This page is a "brief" summary of some of the huge number of improvements in GCC 12. You may also want to check out our Porting to GCC 12 page and the full GCC documentation.
struct S { float a; int : 0; float b; } or struct T { float c; int : 0; } by value could differ between C and C++. Starting with GCC 12 the C++ front-end no longer removes those bit-fields from the internal representation and per clarified psABI some targets have been changed, so that they either ignore those bit-fields in the argument passing by value decisions in both C and C++, or they always take them into account. x86-64, ARM and AArch64 will always ignore them (so there is a C ABI incompatibility between GCC 11 and earlier with GCC 12 or later), PowerPC64 ELFv2 and S/390 always take them into account (so there is a C++ ABI incompatibility, GCC 4.4 and earlier compatible with GCC 12 or later, incompatible with GCC 4.5 through GCC 11). RISC-V has changed the handling of these already starting with GCC 10. As the ABI requires, MIPS takes them into account handling function return values so there is a C++ ABI incompatibility with GCC 4.5 through 11. For function arguments on MIPS, refer to the MIPS specific entry. GCC 12 on the above targets will report such incompatibilities as warnings or other diagnostics unless -Wno-psabi is used. std::pair constructors have been deprecated. These allowed the use of an rvalue and a literal 0 to construct a pair containing a move-only type and a pointer. The nullptr keyword should be used to initialize the pointer member instead of a literal 0, as this is portable to other C++ implementations. --enable-libstdcxx-allocator no longer supports the bitmap, mt, and pool arguments. Those configurations had been broken for some time. --enable-libphobos is used. Other targets may require a more recent version of GCC to bootstrap. Specifics are documented for affected targets in the Installing GCC manual. omp_lib.h include file can no longer be compiled with -std=f95 but now requires at least -std=f2003. Alternatively, use the omp_lib module, which still supports -std=f95 and is recommended to be used instead in general. cr16 target with the cr16-*-* configuration has been obsoleted and will be removed in a future release. hppa[12]*-*-hpux10* and hppa[12]*-*-hpux11* configurations targeting 32-bit PA-RISC with HP-UX have been obsoleted and will be removed in a future release. m32c*-*-rtems* configuration has been obsoleted and will be removed in a future release. m32r-*-linux*, m32rle-*-linux*, m68k*-*-openbsd* and vax-*-openbsd* configurations has been removed. -Ofast now implies -fno-semantic-interposition. -O2 which is now equivalent to what would have been -O2 -ftree-vectorize -fvect-cost-model=very-cheap in the past. Note that the default vectorizer cost model has been changed; it used to behave as if -fvect-cost-model=cheap had been specified. -fsanitize=shadow-call-stack. This sanitizer currently only works on AArch64 targets and it requires an environment in which all code has been compiled with -ffixed-r18. Its primary initial user is the Linux kernel. close map modifier and the affinity clause are now supported. In addition, Fortran gained the following features which were available in C and C++ before: declare variant is now available, depobj, mutexinoutset and iterator can now also be used with the depend clause, defaultmap has been updated for OpenMP 5.0, and the loop directive and combined directives involving the master directive have been added.masked and scope constructs, the nothing and error directives, and using primary with the proc_bind clause and OMP_PROC_BIND environment variable, the reproducible and unconstrained modifiers to the order clause, and, for C/C++ only, the align and allocator modifiers to the allocate clause and the atomic extensions are now available. The OMP_PLACE environment variable supports the OpenMP 5.1 features. In addition, the OMP_NUM_TEAMS and OMP_TEAMS_THREAD_LIMIT environment variables and their associated API routines are now supported as well as the memory-allocation routines added for Fortran and extended for C/C++ in OpenMP 5.1. In Fortran code, strictly structured blocks can be used.--param openacc-kernels=decompose). -Wopenacc-parallelism to warn about potentially suboptimal choices related to OpenACC parallelism. -foffload-options= flag and the pre-existing but now documented -foffload= flag. -gnat2022 flag to indicate strict Ada 2022 compliance. The old -gnat2020 flag is now deprecated.with Static aspect and can be used in more contexts.-gnatX flag is necessary to access these features as they are not considered stable or standard. type Matrix is array (Natural range 0 .. <>, Natural range 0 .. <>) of Integer; is now valid.subtype String_1 is String (1 .. <>);. Boundaries from slices will "slide" to the correct lower bound of the subtype.Object.Operand notation. The following code is now valid V.Add_Element(42);, with V being a vector, for example.when constructs. Keywords return, goto and raise can now use when in addition to the existing exit when. The following expression is therefore now valid raise Constraint_Error with "Element is null" when Element = null;case statement has been extended to cover records and arrays as well as finer grained casing on scalar types. In the future it is expected to provide more compile time guarantees when accessing discriminated fields. Case exhaustion is supported for pattern matching. An example would be type Sign is (Neg, Zero, Pos); function Multiply (S1, S2 : Sign) return Sign is (case (S1, S2) is when (Neg, Neg) | (Pos, Pos) => Pos, when (Zero, <>) | (<>, Zero) => Zero, when (Neg, Pos) | (Pos, Neg) => Neg);
