GCC 12 Release Series
Changes, New Features, and Fixes
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.
Caveats
* An ABI incompatibility between C and C++ when passing or returning by
value certain aggregates containing zero width bit-fields has been
discovered on various targets. As mentioned in PR102024, since the
PR42217 fix in GCC 4.5 the C++ front-end has been removing zero width
bit-fields from the internal representation of the aggregates after the
layout of those aggregates, but the C front-end kept them, so passing
e.g. 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.
* C: Computed gotos require a pointer type now.
* C++: Two non-standard 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.
* The configuration option --enable-libstdcxx-allocator no longer supports
the bitmap, mt, and pool arguments. Those configurations had been broken
for some time.
* Fortran: OpenMP code using the 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.
* OpenMP offloading to Intel MIC has been deprecated and will be removed in
a future release.
* The cr16 target with the cr16-*-* configuration has been obsoleted and
will be removed in a future release.
* The 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.
* The m32c*-*-rtems* configuration has been obsoleted and will be removed
in a future release.
* The support for the m32r-*-linux*, m32rle-*-linux*, m68k*-*-openbsd* and
vax-*-openbsd* configurations has been removed.
* STABS: Support for emitting the STABS debugging format is deprecated and
will be removed in the next release. All ports now default to emit DWARF
(version 2 or later) debugging info or are obsoleted.
* The optimization level -Ofast now implies -fno-semantic-interposition.
General Improvements
* Vectorization is enabled at -O2 which is now equivalent to the original -
O2 -ftree-vectorize -fvect-cost-model=very-cheap. Note that default
vectorizer cost model has been changed which used to behave as -fvect-
cost-model=cheap were specified.
* GCC now supports the ShadowCallStack sanitizer, which can be enabled
using the command-line option -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.
New Languages and Language specific improvements
* OpenMP
o OpenMP 5.0 support has been extended: The close map modifier and
the affinity clause are now supported. In addition Fortran gained
additionally 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.
o The following OpenMP 5.1 features have been added: support for
expressing OpenMP directives as C++ 11 attributes, the masked and
scope construct, 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.
o The OpenMP_Implementation_Status can be found in the libgomp
manual.
* Version 2.6 of the OpenACC specification continues to be maintained and
improved in the C, C++ and Fortran compilers. See the implementation
status section on the OpenACC wiki page and the run-time_library
documentation for further information. In addition to general performance
tuning and bug fixing, new features include:
o OpenACC worker parallelism for AMD_GPUs (already for a long time
supported for Nvidia_GPUs).
o Data privatization/sharing at the OpenACC gang level.
o Considerable improvements for the experimental OpenACC 'kernels'
decomposition (--param_openacc-kernels=decompose).
o A new warning flag -Wopenacc-parallelism to warn about potentially
suboptimal choices related to OpenACC parallelism.
* The offload target code generation for OpenMP and OpenACC can now be
better adjusted using the new -foffload-options= flag and the pre-
existing but now documented -foffload= flag.
Ada
* Ada 2022
o Added the -gnat2022 flag to indicate strict Ada 2022 compliance.
The old -gnat2020 flag is now deprecated.
o Support for Big Numbers (Annex G) has seen continuous improvements.
It is now considered complete. It is compatible with SPARK, i.e.
can be used from SPARK code.
o Continuous improvements to the Ada 2022 standard since GCC 11.
o Greatly improved compile time support. More functions can now have
the with Static aspect and can be used in more contexts.
* Ada 2022 extensions. The use of the -gnatX flag is necessary to access
these features as they are not considered stable or standard.
o Fixed lower bound for unconstrained arrays.
# type Matrix is array (Natural range 0 .. <>, Natural range 0
.. <>) of Integer; is now valid.
# Subtypes can also specify a lower bound: subtype String_1 is
String (1 .. <>);. Boundaries from slices will "slide" to the
correct lower bound of the subtype.
o Generalized Object.Operand notation. The follwing code is now valid
V.Add_Element(42);, with V being a vector, for example.
o Additional 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;
o Pattern matching
# The 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.
* Greatly expanded code covered by contracts. Thanks to this work, there
are now several Ada standard libraries fully proven in SPARK which means
they have no runtime nor logical errors. They are mostly numeric and
string handling libraries.
