GCC 16 Release Series
Changes, New Features, and Fixes

This page is a "brief" summary of some of the huge number of improvements in GCC 16. You may also want to check out our Porting to GCC 16 page and the full GCC documentation.

Caveats

• int8_t etc. are now signed char on Solaris for conformance with the C99 standard. However, this is an incompatible change. See the porting notes for more information.
• The -pthread option no longer predefines _REENTRANT on Solaris. See the porting notes for more information.
• The so-called "json" format for -fdiagnostics-format= has been removed in this release. Users seeking machine-readable diagnostics from GCC should use SARIF.

General Improvements

• Link-Time Optimization now supports better handling of toplevel asm statements with -flto-toplevel-asm-heuristics.
• Speculative devirtualization now handles general indirect function calls and supports speculating of more than one target.
• The vectorizer is now more flexible in identifying in-loop parallelism of reductions.
• The vectorizer now supports vectorizing uncounted loops or loops for which the number of iterations could not be determined.
• The vectorizer now supports peeling for alignment for vector length agnostic loops using masking.
• The vectorizer now supports mutual peeling for alignment.
• The vectorizer now generates more efficient code for loops with early breaks by eliminating the vector induction computations.

Documentation

• The documentation for GCC Command Options and the option index have been corrected to include many previously missing options.
• The documentation for GCC-specific attributes has been modernized to put more emphasis on the standard attribute syntax, which GCC accepts in all supported dialects of C and C++. The material has also been reorganized to be less repetitive, and there is a new index for attributes.
• Documentation for parameters and option spec files has been moved from the main GCC manual to the GCC internals manual. These features are intended for use by GCC developers and those who need to build custom GCC configurations.

New Languages and Language specific improvements

OpenMP

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

• The memory allocation support has been enhanced: for allocators with the pinned trait, including ompx_gnu_pinned_mem_alloc, the CUDA API (if available) is used; this improves the performance when accessing this memory on Nvidia GPUs. The newly added ompx_gnu_managed_mem_alloc allocator and the ompx_gnu_managed_mem_space (both GNU extensions) allocate device-accessible memory on the host. Such memory is device accessible even when unified-shared memory is not supported and might have different page-migration behavior than other memory on systems even if all host memory is device accessible.
• OpenMP 5.0: Limited support for declare mapper has been added for C and C++, only. The uses_allocators clause is now supported, including the OpenMP 5.2 syntax changes and supporting semicolons (OpenMP 6.0); for now, only predefined allocators are supported.
• OpenMP 5.1: Initial support for the iterator modifier in map clauses and the target update construct has been added for C and C++.
• OpenMP 5.2: The begin declare variant directive for C and C++ is now supported.
• OpenMP 6.0: The omp_target_memset and omp_target_memset_async API routines have been added. The no_openmp_constructs assumptions clause can now be used.
• OpenMP Technical Report 14 (TR14): The named omp_default_device constant has been added to denote the default-device number.
• For OpenMP directives and clauses that have been deprecated in OpenMP 5.0, 5.1, or 5.2, a deprecation warning is shown, hinting at the to-be-used syntax; the warning is on by default and can be silenced using -Wno-deprecated-openmp. Additionally, a deprecation warning is shown when using a deprecated named constant or API routine; this warning can be silenced using -Wno-deprecated-declarations.

OpenACC

See the GCC OpenACC wiki page for general information.

• The acc_memcpy_device and acc_memcpy_device_async API routines have been added for C, C++ and Fortran.
• OpenACC 3.0: The wait directive now accepts the if clause.
• OpenACC 3.3: The Fortran API routines acc_attach and acc_detach now augment their OpenACC 2.6 C/C++ counterparts.
• OpenACC 3.4: In Fortran, named constants (PARAMETER) used as var in data clauses are now permitted by the specification and GCC for better compatibility with existing code; however, with GCC, specifying them in data clauses affects neither compile-time nor runtime behavior.

Ada

GNAT Extensions

• The Constructor [RFC] and Destructor [RFC] extensions add new construction/finalization mechanisms that differ significantly from standard Ada. Those features are inspired by object-oriented programming in other widely used languages (such as C++).
• Implicit with allows a stand-alone use clause in the context clause of a compilation unit to imply an implicit with of the same library unit where an equivalent with clause would be allowed.
• Structural Generic instantiation [RFC] allows reference to an implicit instance of a generic unit, that is denoted directly by the unit’s name and actual parameters, rather than by a separately declared name.
• The Extended_Access aspect can be specified on a general access type declaration designating an unconstrained array subtype. It changes the pointer representation and allows easier interfacing with foreign languages when memory for the designated object is not allocated by Ada. In particular, it allows the creation of access to an array slice [RFC].

