Documentation of `force_align_arg_pointer` attribute seems to say that it realigns the stack to 16-byte boundaries, so it now has a side effect to increase the preferred boundary to at least 16 bytes (128 bits).
I have booststrapped GCC 15.1 with this patch on i686-w64-mingw32, and verified that ESP is properly aligned with all these configurations:
------------------------------------------------------------------
-O? local variable force_align_arg_pointer runtime
alignment realign to
------------------------------------------------------------------
0 4 no 4 (no-op)
0 8 no 8
0 16 no 16
0 32 no 32
0 4 yes 16
0 8 yes 16
0 16 yes 16
0 32 yes 32
2 4 no 4 (no-op)
2 8 no 8
2 16 no 16
2 32 no 32
2 4 yes 16
2 8 yes 16
2 16 yes 16
2 32 yes 32
s 4 no 4 (no-op)
s 8 no 8
s 16 no 16
s 32 no 32
s 4 yes 16
s 8 yes 16
s 16 yes 16
s 32 yes 32
------------------------------------------------------------------
This patch may affect other 32-bit targets where the stack is not always aligned to 16 bytes, but I don't
have any system with such a configuration, so can't test that for now.
-- Best regards, LIU Hao From 1c101f4903a9be7d56efa8d97be603284f6bd4d4 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. 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. 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. * config/i386/i386.cc (ix86_compute_frame_layout): Force minimum 128-bit alignment if `force_align_arg_pointer`. Signed-off-by: LIU Hao <lh_mo...@126.com> --- gcc/config/i386/cygming.h | 4 ++++ gcc/config/i386/i386.cc | 9 +++++++++ 2 files changed, 13 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 diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc index 38df84f7db24..61e2acc53b7d 100644 --- a/gcc/config/i386/i386.cc +++ b/gcc/config/i386/i386.cc @@ -7920,6 +7920,15 @@ 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 -- 2.49.0
From 1c101f4903a9be7d56efa8d97be603284f6bd4d4 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. 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. 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. * config/i386/i386.cc (ix86_compute_frame_layout): Force minimum 128-bit alignment if `force_align_arg_pointer`. Signed-off-by: LIU Hao <lh_mo...@126.com> --- gcc/config/i386/cygming.h | 4 ++++ gcc/config/i386/i386.cc | 9 +++++++++ 2 files changed, 13 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 diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc index 38df84f7db24..61e2acc53b7d 100644 --- a/gcc/config/i386/i386.cc +++ b/gcc/config/i386/i386.cc @@ -7920,6 +7920,15 @@ 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 -- 2.49.0
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