From: LIU Hao <lh_mouse@126.com>
Date: Tue, 29 Apr 2025 02:43:06 +0000 (+0800)
Subject: i386/cygming: Decrease default preferred stack boundary for 32-bit targets
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=ee7c0a5b70dc316477f45abc0f09dd2af9abe5cb;p=thirdparty%2Fgcc.git

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.

For compatibility reasons, when the attribute `force_align_arg_pointer` is
attached to a function, it continues to ensure the stack is at least aligned
to a 16-byte boundary, as the documentation seems to suggest.

After this change, `STACK_REALIGN_DEFAULT` no longer has an effect on this
target, so it is removed.

Reference: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111107#c9
Signed-off-by: LIU Hao <lh_mouse@126.com>
Signed-off-by: Jonathan Yong <10walls@gmail.com>

gcc/ChangeLog:

	PR target/111107
	* config/i386/cygming.h (PREFERRED_STACK_BOUNDARY_DEFAULT): Override
	definition from i386.h.
	(STACK_REALIGN_DEFAULT): Undefine, as it no longer has an effect.
	* config/i386/i386.cc (ix86_update_stack_boundary): Force minimum
	128-bit alignment if `force_align_arg_pointer`.
---

diff --git a/gcc/config/i386/cygming.h b/gcc/config/i386/cygming.h
index d587d25a58a..743cc38f585 100644
--- a/gcc/config/i386/cygming.h
+++ b/gcc/config/i386/cygming.h
@@ -28,16 +28,15 @@ 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
 #define MAX_STACK_ALIGNMENT  (TARGET_SEH ? 128 : MAX_OFILE_ALIGNMENT)
 
-/* 32-bit Windows aligns the stack on a 4-byte boundary but SSE instructions
-   may require 16-byte alignment.  */
-#undef STACK_REALIGN_DEFAULT
-#define STACK_REALIGN_DEFAULT (TARGET_64BIT ? 0 : 1)
-
 /* Support hooks for SEH.  */
 #undef  TARGET_ASM_UNWIND_EMIT
 #define TARGET_ASM_UNWIND_EMIT  i386_pe_seh_unwind_emit
diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index fd36ea802c0..9c24a926a89 100644
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -7942,6 +7942,15 @@ ix86_update_stack_boundary (void)
   if (ix86_tls_descriptor_calls_expanded_in_cfun
       && crtl->preferred_stack_boundary < 128)
     crtl->preferred_stack_boundary = 128;
+
+  /* For 32-bit MS ABI, both the incoming and preferred stack boundaries
+     are 32 bits, but if force_align_arg_pointer is specified, it should
+     prefer 128 bits for a backward-compatibility reason, which is also
+     what the doc suggests.  */
+  if (lookup_attribute ("force_align_arg_pointer",
+			TYPE_ATTRIBUTES (TREE_TYPE (current_function_decl)))
+      && crtl->preferred_stack_boundary < 128)
+    crtl->preferred_stack_boundary = 128;
 }
 
 /* Handle the TARGET_GET_DRAP_RTX hook.  Return NULL if no DRAP is