linux-kernel - Re: [Announce] Linux-tiny project revival

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Date:	Fri, 28 Sep 2007 14:36:03 -0000
From:	dick.streefland@...ium.nl (Dick Streefland)
To:	linux-kernel@...r.kernel.org
Subject: Re: [Announce] Linux-tiny project revival

Rob Landley <rob@...dley.net> wrote:
| The "change every printk in the kernel" suggestion came from me trying to 
| figure out how to get the printk() calls below a certain log level to 
| optimize out and not take up space in the binary.
| 
| The above doesn't address the original cause of the thread, as far as I can 
| tell.

It does, provided you change the macro definition to:

#define _printk(level, str, ...) \
   do { \
     if (sizeof(level) == 1) /* continued printk */\
      actual_printk(str, __VA_ARGS__); \
     else if ((level[1] - '0') < CONFIG_PRINTK_DOICARE) \
       actual_printk(level str, __VA_ARGS__); \
   } while(0);
#define printk(level_str, ...)	_printk(level_str, __VA_ARGS__)

Gcc will do constant folding on the string subscripts, and remove the
code and the string constants for calls above the desired level.

This is basically the same as I proposed in my earlier message [*],
but with the disadvantage that you need to modify all printk() calls
without an explicit level and add the ugly comma to the KERN_* macros.
Just compile the code in my message with -O and -S and you will see
that the KERN_NOTICE call and the corresponding string literal are
optimized away.

[*] http://lkml.org/lkml/2007/9/21/151

-- 
Dick Streefland                      ////                      Altium BV
dick.streefland@...ium.nl           (@ @)          http://www.altium.com
--------------------------------oOO--(_)--OOo---------------------------

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