linux-kernel - Re: [patch 1/3] move WARN

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Date:	Thu, 03 Jan 2008 17:22:30 +0100
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch 1/3] move WARN_ON() out of line

Ingo Molnar wrote:
> * Arjan van de Ven <arjan@...ux.intel.com> wrote:
> 
>> +#define WARN_ON(condition) do_warn_on((unsigned long)(condition), __FILE__, \
>> +					 __LINE__, __FUNCTION__)
> 
> hm. This passes in 4 arguments to do_warn_on().
> 
> i think we could get away with no arguments (!), by using section 
> tricks. Firstly, we can get rid of __FUNCTION__ and replace it with a 
> ksyms lookup - that is fine enough.

I can see that; I'll play with that

> Secondly, we could put __FILE__ and 
> __LINE__ into a text section and key it up to the return address from 
> do_warn_on().

the asm generated for this is 2 movl instructions for immediate to register.
Doing fancy tricks ... it may well end up bigger and gain nothing.
> 
> the condition code should not be passed in at all i think - it creates 
> extra function calls to do_warn_on() all the time.

function calls are *CHEAP*.

passing the condition is actually near free (remember we have regparm!), it's likely to be
in a register already anyway.

Doing the test inline makes stuff bigger, and also spreads the branch prediction pain around
rather than having one nicely predictable place...


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