gnatfind and gnatxref, which were already deprecated, have been removed.Pure functions.__builtin_shufflevector compatible with the clang language extension was added.unavailable was added.__builtin_assoc_barrier, was added. It can be used to inhibit re-association of floating-point expressions.__builtin_dynamic_object_size compatible with the clang language extension was added.-Wbidi-chars warns about potentially misleading UTF-8 bidirectional control characters. The default is -Wbidi-chars=unpaired (PR103026)-Warray-compare warns about comparisons between two operands of array type (PR97573)-Wattributes has been extended so that it's possible to use -Wno-attributes=ns::attr or -Wno-attributes=ns:: to suppress warnings about unknown scoped attributes (in C++11 and C2X). Similarly, #pragma GCC diagnostic ignored_attributes "vendor::attr" can be used to achieve the same effect (PR101940)-std=c2x and -std=gnu2x. Some of these features are also supported as extensions when compiling for older language versions. In addition to the features listed, some features previously supported as extensions and now added to the C standard are enabled by default in C2X mode and not diagnosed with -std=c2x -Wpedantic. #elifdef and #elifndef preprocessing directives are now supported.printf and scanf format checking with -Wformat now supports the %b format specified by C2X for binary integers, and the %B format recommended by C2X for printf. if consteval (PR100974)auto(x): decay-copy in the language (PR103049)elifdef and elifndef (PR102616)auto specifier for pointers and references to arrays (PR100975)void{}enum direct-list-initializationauto specifier for pointers and references to arrays-fimplicit-constexpr can be used to make inline functions implicitly constexpr (git)-ffold-simple-inlines can be used to fold calls to certain trivial inline functions (currently std::move, std::forward, std::addressof and std::as_const). In contrast to inlining such calls, folding means that no intermediate code or debug information will be generated for them; this minimizes the abstraction penalty incurred for using these functions versus using the fundamental operations from which they're defined (e.g. std::move versus static_cast). This flag is enabled by default when -fno-inline is not active.-Wuninitialized warns about using uninitialized variables in member initializer lists (PR19808) -Wint-in-bool-context is now disabled when instantiating a template (git)-Wc++11-extensions, -Wc++14-extensions, -Wc++17-extensions, -Wc++20-extensions, and -Wc++23-extensions. They are enabled by default and can be used to control existing pedwarns about occurrences of new C++ constructs in code using an old C++ standard dialect.-Wmissing-requires warns about missing requires (git)std::is_constant_evaluated in if warning was extended to warn in more cases (PR100995)-Waddress has been enhanced so that it now warns about, for instance, comparing the address of a nonstatic member function to null (PR102103)__cpp_lib_is_pointer_interconvertible and __cpp_lib_is_layout_compatible to help the C++ library implement P0466, Layout-compatibility and Pointer-interconvertibility Traits (PR101539)constinit thread_local variables are optimized better (PR101786)std::hardware_destructive_interference_size was added, along with the -Winterference-size warning (git)-fconstexpr-fp-except flag allows IEC559 floating point exceptions in constant-expressions.std::vector, std::basic_string, std::optional, and std::variant can be used in constexpr functions.std::make_shared for arrays with default initialization, and std::atomic<std::shared_ptr<T>>.std::optional.std::expectedstd::move_only_function<spanstream>std::basic_string::resize_and_overwritestd::unique_ptr can be used in constexpr functions.<stacktrace> (not built by default, requires linking to an extra library).<stdatomic.h>std::invoke_rconstexpr std::type_info::operator==__traits(compiles) expression can now be used to determine whether a target-specific built-in is available without error during CTFE (PR101127). pragma(inline, true) are now compiled into every module where they are used from (PR106563). -fcheck=, enables or disables the code generation of specific run-time contract checks. -fcheckaction=, controls the run-time behavior on an assert, array bounds check, or final switch contract failure. The default is -fcheckaction=throw. -fdump-c++-spec=, dumps all compiled extern(C++) declarations as C++ code to the given file. The supplementary option -fdump-c++-spec-verbose turns on emission of comments for ignored declarations in the generated spec. -fextern-std=, controls which C++ standard extern(C++) declarations are compiled to be compatible with. The default is -fextern-std=c++17. -fpreview=, added to enable upcoming