Date: Mon, 30 Jun 2003 22:27:52 +0200
From: Adrian Bunk <bunk@fs.tum.de>
To: Andries Brouwer <aebr@win.tue.nl>
Cc: Margit Schubert-While <margitsw@t-online.de>, linux-kernel@vger.kernel.org
Subject: Re: 2.5.73 compile warnings
Message-ID: <20030630202752.GJ282@fs.tum.de>
References: <5.1.0.14.2.20030630170916.00afd930@pop.t-online.de> <20030630160319.GA15506@win.tue.nl>
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On Mon, Jun 30, 2003 at 06:03:19PM +0200, Andries Brouwer wrote:
> On Mon, Jun 30, 2003 at 05:13:05PM +0200, Margit Schubert-While wrote:
> > 2.5.73 + latest cset
> > GCC 3.3
> > 
> > drivers/char/vt_ioctl.c: In function `do_kdsk_ioctl':
> > drivers/char/vt_ioctl.c:85: warning: comparison is always false due to 
> > limited range of data type
> > drivers/char/vt_ioctl.c:85: warning: comparison is always false due to 
> > limited range of data type
> > drivers/char/vt_ioctl.c: In function `do_kdgkb_ioctl':
> > drivers/char/vt_ioctl.c:211: warning: comparison is always false due to 
> > limited range of data type
> > 
> > drivers/char/keyboard.c: In function `k_fn':
> > drivers/char/keyboard.c:665: warning: comparison is always true due to 
> > limited range of data type
> 
> These are checks of the "cannot happen" type, where "cannot happen"
> can be seen by the compiler, so that it can optimize the tests away.
> 
> As it is now, correctness of the code can be seen locally.
> If the tests are removed, a human reader must look up the values
> of these constants to conclude that the code is correct.

There are cases where a comparison is only on 32 bit architectures 
always true/false (if longs are involved).

There are cases where a #define is set that might be set differently
under certain circumstances:

<--  snip  -->

#define MAX_VALUE 10000

u8 tmp;

if (tmp > MAX_VALUE)
    return -ESOMEERROR;

<--  snip  -->


These are cases where the warning is just noise. But this new warning 
already found several bugs, there are several places where I sent 
patches for things like:

<--  snip  -->

u8 i;

for (i = 0; i <= 0xFF; i++)
    do_something;

<--  snip  -->


In these cases the warning found an actual bug.


Is there any way to tell gcc to disable such warnings locally?


cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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