GCC 17 Release Series
Changes, New Features, and Fixes

This page is a "brief" summary of some of the huge number of improvements in GCC 17.

Note: GCC 17 has not been released yet, so this document is a work-in-progress.

Caveats

• ...

General Improvements

New Languages and Language specific improvements

OpenMP

See the GNU Offloading and Multi-Processing Project (GOMP) page for general information.

• OpenMP 5.0: Multiple clauses mapping the same variable is now supported. Handling used-defined map operation (declare mapper) in C and C++ has been improved. The uses_allocators clause now accepts traits (also with the OpenMP 5.1 syntax).
• OpenMP 5.1: The iterator modifier to the map, to, and from clauses has been improved for C and C++ and initial support for Fortran added.
• OpenMP 6.0: The device_type clause is now supported on the target construct.
• TR15: The omp_get_device_distances API routine has been added that can be used to find the offload devices that are closest, in terms of NUMA node, to the thread that called the routine. The multidimensional-information API routines have been added, albeit GCC currently only supports one spatial dimension; already useful are the new omp_get_supported_teams_dim and omp_get_supported_threads_dim routines that return, respectively, the maximal number of teams and threads that GCC supports on a given device.

C

• Improvements to GNU extensions including:
  • The new built-in functions __builtin_call_static_chain and __builtin_call_code_address can be used to obtain the static chain and the underlying code address of a nested function. This allows calling of the nested function indirectly without the use of a trampoline.

C++

Runtime Library (libstdc++)

• Improved experimental C++26 support including:
  • std::format in constexpr functions.
• std::filesystem support for Windows symlinks, thanks to Adam Wood.

Fortran

• The Fortran 2023 c_f_strpointer intrinsic is now supported. Additionally, the f_c_string intrinsic has been reimplemented to be more efficient.
• The -Wunused-but-set-variables option is now also effective for Fortran.
• The new option -Wundefined-vars, enabled by -Wall, warns about variables that are used that have not been defined (i.e. have no value assigned to them).
• The new options -Wunused-read and -Wunused-intent-out, both enabled by -Wextra, warn about variables in READ statements and variables passed to INTENT(OUT) variables, respectively, whose values are never used.

New Targets and Target Specific Improvements

AMD GPU (GCN)

• For offloading to AMD GPUs: The launch overhead of OpenMP target regions and OpenACC compute regions has been further reduced compared with GCC 16.

AVR

• The code size of an inline asm construct — or parts of it — can be specified with [[len=spec]] annotations. The code size is specified in units of 16-bit words and is set to the sum over all such notes. The notes can be deactivated with the new command-line option -mno-asm-len-notes so that the default length applies. The diagnostic behavior can be adjusted with -Wasm-len-notes.

S/390, System z, IBM z Systems

• Support for -m31 was removed and as a consequence -mesa support.

SH

• The SH backend has been switched permantenly to LRA. The option -mlra is now ineffective but kept around for backwards compatibility

Operating Systems

Other significant improvements