D language features in the compiler. -frevert=, added to revert D language changes to support older D codebases that need more time to transition. -fsave-mixins=, saves mixins expanded at compile-time to a file. -Wtemplates compiler switch has been removed, as it had been superceded by -ftransition=templates, which more accurately reports on which templates have been instantiated. -ftransition=dip25 and -ftransition=dip1000 compiler switches have been renamed to -fpreview=dip25 and -fpreview=dip1000. bind(c) and gaps in the handling for assumed-rank arrays, numerous other bugs have been fixed, and an extensive set of new conformance test cases has been added. OPERATION as the name of the function to the CO_REDUCE intrinsic for the pairwise reduction, thus conforming to the Fortran 2018 standard. Previous versions used OPERATOR which conforms to TS 18508. -mabi=ieeelongdouble option now selects the IEEE 128-bit floating point format for REAL(KIND=16). R16_IBM and R16_IEEE have been added to the -fconvert option, the CONVERT specifier of the OPEN statement and the GFORTRAN_CONVERT_UNIT environment variable. LIBGCCJIT_ABI_16) gcc_jit_lvalue_set_tls_model for supporting thread-local variables (LIBGCCJIT_ABI_17) gcc_jit_lvalue_set_link_section for setting the link section of global variables, analogous to __attribute__((section(".section"))) (LIBGCCJIT_ABI_18) LIBGCCJIT_ABI_19) LIBGCCJIT_ABI_20) gcc_jit_context_new_bitcast for reinterpreting the bits of an rvalue as a different type (LIBGCCJIT_ABI_21) gcc_jit_lvalue_set_register_name for setting a specific register for a variable (LIBGCCJIT_ABI_22) gcc_jit_context_set_bool_print_errors_to_stderr (LIBGCCJIT_ABI_23) LIBGCCJIT_ABI_24) -march option: armv8.7-a, armv8.8-a, armv9-a.cortex-a510 argument to the -mcpu and -mtune options. -mcpu and -mtune options (GCC identifiers in parentheses). ampere1).cortex-a710).cortex-x2).+ls64 option extension.memcpy,memmove and memset standard functions. These instructions can be generated when compiling with the +mopsoption extension.arm_neon.h header have been significantly reimplemented and generate higher-performing code than previous GCC versions.-mtune=neoverse-512tvb is added to tune for Arm Neoverse cores that have a total vector bandwidth of 512 bits. Please refer to the documentation for more details.__int128_t/integer(kind=16) was added.-fstack-protector-guard=tls and -mstack-protector-guard-offset= options. This intended for use in Linux kernel development. Please refer to the documentation for more details.-mavx512fp16 compiler switch. _Float16 type is supported on x86 systems with SSE2 enabled. Without {-mavx512fp16}, all operations will be emulated in software and float instructions. -mharden-sls=[none|all|return|indirect-jmp]. -mindirect-branch-cs-prefix. -mno-direct-extern-access command-line option is specified. -march= and -mtune= options (GCC identifiers in parentheses). la464).loongarch64).-Wno-psabi is used. -Wno-psabi is used. -march flag has been added. The -misa flag is now considered an alias of the -march flag.sm_53, sm_70, sm_75 and sm_80 has been added. These can be specified using the -march flag.sm_30, to fix support for sm_30 boards.-march-map flag has been added. The -march-map value will be mapped to an valid -march flag value. For instance, -march-map=sm_50 maps to -march=sm_35. This can be used to specify that generated code is to be executed on a board with at least some specific compute capability, without having to know the valid values for the -march flag.-mptx flag has been added to specify the PTX ISA version for the generated code; permitted values are 3.1 (matches previous GCC versions), 6.0, 6.3, and 7.0. If not specified, the used version is the minimal version required for -march but at least 6.0. mptx-3.1 multilib was added. This allows using older drivers which do not support PTX ISA version 6.0.__PTX_SM__ predefined macro allows code to check the PTX ISA target architecture being targeted by the compiler.__PTX_ISA_VERSION_MAJOR__ and __PTX_ISA_VERSION_MINOR__ predefined macros allows code to check the PTX ISA version being targeted by the compiler.__builtin_get_texasr, __builtin_get_texasru, __builtin_get_tfhar, __builtin_get_tfiar, __builtin_set_texasr, __builtin_set_texasru, __builtin_set_tfhar, and __builtin_set_tfiar now behave as documented in all supported configurations. On prior releases, the arguments and return values of these functions were treated as unsigned long long instead of as unsigned long, when the options -m32 -mpowerpc64 were in effect. vec_cntlz_lsbb and vec_cnttz_lsbb now behave as documented. On prior releases, these built-in functions had incorrect semantics on little-endian targets. __regio_symbol variable qualifier has been added. It allows easier access in C programs to the __R30 and __R31 CPU I/O registers. -march= parsing.-mtune=thead-c906 is added to tune for T-HEAD c906 cores.-latomic should check their ABIs again.-Wanalyzer-use-of-uninitialized-value warning, similar to -Wuninitialized and -Wmaybe-uninitialized, but based on an interprocedural path-sensitive analysis (PR95006). Such warnings are not disabled by the new -ftrivial-auto-var-init (see below), as the latter is considered a mitigation option.
