[Bug c/108849] New: __declspec(code_seg("segname")) does not work

[Bug c/108849] New: __declspec(code_seg("segname")) does not work

[pali at kernel dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108849

            Bug ID: 108849
           Summary: __declspec(code_seg("segname")) does not work
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pali at kernel dot org
  Target Milestone: ---

Originally reported on: https://sourceware.org/bugzilla/show_bug.cgi?id=30005

GCC/LD does not support __declspec(code_seg("segname")) declarator for
specifying name of PE/COFF segment name.

Instead GCC/LD supports different and custom syntax
__declspec(section("segname")) incompatible with other compilers, like MSVC.

Please add support for de-facto standard "code_seg" declarator into the PE/COFF
__declspec keyword and not custom declarator. It does not bring any value, just
make code incompatible with gcc.

Test case on Debian sid:

$ x86_64-w64-mingw32-ld -v
GNU ld (GNU Binutils) 2.39.90.20230110
$
$ x86_64-w64-mingw32-gcc -v
Using built-in specs.
COLLECT_GCC=x86_64-w64-mingw32-gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-w64-mingw32/12-win32/lto-wrapper
Target: x86_64-w64-mingw32
Configured with: ../../src/configure --build=x86_64-linux-gnu --prefix=/usr
--includedir='/usr/include' --mandir='/usr/share/man'
--infodir='/usr/share/info' --sysconfdir=/etc --localstatedir=/var
--disable-option-checking --disable-silent-rules
--libdir='/usr/lib/x86_64-linux-gnu' --libexecdir='/usr/lib/x86_64-linux-gnu'
--disable-maintainer-mode --disable-dependency-tracking --prefix=/usr
--enable-shared --enable-static --disable-multilib --with-system-zlib
--libexecdir=/usr/lib --without-included-gettext --libdir=/usr/lib
--enable-libstdcxx-time=yes --with-tune=generic --with-headers
--enable-version-specific-runtime-libs --enable-fully-dynamic-string
--enable-libgomp --enable-languages=c,c++,fortran,objc,obj-c++,ada --enable-lto
--enable-threads=win32 --program-suffix=-win32
--program-prefix=x86_64-w64-mingw32- --target=x86_64-w64-mingw32
--with-as=/usr/bin/x86_64-w64-mingw32-as
--with-ld=/usr/bin/x86_64-w64-mingw32-ld --enable-libatomic
--enable-libstdcxx-filesystem-ts=yes --enable-dependency-tracking SED=/bin/sed
Thread model: win32
Supported LTO compression algorithms: zlib
gcc version 12-win32 (GCC)
$
$ cat test-code-seg.c
__declspec(code_seg("segname"))
int test(void) { return 0; }
$
$ x86_64-w64-mingw32-gcc -c test-code-seg.c -o test-code-seg.o
test-code-seg.c:2:1: warning: 'code_seg' attribute directive ignored
[-Wattributes]
    2 | int test(void) { return 0; }
      | ^~~
$
$ x86_64-w64-mingw32-objdump -h test-code-seg.o | grep segname
$

[Bug target/108849] __declspec(code_seg("segname")) does not work

[arsen at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108849

Arsen Arsenović <arsen at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |arsen at gcc dot gnu.org

--- Comment #1 from Arsen Arsenović <arsen at gcc dot gnu.org> ---
I think that declspec is currently simply an alias to attribute, IIUC (though,
I've not used or written code for w32 in many years):

  /* Support the __declspec keyword by turning them into attributes.
     Note that the current way we do this may result in a collision
     with predefined attributes later on.  This can be solved by using
     one attribute, say __declspec__, and passing args to it.  The
     problem with that approach is that args are not accumulated: each
     new appearance would clobber any existing args.  */
  if (TARGET_DECLSPEC)
    builtin_define ("__declspec(x)=__attribute__((x))");

GCC currently does not assign code_seg to any attribute, so perhaps it could be
aliased on TARGET_DECLSPEC platforms to `section'?  I'm not sure if that is
correct, I've never looked at the PE spec either.

[Bug target/108849] __declspec(code_seg("segname")) does not work

[pali at kernel dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108849

--- Comment #2 from Pali Rohár <pali at kernel dot org> ---
`section` is the best option. MS says about it:

https://learn.microsoft.com/en-us/cpp/cpp/code-seg-declspec

> The code_seg declaration attribute names an executable text segment in the .obj file in which the object code for the function or class member functions is stored.

> A segment is a named block of data in an .obj file that is loaded into memory as a unit. A text segment is a segment that contains executable code. The term section is often used interchangeably with segment.

> By default, when no code_seg is specified, object code is put in a segment named .text.

[Bug target/108849] __declspec(code_seg("segname")) does not work

[pali at kernel dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108849

--- Comment #3 from Pali Rohár <pali at kernel dot org> ---
Arsen, so based on my understooding (please correct me if I'm wrong), gcc's
"section" can be used on both code (functions) and data (global variables). And
ms's "code_seg" can be used only on code (functions).

So if gcc adds __declspec(code_seg("segname")) as alias to
__declspec(section("segname")) for TARGET_DECLSPEC then it should be OK for
valid source code. However it does not throws an compile error if
__declspec(code_seg("segname")) is specified on data. But I think it is
acceptable. Primary motivation is support for compiling valid source code.

Are you able to add this alias?