linux-kernel - Re: [10 PATCHES] inline functions to avoid stack overflow

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Date:	Thu, 26 Jun 2008 08:32:35 +0200
From:	"Bart Van Assche" <bart.vanassche@...il.com>
To:	"David Miller" <davem@...emloft.net>
Cc:	mpatocka@...hat.com, helge.hafting@...el.hist.no,
	sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
	gcc@....gnu.org
Subject: Re: [10 PATCHES] inline functions to avoid stack overflow

On Thu, Jun 26, 2008 at 12:09 AM, David Miller <davem@...emloft.net> wrote:
> From: Mikulas Patocka <mpatocka@...hat.com>
> Date: Wed, 25 Jun 2008 08:53:10 -0400 (EDT)
>
>> Even worse, gcc doesn't use these additional bytes. If you try this:
>>
>> extern void f(int *i);
>> void g()
>> {
>>          int a;
>>          f(&a);
>> }
>>
>> , it allocates additional 16 bytes for the variable "a" (so there's total
>> 208 bytes), even though it could place the variable into 48-byte
>> ABI-mandated area that it inherited from the caller or into it's own
>> 16-byte padding that it made when calling "f".
>
> The extra 16 bytes of space allocated is so that GCC can perform a
> secondary reload of a quad floating point value.  It always has to be
> present, because we can't satisfy a secondary reload by emitting yet
> another reload, it's the end of the possible level of recursions
> allowed by the reload pass.

Is there any floating-point code present in the Linux kernel ? Would
it be a good idea to add an option to gcc that tells gcc that the
compiled code does not contain floating-point instructions, such that
gcc knows that no space has to be provided for a quad floating point
value ?

Bart.
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