Other

• VAST (Verifier for the Ada Semantic Tree), enabled with -gnatd_V (or -gnatd_W for verbose mode), can be used to debug the compiler. It checks various properties of the produced Ada Semantic Tree and reports detected violations.
• The semantic analysis of Ada 2022’s Reduction Expressions has been enhanced.
• The Ada.Containers.Bounded_Indefinite_Holders unit has been added.
• Various loopholes in the implementation of accessibility rules have been plugged.
• Android support has been improved.

C++

• C++20 by default: GCC 16 changes the default language version for C++ compilation from -std=gnu++17 to -std=gnu++20. If your code relies on older versions of the C++ standard, you will need to either add -std= to your build flags, or port your code; see the porting notes. N.B. C++20 modules support is still experimental and must be enabled by -fmodules.
• Several C++26 features have been implemented:
  • P2996R13, Reflection (PR120775, enabled by -std=c++26 -freflection)
  • P3394R4, Annotations for Reflection
  • P3293R3, Splicing a base class subobject
  • P3096R12, Function Parameter Reflection
  • P3491R3, define_static_{string,object,array} (PR120783)
  • P3560R2, Error Handling in Reflection
  • P1306R5, Expansion statements (PR120776)
  • P2900R14, Contracts (PR119061)
  • P2795R5, Erroneous behavior for uninitialized reads (PR114457)
  • P1061R10, Structured bindings can introduce a pack (PR117783)
  • P3068R5, constexpr exceptions (PR117785)
  • P3533R2, constexpr virtual inheritance (PR120777)
  • P1494R5, Partial program correctness (PR119060)
  • P3618R0, Allow attaching main to the global module (PR120773)
  • P2843R3, Preprocessing is never undefined (PR120778)
  • P2686R4, constexpr structured bindings and references to constexpr variables (PR117784, only partially, structured bindings can be constexpr but references to constexpr automatic variables still not allowed)
• Several C++23 features have been implemented:
  • P2036R3, Change scope of lambda trailing-return-type (PR102610)
  • P2590R2, Explicit lifetime management (PR106658)
  • P2246R1, Character encoding of diagnostic text (PR102613)
• Various C++ error messages (such as for problems involving templates) now have a hierarchical structure. This nesting of messages is presented using indentation and bullet points. The old behavior can be restored via -fno-diagnostics-show-nesting or -fdiagnostics-plain-output.
• Improved experimental C++20 modules support:
  • New command line option --compile-std-module that conveniently builds the <bits/stdc++.h> header unit and the std and std.compat modules before compiling any source files explicitly specified on the command line.
  • Whenever the <bits/stdc++.h> header unit has been built, GCC now transparently translates an #include of any importable standard library header into an import of <bits/stdc++.h>.
  • Many reported bugs have been fixed, thanks to Nathaniel Shead.
• Constraint failure diagnostics for standard library type traits such as is_constructible_v and is_invocable_v are improved to further elaborate why the trait is false instead of just reporting expression is_foo_v<...> evaluated to false, thanks to Nathaniel Shead.

Runtime Library (libstdc++)

• For targets that support 128-bit integers, std::is_integral<__int128> and similar traits are always true. Previously this was only the case when compiling with GNU dialects (-std=gnu++17, -std=gnu++14, etc.) and not with strict dialects (-std=c++17, etc.)
• The proposal P0952R2: A new specification for std::generate_canonical was implemented in all affected modes (since C++11), impacting the observed output. The previous behavior can be restored by defining _GLIBCXX_USE_OLD_GENERATE_CANONICAL.
• The std::variant ABI was updated to make it conforming and consistent with C++20 and later modes. This impacts the layout of classes which have a std::variant as the first member and a base class of the same type as one of the variant's alternatives, if that type is an empty class and has a non-trivial destructor:

  struct E { ~E(); };
  struct Affected : E
  {
    std::variant<E, int> mem; // previously stored at offset zero,
  			    // uses non-zero offset now
  };

