https://gcc.gnu.org/g:ee7c0a5b70dc316477f45abc0f09dd2af9abe5cb
commit r16-511-gee7c0a5b70dc316477f45abc0f09dd2af9abe5cb
Author: LIU Hao <lh_mo...@126.com>
Date: Tue Apr 29 10:43:06 2025 +0800
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_mo...@126.com>
Signed-off-by: Jonathan Yong <10wa...@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:
---
gcc/config/i386/cygming.h | 9 ++++-----
gcc/config/i386/i386.cc | 9 +++++++++
2 files changed, 13 insertions(+), 5 deletions(-)
diff --git a/gcc/config/i386/cygming.h b/gcc/config/i386/cygming.h
index d587d25a58a8..743cc38f5852 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 fd36ea802c00..9c24a926a890 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