* Enable return-slot optimization for Pure functions.
* General optimizations, improvements and additions to the standard
library. Performance, correctness and in some cases stability was
improved. Memory pools have also seen some minor enhancements.
* Improvements to embedded-RTOS targets such as RTEMS, VxWorks and QNX.
Older targets were removed or cleaned.
* Added some hardening_features.
C family
* Support for __builtin_shufflevector compatible with the clang language
extension was added.
* Support for attribute unavailable was added.
* A new built-in function, __builtin_assoc_barrier, was added. It can be
used to inhibit re-association of floating-point expressions.
* Support for __builtin_dynamic_object_size compatible with the clang
language extension was added.
* New warnings:
o -Wbidi-chars warns about potentially misleading UTF-8 bidirectional
control characters. The default is -Wbidi-chars=unpaired (PR103026)
o -Warray-compare warns about comparisons between two operands of
array type (PR97573)
* Enhancements to existing warnings:
o -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)
C
* Some new features from the upcoming C2X revision of the ISO C standard
are supported with -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.
o Digit separators (as in C++) are supported for C2X.
o The #elifdef and #elifndef preprocessing directives are now
supported.
o The 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.
C++
* Several C++23 features have been implemented:
o P1938R3, if consteval (PR100974)
o P0849R8, auto(x): decay-copy in the language (PR103049)
o P2242R3, Non-literal variables (and labels and gotos) in constexpr
functions (PR102612)
o P2334R1, Support for preprocessing directives elifdef and elifndef
(PR102616)
o P2360R0, Extend init-statement to allow alias-declaration
(PR102617)
o P2128R6, Multidimensional subscript operator
o DR_2397, auto specifier for pointers and references to arrays
(PR100975)
* Several C++ Defect Reports have been resolved, e.g.:
o DR_960, Covariant functions and lvalue/rvalue references
o DR_1227, Mixing immediate and non-immediate contexts in deduction
failure
o DR_1315, Restrictions on non-type template arguments in partial
specializations
o DR_2082, Referring to parameters in unevaluated operands of default
arguments
o DR_2351, void{}
o DR_2374, Overly permissive specification of enum direct-list-
initialization
o DR_2397, auto specifier for pointers and references to arrays
o DR_2446, Questionable type-dependency of concept-ids
* New command-line option -fimplicit-constexpr can be used to make inline
functions implicitly constexpr (git)
* New command-line option -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.
* Deduction guides can be declared at class scope (PR79501)
* -Wuninitialized warns about using uninitialized variables in member
initializer lists (PR19808)
* -Wint-in-bool-context is now disabled when instantiating a template (git)
* Stricter checking of attributes on friend declarations: if a friend
declaration has an attribute, that declaration must be a definition.
Moreover, a C++11 attribute cannot appear in the middle of the decl-
specifier-seq. (PR99032)
* New warning options for C++ language mismatches: -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.
* New warning -Wmissing-requires warns about missing requires (git)
* The existing 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)
* Errors about narrowing are no longer hidden if they occur in system
headers
* Ordered comparison of null pointers is now rejected (PR99701)
* Anonymous structs with bases are now rejected (git)
* The compiler rejects taking the address of an immediate member function
(PR102753)
* The compiler has support for C++20 __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)
* Memory usage of constraint subsumption has been improved (PR100828)
* constinit thread_local variables are optimized better (PR101786)
* Support for C++17 std::hardware_destructive_interference_size was added,
along with the -Winterference-size warning (git)
* Many bugs in the CTAD handling have been fixed (PR101344, PR101883,
PR89062, PR101233, PR88252, PR86439, PR98832, PR102933 ...)
* Two-stage name lookup for dependent operator expressions has been
corrected (PR51577)
* Several issues with constrained variable templates have been fixed
(PR98486)
* The compiler performs less instantiating when doing speculative constant
evaluation (git)
* Various diagnostic improvements; e.g., a more precise caret location for
pointer-to-member expressions
* The new -fconstexpr-fp-except flag allows IEC559 floating point
exceptions in constant-expressions.
