__builtin_thread_pointer

[Brian Inglis]

On 2025-02-11 13:58, Dimitry Andric wrote:
>> On 11 Feb 2025, at 21:45, Brian Inglis via Cygwin 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)?

 > 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();
 > }

If you read the docs at `info gcc tls` it uses the __thread storage class to 
allow you to declare auto, extern, or static thread variables like any other.

Whether that would run as expected is more than I can deduce.

-- 
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