RFC: -msse/-mavx change x86 ABI

RFC: -msse/-mavx change x86 ABI

[H.J. Lu]

Hi,

When passing a parameter with alignment >= 16byte on stack,
 -msse/-mavx may change its alignment on stack. See:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44948

Before gcc 4.4, we couldn't align the stack properly. The
maximum alignment we could generate for parameters on
stack we supported is the alignment of incoming stack. Now
we can properly align the stack.

How should we fix this bug? There are several different
approaches in the PR. Any comments?

Thanks.

-- 
H.J.

Re: RFC: -msse/-mavx change x86 ABI

[H.J. Lu]

On Mon, Jul 19, 2010 at 7:50 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> Hi,
>
> When passing a parameter with alignment >= 16byte on stack,
>  -msse/-mavx may change its alignment on stack. See:
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44948
>
> Before gcc 4.4, we couldn't align the stack properly. The
> maximum alignment we could generate for parameters on
> stack we supported is the alignment of incoming stack. Now
> we can properly align the stack.
>
> How should we fix this bug? There are several different
> approaches in the PR. Any comments?
>

CC Intel compiler people.  David, Milind, do you have any
comments?

Thanks.


-- 
H.J.

RE: RFC: -msse/-mavx change x86 ABI

[Kreitzer, David L]

Hi HJ,

In the Intel compiler, we guarantee that outgoing arguments are aligned properly except in the case of 8-byte alignment on IA-32.  We will dynamically align the stack and/or insert padding in the outgoing argument block as necessary to make this happen.

Note that this includes both the alignment requirements of the built-in types as specified by the ABI and alignment requirements specified by the programmer via __attribute__((aligned(N))) or __declspec(align(N)).

To summarize

1) For Intel64, always properly align outgoing parameter.
2) For IA-32, properly align parameter if alignment is >= 16 bytes.  Otherwise, align to 4 bytes.

Dave Kreitzer

-----Original Message-----
From: H.J. Lu [mailto:hjl.tools@gmail.com] 
Sent: Friday, August 20, 2010 10:53 AM
To: GCC Patches; Uros Bizjak; Jan Hubicka; Kreitzer, David L; Girkar, Milind
Subject: Re: RFC: -msse/-mavx change x86 ABI

On Mon, Jul 19, 2010 at 7:50 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> Hi,
>
> When passing a parameter with alignment >= 16byte on stack,
>  -msse/-mavx may change its alignment on stack. See:
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44948
>
> Before gcc 4.4, we couldn't align the stack properly. The
> maximum alignment we could generate for parameters on
> stack we supported is the alignment of incoming stack. Now
> we can properly align the stack.
>
> How should we fix this bug? There are several different
> approaches in the PR. Any comments?
>

CC Intel compiler people.  David, Milind, do you have any
comments?

Thanks.


-- 
H.J.