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Subject: Re: __builtin_thread_pointer
In-Reply-To: <6faedefc-eb17-4b02-89b9-44b73e82d43f@SystematicSW.ab.ca>
Date: Tue, 11 Feb 2025 21:58:17 +0100
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 <D47E777E-C732-47BD-81C8-5A488E38E1F3 AT unified-streaming DOT com>
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To: The Cygwin Mailing List <cygwin AT cygwin DOT com>
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From: Dimitry Andric via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Dimitry Andric <dimitry AT unified-streaming DOT com>
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It does not look like it's supported by Cygwin's gcc:

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-cygwin/12/lto-wrapper.exe
Target: x86_64-pc-cygwin
Configured with: /mnt/share/cygpkgs/gcc/gcc.x86_64/src/gcc-12.4.0/configure --srcdir=/mnt/share/cygpkgs/gcc/gcc.x86_64/src/gcc-12.4.0 --prefix=/usr --exec-prefix=/usr --localstatedir=/var --sysconfdir=/etc --docdir=/usr/share/doc/gcc --htmldir=/usr/share/doc/gcc/html -C --build=x86_64-pc-cygwin --host=x86_64-pc-cygwin --target=x86_64-pc-cygwin --without-libiconv-prefix --without-libintl-prefix --libexecdir=/usr/lib --with-gcc-major-version-only --enable-shared --enable-shared-libgcc --enable-static --enable-version-specific-runtime-libs --enable-bootstrap --enable-__cxa_atexit --enable-clocale=newlib --with-dwarf2 --with-tune=generic --enable-languages=ada,c,c++,fortran,lto,objc,obj-c++,jit --enable-graphite --enable-threads=posix --enable-libatomic --enable-libgomp --enable-libquadmath --enable-libquadmath-support --disable-libssp --enable-libada --disable-symvers --disable-multilib --with-gnu-ld --with-gnu-as --with-cloog-include=/usr/include/cloog-isl --without-libiconv-prefix --without-libintl-prefix --with-system-zlib --enable-linker-build-id --with-default-libstdcxx-abi=gcc4-compatible --enable-libstdcxx-filesystem-ts
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.4.0 (GCC)

$ cat test-builtin-thread-pointer.c
void f(void)
{
  void *p = __builtin_thread_pointer();
}

$ gcc -c test-builtin-thread-pointer.c
test-builtin-thread-pointer.c: In function ‘f’:
test-builtin-thread-pointer.c:3:13: warning: implicit declaration of function ‘__builtin_thread_pointer’; did you mean ‘__builtin_extend_pointer’? [-Wimplicit-function-declaration]
    3 |   void *p = __builtin_thread_pointer();
      |             ^~~~~~~~~~~~~~~~~~~~~~~~
      |             __builtin_extend_pointer
test-builtin-thread-pointer.c:3:13: warning: initialization of ‘void *’ from ‘int’ makes pointer from integer without a cast [-Wint-conversion]

On Ubuntu 24.04 this program compiles without warnings.

It looks like the gcc configure script checks for TLS here: https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/configure.ac;h=8fab93c9365496062583c5bd8ace402c8a797817;hb=HEAD#l3846, by running some code through the assembler. I would guess that doesn't work on Cygwin.

-Dimitry

> On 11 Feb 2025, at 21:45, Brian Inglis via Cygwin <cygwin AT cygwin DOT com> wrote:
> 
> 
> On 2025-02-11 12:41, Dimitry Andric via Cygwin wrote:
>>> On 11 Feb 2025, at 20:26, Ken Brown via Cygwin wrote:
>>> Does Cygwin support __builtin_thread_pointer?  I'm guessing not, because
>>> I'm getting a link error (undefined reference to
>>> `__builtin_thread_pointer') when I try to build some software that uses
>>> it.  Or is there something that would have to be added to the link command
>>> line?  The full command line and error message are quoted below, in case
>>> anyone spots something obvious.
> 
>> It's a gcc builtin function, not something implemented in an external library. Therefore, there is no linker option that can fix this.
>> Also, as far as I can see from gcc's documentation,
>> __builtin_thread_pointer() is only supported for the RISCV and SH
>> architectures. Then again, gcc's implementation seems to hinge this on
>> whether the platform supports TLS or not.
>> In any case, it is probably better to not use this function if your gcc does
>> not support it.
> Does `info gcc tls` help and do we know if that is supported by our gcc(s)?
> 
> -- 
> Take care. Thanks, Brian Inglis              Calgary, Alberta, Canada
> 
> La perfection est atteinte                   Perfection is achieved
> non pas lorsqu'il n'y a plus rien à ajouter  not when there is no more to add
> mais lorsqu'il n'y a plus rien à retrancher  but when there is no more to cut
>                                -- Antoine de Saint-Exupéry
> 
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