  The previous behavior can be restored by defining _GLIBCXX_USE_VARIANT_CXX17_OLD_ABI. This impacts only C++17 mode.
• std::regex execution has been rewritten to use a heap-based stack instead of the system stack, avoiding stack overflows when matching larger strings.
• Improved support for C++20, including:
  • The C++20 implementation is no longer experimental.
  • Working std::chrono::current_zone() on Windows (thanks to Björn Schäpers).
• There are several changes to C++20 components which are incompatible with the experimental C++20 support in previous releases. The following C++20 components have ABI changes in GCC 16:
  • Atomic waiting/notifying functions in <atomic> and semaphore types in <semaphore>. Synchronization for <syncstream>.
  • The representation of std::format args and std::formatter specializations.
  • The representation of the std::partial_ordering type in <compare>.
  • Semantics of std::variant with std::jthread, std::stop_token, and std::stop_source alternatives.
  • Representation of some range adaptors in <ranges>.
  This list is not necessarily complete. As C++20 support was experimental before GCC 16, programs using C++20 components should assume that those components are not compatible with older releases.
• Improved experimental support for C++23, including:
  • std::mdspan, thanks to Luc Grosheintz.
  • ranges::starts_with and ranges::ends_with.
  • ranges::shift_left and ranges::shift_right.
  • std::allocator_traits::allocate_at_least.
• Improved experimental support for C++26, including:
  • std::simd.
  • std::inplace_vector.
  • std::optional<T&>.
  • std::copyable_function and std::function_ref.
  • std::indirect and std::polymorphic.
  • std::owner_equal for shared pointers, thanks to Paul Keir.
  • <debugging> header and contents.
  • New std::stringstream and std::bitset member functions accepting std::string_view arguments, thanks to Nathan Myers.
  • Padded mdspan layouts, aligned accessor, std::dims, std::constant_wrapper, and std::submdspan thanks to Luc Grosheintz.
  • std::philox_engine, thanks to 1nfocalypse.
  • std::atomic_ref::address(), thanks to Yuao Ma.

Fortran

• Coarrays using native shared memory mulithreading on single node machines and handling Fortran 2018's TEAM feature.
• Fortran 2003: Parameterized Derived Types support is improved. Handling of LEN parameters works but still requires a future change of representation (see PR82649).
• Fortran 2018: Support the extensions to the IMPORT statement, the REDUCE intrinsic and the new GENERIC statement.
• The Fortran 2023 additions to the trigonometric functions are now supported (such as the sinpi intrinsic).
• Fortran 2023: The split intrinsic subroutine is now supported and c_f_pointer now accepts an optional lower bound as a argument.
• The -fexternal-blas64 option has been added to call external BLAS routines with 64-bit integer arguments for MATMUL. This option is only valid for 64-bit systems and when -ffrontend-optimize is in effect.

Modula-2

• Spelling hints have been implemented. Currently spelling hints are issued when processing: import lists, module names and all symbols within nested scopes.
• A new implementation of wide set is used and this is accompanied with a library module M2WIDESET. This has changed the ABI and may lead to link-time errors with object files generated with a previous GCC version.
• A binary dictionary module BinDict has been added to the base libraries.
• The procedures Write and WriteLn are available in the modules: ARRAYOFCHAR, CFileSysOp, CHAR, FileSysOp, String and StringFileSysOp.
• The -fm2-pathname-root= option has been added to improve access to external library modules.

Algol 68

• GCC now includes an experimental Algol 68 compiler, ga68. It aims to implement the language described by the Revised Report, including all errata approved by the Algol 68 Support subcommittee of IFIP WG2.1. Some GNU extensions and a POSIX prelude are also implemented.

  More information about the language can be found on the Algol 68 website.

  More information about the front end can be found on the wiki.

New Targets and Target Specific Improvements

IA-32/x86-64

• GCC now supports AMD CPUs based on the Zen6 core via -march=znver6. This switch enables the AVX512_BMM, AVX_NE_CONVERT, AVX_IFMA, AVX_VNNI_INT8 and AVX512_FP16 ISA extensions on top of ISA extensions enabled for Zen5.
• Auto-vectorization will now try to use a masked vector epilog when AVX512 support is enabled and tuning for znver4, znver5 or znver6, saving code size and improving performance.
• GCC now supports the Intel CPU named Wildcat Lake through -march=wildcatlake. Wildcat Lake is based on Panther Lake.
• GCC now supports the Intel CPU named Nova Lake through -march=novalake. Based on ISA extensions enabled on Panther Lake, the switch in addition enables the APX_F, AVX10.1, AVX10.2 and PREFETCHI ISA extensions.
• Since GCC 16, AMX-TRANSPOSE and USER_MSR are not enabled through the compiler switch -march=diamondrapids any longer. CLDEMOTE is not enabled through the compiler switches -march=alderlake, -march=arrowlake, -march=arrowlake-s, -march=gracemont, -march=lunarlake, -march=meteorlake, -march=pantherlake and -march=raptorlake any longer. KL and WIDEKL are not enabled through the compiler switches -march=clearwaterforest and -march=pantherlake any longer. PREFETCHI is not enabled through the compiler switch -march=pantherlake any longer.
• -mavx10.1-256, -mavx10.1-512, and -mevex512 were removed together with the warning for the behavior change on -mavx10.1. -mavx10.1 has enabled AVX10.1 intrinsics with 512-bit vector support since GCC 15.
• Support for AMX-TRANSPOSE was removed in GCC 16. GCC will no longer accept -mamx-transpose,
• The new --enable-x86-64-mfentry configure option enables -mfentry which uses __fentry__, instead of mcount for profiling on x86-64. This option is enabled by default for glibc targets.
• --enable-tls=DIALECT is now supported to control the default TLS dialect. The default remains gnu. The accepted values are gnu and gnu2 (for TLS descriptors).

