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

Andi Kleen <[email protected]> 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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References:
- [PATCH] [0/58] First batch of x86 patches for .23
  - From: Andi Kleen <[email protected]>
- [PATCH] [2/58] x86_64: Tell gcc to only align stack to 8 bytes
  - From: Andi Kleen <[email protected]>

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