarb.net>
Date:	Sun, 27 Sep 2009 14:48:51 -0300
From:	Cesar Eduardo Barros <cesarb@...arb.net>
To:	Daniel Walker <dwalker@...o99.com>
CC:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Roland Dreier <rolandd@...co.com>
Subject: Re: [PATCH] WARN_ONCE(): use bool for boolean flag

Daniel Walker escreveu:
> On Sun, 2009-09-27 at 14:24 -0300, Cesar Eduardo Barros wrote:
> 
>> I took a quick look, and all uses seem to be directly in a boolean 
>> context (within an if()), so there would be no problem. Besides, the 
>> unlikely() all these macros end with does a double negation, meaning 
>> even if it is an int, it will be either 0 or 1 (but I am not sure I am 
>> reading these macros right - it seems CONFIG_TRACE_BRANCH_PROFILING 
>> turns all unlikely() into likely()).
>>
>> In fact, I was expecting no change at all, since gcc should be able to 
>> see it is being treated as a boolean (perhaps I am trusting gcc too 
>> much). And to make matters even more confusing, my own test changing all 
>> __ret_warn_once to bool and dropping the !! caused an _increase_ of 598 
>> bytes (x86-64 defconfig).
>>
>>     text    data     bss     dec     hex filename
>> 8100553 1207148  991988 10299689         9d2929 vmlinux.warnret.before
>> 8101119 1207180  991988 10300287         9d2b7f vmlinux.warnret.after
>>
>> (And yes, data increased again.)
> 
> Did you have the CONFIG_TRACE_BRANCH_PROFILING option enabled for the
> test above?

CONFIG_BRANCH_PROFILE_NONE=y

CONFIG_TRACE_BRANCH_PROFILING does not even appear in the .config.

> If this was just your regular base line config , then that is odd .. I
> also would think worse case would be no size reduction .. I did my
> compile test on x86-32 btw..

Yes, it is very odd. And I tried compiling a small test module to see if 
I could see the changes in the assembly output:

#include <linux/module.h>
#include <linux/kernel.h>

void test(int value)
{
         WARN_ON_ONCE(value);
}
EXPORT_SYMBOL_GPL(test);

But the assembly output is identical.

I will try looking at the first function which shows a difference in 
size (which appears to be handle_irq) and see what I can find.

-- 
Cesar Eduardo Barros
cesarb@...arb.net
cesar.barros@...il.com
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