AMD GPU (GCN)

• For offloading to AMD GPUs: The launch overhead of OpenMP target regions and OpenACC compute regions has been drastically reduced.
• Experimental support for AMD Instinct MI300 (gfx942) devices has been added, including the generic gfx9-4-generic and mostly compatible gfx950.
• By default, the following multilibs are now built: gfx908, gfx90a, gfx9-generic, gfx9-4-generic, gfx10-3-generic, and gfx11-generic. Multilibs for specific devices are no longer built by default if a generic arch exists. Note:
  • When compiling for a specific arch and the multilib only exists for the associated generic arch, GCC's error message suggests the command-line option to do so.
  • Generic architectures require ROCm 6.4.0 or newer.
  • The new default-built set of multilibs now requires the assembler and linker of LLVM 20 or newer.
  • Consult GCC's AMD installation notes and configuration notes for setting the multilibs to be build.

LoongArch

• Bit-precise integer types (_BitInt (N) and unsigned _BitInt (N)) are supported.
• Added FunctionMulti-Versioning (FMV) support. The target_clones attribute can be used to generate multiple function versions for different LoongArch CPU features (e.g., lsx, lasx), with automatic runtime selection of the optimal version based on CPU capabilities.
• Added support for the LoongArch32 architecture, including the ilp32d (default), ilp32f, and ilp32s ABIs.
  This covers both the standard 32-bit version (LA32) and the reduced 32-bit version (LA32R), enabling GCC to generate 32-bit target code for a wider range of embedded applications.
  (Note: This feature depends on corresponding Binutils and glibc support.)

S/390, System z, IBM z Systems

• Bit-precise integer types (_BitInt (N) and unsigned _BitInt (N)) are supported, now.
• Floating-point type _Float16 is supported. All operations are carried out in software or by float instructions.
• Global stack protector support has been added and exported via -mstack-protector-guard=global. Option -mstack-protector-guard-record was added, too. The primary use is for the Linux kernel in order to support run-time patching of the address loading of the canary.
• Support for -m31 is deprecated and will be removed in a future release.

Operating Systems

Solaris

• GCC now supports the easy generation of Solaris CTF (Compact C Type Format) with the -gsctf option. More information can be found in the ctf(5) manual page.

Windows

• GCC now supports native TLS (Thread-Local Storage) on Windows. In order to enable it, --enable-tls must be specified at configure time and recent GNU binutils must be used (version 2.44 or later).

Improvements to GCC Diagnostics

• GCC can now output diagnostics in HTML form via -fdiagnostics-add-output=experimental-html
• GCC's SARIF output now respects the dump directory. For example, given

  
        gcc \
          -o build-dir/foo.o \
          -fdiagnostics-add-output=sarif
          foo.c
      

  GCC 15 would write the SARIF to foo.c.sarif, whereas GCC 16 now writes it to build-dir/foo.c.sarif.
• GCC's SARIF output now captures the nesting of logical locations.
• In GCC's SARIF output, fix objects now contain description properties in many cases.
• GCC's SARIF output has gained 5 new values for the §3.38.8 kinds property of threadFlowLocation, for expressing non-standard control flow:
  • throw for throwing an exception
  • catch for catching an exception
  • unwind for unwinding stack frame(s) during exception-handling
  • setjmp for calls to setjmp
  • longjmp for calls to longjmp that rewind the program counter/stack to the location of a previous setjmp call
• GCC diagnostics can now have directed graphs associated with them, and can also report "global" directed graphs. Graphs are ignored by text sinks, but are captured by SARIF sinks, and the "experimental-html" renders any such graphs in SVG-based form using dot. For example, setting cfgs=yes on a SARIF or HTML diagnostic sink will enable capturing GCC's intermediate representation of every function at every optimization pass.
• GCC diagnostics can now refer to logical locations inside XML and JSON files (such as via libgdiagnostics). The sarif-replay tool now uses this to provide JSON pointers when it reports on issues in its SARIF input.
• If GCC_DIAGNOSTICS_LOG is set in the environment, GCC's diagnostic subsystem will emit a text log to stderr (or a named file) to track what it's doing and the decisions it's making (e.g. exactly when and why a diagnostic is being rejected).
• If EXPERIMENTAL_SARIF_SOCKET is set in the environment, GCC will attempt to connect to that socket on startup and send JSON-RPC notifications to it for every diagnostic emitted.

