linux-kernel - Re: [PATCH] perf tools: Kill off -Wcast-align

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Message-Id: <20091026.154055.07992945.davem@davemloft.net>
Date:	Mon, 26 Oct 2009 15:40:55 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	lethal@...ux-sh.org
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf tools: Kill off -Wcast-align

From: Paul Mundt <lethal@...ux-sh.org>
Date: Mon, 26 Oct 2009 18:47:30 +0900

> On Mon, Oct 26, 2009 at 07:25:20AM +0100, Ingo Molnar wrote:
>> 
>> * Paul Mundt <lethal@...ux-sh.org> wrote:
>> 
>> > The present use of -Wcast-align causes the build to blow up on SH due to
>> > generating a "cast increases required alignment of target type" error on
>> > each invocation of list_for_each_entry().
>> > 
>> > It seems that this was previously reported and killed off in the ia64
>> > support patch, but nothing seems to have happened with that. Presumably
>> > the same problem still remains there, too.
>> > 
>> > Signed-off-by: Paul Mundt <lethal@...ux-sh.org>
>> 
>> Is this a GCC bug producing false positive warnings? The GCC manpage 
>> says:
>> 
>>        -Wcast-align
>>            Warn whenever a pointer is cast such that the required alignment of the
>>            target is increased.  For example, warn if a "char *" is cast to an
>>            "int *" on machines where integers can only be accessed at two- or
>>            four-byte boundaries.
>> 
>> Which looks moderately useful - if it works.
>> 
> Well, both ia64 and sh have hit this in the current compilers, and it
> doesn't seem to pose any code generation issues. In the areas where it is
> generated it seems to relate to 64-bit data types in the data structures,
> which in itself doesn't seem inherently problematic.

sparc64 hits this too when building 32-bit perf binary, the first
thing I do after a pull is remove this warning option from the
Makefile :-)
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