Re: [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_mo...@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_mo...@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)

+#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

+#undef PREFERRED_STACK_BOUNDARY_DEFAULT

--
Best regards,
LIU Hao

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