Improvements for plugin authors

• GCC has gained a "publish/subscribe" framework, allowing for loosely-coupled senders and receivers, with strongly-typed messages passing between them. In this release the only topics for plugins to subscribe to are:
  • events relating to optimization passes starting/stopping on particular functions
  • events relating to the static analyzer
• GCC diagnostic sinks can now have extension objects associated with them, with a finalizer hook. Plugins can use this to capture additional information in SARIF output files.
• GCC's diagnostic machinery has been substantially cleaned up in GCC 16. This should not affect plugins that use just the diagnostic-core.h header, but maintainers of plugins making more sophisticated uses of diagnostics may need to refer to the porting guide.

Improvements to Static Analyzer

• The analyzer is now usable on simple C++ examples, as it now handles C++'s Named Return Value Optimization and has some initial support for exceptions. However due to scaling issues it is not likely to be usable on production C++ code in this release.
• With the added support for exception-handling, -fanalyzer assumes that a call to an external function not marked with attribute nothrow could throw an exception if -fexceptions is enabled. GCC 16 adds a new option -fanalyzer-assume-nothrow, for disabling this assumption. This is intended as a workaround for projects where the exception-handling generates large numbers of new warnings, such as C code where -fexceptions is used for interoperability with C++ but where the C APIs in use are unlikely to throw exceptions.
• The data structure used by -fanalyzer for representing the user's code has been rewritten in a way that makes it easier to understand and debug, and slightly improves locations used when reporting diagnostics. This comes at the cost of increasing the memory usage of the analyzer.
• The data structure used by -fanalyzer for simulating the contents of memory in the user's program has been reimplemented. The new implementation is faster and easier to maintain.
• The analyzer has started to make use of GCC's value_range machinery, eliminating some false positives.

Improvements to libgdiagnostics

libgdiagnostics has gained 37 entrypoints:

• 5 new entrypoints for working with logical locations
  • diagnostic_logical_location_get_kind()
  • diagnostic_logical_location_get_parent()
  • diagnostic_logical_location_get_short_name()
  • diagnostic_logical_location_get_fully_qualified_name()
  • diagnostic_logical_location_get_decorated_name()
• 2 new entrypoints for supporting command-line options and SARIF playback
  • diagnostic_manager_add_sink_from_spec()
  • diagnostic_manager_set_analysis_target()
• 12 new entrypoints for working with directed graphs:
  • diagnostic_manager_new_graph()
  • diagnostic_manager_take_global_graph()
  • diagnostic_take_graph()
  • diagnostic_graph_release()
  • diagnostic_graph_set_description()
  • diagnostic_graph_add_node()
  • diagnostic_graph_add_edge()
  • diagnostic_graph_get_node_by_id()
  • diagnostic_graph_get_edge_by_id()
  • diagnostic_node_set_label()
  • diagnostic_node_set_location()
  • diagnostic_node_set_logical_location()
• 17 new entrypoints for building up text for a diagnostic via a buffer:
  • diagnostic_message_buffer_new()
  • diagnostic_message_buffer_release()
  • diagnostic_message_buffer_append_str()
  • diagnostic_message_buffer_append_text()
  • diagnostic_message_buffer_append_byte()
  • diagnostic_message_buffer_append_printf()
  • diagnostic_message_buffer_append_event_id()
  • diagnostic_message_buffer_begin_url()
  • diagnostic_message_buffer_end_url()
  • diagnostic_message_buffer_begin_quote()
  • diagnostic_message_buffer_end_quote()
  • diagnostic_message_buffer_begin_color()
  • diagnostic_message_buffer_end_color()
  • diagnostic_message_buffer_dump()
  • diagnostic_finish_via_msg_buf()
  • diagnostic_add_location_with_label_via_msg_buf()
  • diagnostic_execution_path_add_event_via_msg_buf()
• diagnostic_manager_set_debug_physical_locations()

Other significant improvements

This is the list of problem reports (PRs) from GCC's bug tracking system that are known to be fixed in the 16.1 release. This list might not be complete (that is, it is possible that some PRs that have been fixed are not listed here).