[PATCH] i386/cygming: Decrease default preferred stack boundary for 32-bit targets

[LIU Hao]

在 2025-4-29 13:03, LIU Hao 写道:
> This fixes a long-standing issue that GCC used to assume 16-byte stack alignment on i686-w64-mingw32, 
> which is not always the case for callbacks from system libraries.
> 

CC Zeb Figura

This patch looks a bit risky. The overall effect of `__attribute__((__force_align_arg_pointer__))` seems 
to be that it realigns ESP to the _preferred_ alignment, which is changed by this patch.

If we change it to 4 bytes, then incoming alignment == preferred alignment == 4, then suddenly, 
`__attribute__((__force_align_arg_pointer__))` no longer has an effect, which may break code that it to 
fix stack alignment for SSE.

Maybe we shouldn't apply this patch. This leaves 8-byte alignment unresolved 
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111107#c17). But does it harm in practice? `double` on 
x86-32 requires only 4-byte alignment despite performance pitfalls. It might be an issue if an atomic 
int64 is stored on the stack and is accessed with CMPXCHG8B.



> 
> 9005-i386-cygming-Decrease-default-preferred-stack-bounda.patch
> 
>  From 1b92f8105dbece1694dd3ab398cfb5e3ce2c15d9 Mon Sep 17 00:00:00 2001
> From: LIU Hao<lh_mouse@126.com>
> Date: Tue, 29 Apr 2025 10:43:06 +0800
> Subject: [PATCH] i386/cygming: Decrease default preferred stack boundary for
>   32-bit targets
> 
> This commit decreases the default preferred stack boundary to 4.
> 
> In i386-options.cc, there's
> 
>     ix86_default_incoming_stack_boundary = PREFERRED_STACK_BOUNDARY;
> 
> which sets the default incoming stack boundary to this value, if it's not
> overridden by other options or attributes.
> 
> Previously, GCC preferred 16-byte alignment like other platforms, unless
> `-miamcu` was specified. However, the Microsoft x86 ABI only requires the
> stack be aligned to 4-byte boundaries. Callback functions from MSVC code may
> break this assumption by GCC (see reference below), causing local variables
> to be misaligned.
> 
> Reference:https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111107#c9
> Signed-off-by: LIU Hao<lh_mouse@126.com>
> 
> gcc/ChangeLog:
> 
> 	PR 111107
> 	* config/i386/cygming.h (PREFERRED_STACK_BOUNDARY_DEFAULT): Override
> 	definition from i386.h.
> ---
>   gcc/config/i386/cygming.h | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/gcc/config/i386/cygming.h b/gcc/config/i386/cygming.h
> index 3ddcbecb22fd..b8c396d35793 100644
> --- a/gcc/config/i386/cygming.h
> +++ b/gcc/config/i386/cygming.h
> @@ -28,6 +28,10 @@ along with GCC; see the file COPYING3.  If not see
>   #undef TARGET_SEH
>   #define TARGET_SEH  (TARGET_64BIT_MS_ABI && flag_unwind_tables)
>   
> +#undef PREFERRED_STACK_BOUNDARY_DEFAULT
> +#define PREFERRED_STACK_BOUNDARY_DEFAULT \
> +  (TARGET_64BIT ? 128 : MIN_STACK_BOUNDARY)
> +
>   /* Win64 with SEH cannot represent DRAP stack frames.  Disable its use.
>      Force the use of different mechanisms to allocate aligned local data.  */
>   #undef MAX_STACK_ALIGNMENT
> -- 2.49.0
> 


-- 
Best regards,
LIU Hao
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