-Wanalyzer-write-to-const and -Wanalyzer-write-to-string-literal will now check for __attribute__ ((access, ....)) on calls to externally-defined functions, and complain about read-only regions pointed to by arguments marked with a write_only or read_write attribute (PR104793). -fanalyzer-checker=taint (in addition to -fanalyzer), has gained four new taint-based warnings: -Wanalyzer-tainted-allocation-size for e.g. attacker-controlled malloc and alloca, -Wanalyzer-tainted-divisor for detecting where an attacker can inject a divide-by-zero, -Wanalyzer-tainted-offset for attacker-controlled pointer offsets, -Wanalyzer-tainted-size for attacker-controlled values being used as a size parameter to calls to memset or to functions marked with __attribute__ ((access, ....)). The existing -Wanalyzer-tainted-array-index has been reworded to talk about "attacker-controlled" rather than "tainted" values, for consistency with the new warnings.
A new __attribute__ ((tainted_args)) has been added to the C and C++ front ends, usable on functions, and on function pointer callback fields in structs. The analyzer's taint mode will treat all parameters and buffers pointed to by parameters of such functions as being attacker-controlled, such as for annotating system calls in an operating system kernel as being an "attack surface".
__attribute__((const)): it will treat such functions as returning the same value when given the same inputs (PR104434), and as having no side effects (PR104576). realloc calls by splitting the execution path into three possible outcomes for the call: NULLmalloc etc) and stack (via alloca), though none of the analyzer warnings make use of this yet in GCC 12.-Wuninitialized is active. For best debugging, use of the new command-line option -ftrivial-auto-var-init=pattern can be used to fill variables with a repeated 0xFE pattern, which tends to illuminate many bugs (e.g. pointers receive invalid addresses, sizes and indices are very large). For best production results, the new command-line option -ftrivial-auto-var-init=zero can be used to fill variables with 0x00, which tends to provide a safer state for bugs (e.g. pointers are NULL, strings are NUL filled, and sizes and indices are 0). -gctf enables the generation of CTF. -gbtf This is the list of problem reports (PRs) from GCC's bug tracking system that are known to be fixed in the 12.1 release. This list might not be complete (that is, it is possible that some PRs that have been fixed are not listed here).
This is the list of problem reports (PRs) from GCC's bug tracking system that are known to be fixed in the 12.2 release. This list might not be complete (that is, it is possible that some PRs that have been fixed are not listed here).
-m[check|no-check]-zero-division is changed for optimized code. Now -mno-check-zero-division is the default for all optimization levels but -O0 and -Og. The old behavior can be obtained by explicitly passing -mcheck-zero-division to GCC. znver4 core via -march=znver4. The switch makes GCC consider using 512-bit vectors when auto-vectorizing. This is the list of problem reports (PRs) from GCC's bug tracking system that are known to be fixed in the 12.3 release. This list might not be complete (that is, it is possible that some PRs that have been fixed are not listed here).
This is the list of problem reports (PRs) from GCC's bug tracking system that are known to be fixed in the 12.4 release. This list might not be complete (that is, it is possible that some PRs that have been fixed are not listed here).
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