linux-kernel - Re: [PATCH] [2/58] x86_64: Tell gcc to only align stack to 8 bytes

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Date:	Thu, 19 Jul 2007 15:50:20 +0400
From:	Serge Belyshev <belyshev@...ni.sinp.msu.ru>
To:	Andi Kleen <ak@...e.de>
Cc:	patches@...-64.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [2/58] x86_64: Tell gcc to only align stack to 8 bytes

Andi Kleen <ak@...e.de> writes:

> Don't need 16 byte alignment because kernel doesn't use SSE2
>
...
>  cflags-y += -maccumulate-outgoing-args
> +cflags-y += -mpreferred-stack-boundary=4
>  

>From gcc manpage:

       -mpreferred-stack-boundary=num
           Attempt to keep the stack boundary aligned to a 2 raised to num
           byte boundary.  If -mpreferred-stack-boundary is not specified, the
           default is 4 (16 bytes or 128 bits), except when optimizing for
           code size (-Os), in which case the default is the minimum correct
           alignment (4 bytes for x86, and 8 bytes for x86-64).

So -mpreferred-stack-boundary=4 is the default and to align stack
to 8 bytes you want -mpreferred-stack-boundary=3, not 4, IIUC.
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