Runtime Library (libstdc++)
* Improved experimental C++20 support, including:
o std::vector, std::basic_string, std::optional, and std::variant can
be used in constexpr functions.
o std::make_shared for arrays with default initialization, and std::
atomic<std::shared_ptr<T>>.
o Layout-compatibility and pointer-interconvertibility traits.
* Improved experimental C++23 support, including:
o Monadic operations for std::optional.
o std::move_only_function
o <spanstream>
o std::basic_string::resize_and_overwrite
o std::unique_ptr can be used in constexpr functions.
o <stacktrace> (not built by default, requires linking to an extra
library).
o <stdatomic.h>
o std::invoke_r
o constexpr std::type_info::operator==
Fortran
* WG5/N1942, "TS 29113 Further Interoperability of Fortran with C", is now
fully supported. In addition to implementing previously missing
functionality, such as support for character arguments of length greater
than one in functions marked 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.
* GCC 12 now uses 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.
* On POWER systems which support it, the -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
specifyer of the OPEN statement and the GFORTRAN_CONVERT_UNIT environment
variable.
libgccjit
* The libgccjit API gained 30 new entry points:
o 17 new "reflection" entrypoints for querying functions and types
(LIBGCCJIT_ABI_16)
o gcc_jit_lvalue_set_tls_model for supporting thread-local variables
(LIBGCCJIT_ABI_17)
o gcc_jit_lvalue_set_link_section for setting the link section of
global variables, analogous to __attribute__((section(".section")))
(LIBGCCJIT_ABI_18)
o 4 new entrypoints for initializing global variables and creating
constructors for rvalues (LIBGCCJIT_ABI_19)
o Support for sized integer types, including 128-bit integers and
helper functions for such types (LIBGCCJIT_ABI_20)
o gcc_jit_context_new_bitcast for reinterpreting the bits of an
rvalue as a different type (LIBGCCJIT_ABI_21)
o gcc_jit_lvalue_set_register_name for setting a specific register
for a variable (LIBGCCJIT_ABI_22)
o gcc_jit_context_set_bool_print_errors_to_stderr (LIBGCCJIT_ABI_23)
o 2 new entrypoints for setting the alignment of a variable
(LIBGCCJIT_ABI_24)
* libgccjit has gained support for the use of various atomic builtins
(PR96066, PR96067)
* gcc_jit_context_new_cast is now able to handle truncation and extension
between different integer types (PR95498)
New Targets and Target Specific Improvements
AArch64 & arm
* Newer revisions of the Arm Architecture are supported as arguments to the
-march option: armv8.7-a, armv8.8-a, armv9-a.
* The Arm Cortex-A510 CPU is now supported through the cortex-a510 argument
to the -mcpu and -mtune options.
* GCC can now auto-vectorize operations performing sign-differing dot-
product operations, taking advantage of instructions in the Advanced SIMD
(AArch64/AArch32) and SVE (AArch64) instruction sets.
AArch64
* A number of new CPUs are supported through the -mcpu and -mtune options
(GCC identifiers in parentheses).
o Ampere-1 (ampere1).
o Arm Cortex-A710 (cortex-a710).
o Arm Cortex-X2 (cortex-x2).
* The 64-byte atomic load/store intrinsics to accelerator memory from the
2020_Arm_Architecture_extensions are supported through the +ls64 option
extension.
* Initial code generation support is supported for hardware instructions
used to accelerate the memcpy,memmove and memset standard functions.
These instructions can be generated when compiling with the +mopsoption
extension.
* The ACLE Advanced SIMD intrinsics accessible through the arm_neon.h
header have been significantly reimplemented and generate higher-
performing code than previous GCC versions.
* The option -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.
AMD Radeon (GCN)
* Debug experience with ROCGDB has been improved.
* Support for the type __int128_t/integer(kind=16) was added.
* For offloading, the limitation of using only one wavefront per compute
unit (CU) has been lifted. Up to 40 workgroups per CU and 16 wavefronts
per workgroup are supported (up to a limit of 40 wavefronts in total, per
CU). Additionally, the number of used wavefronts and workgroups was tuned
for performance.
arm
* Support is added for accessing the stack canary value via the TLS
register through the -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.
BPF
* Support for CO-RE (compile-once, run-everywhere) has been added to the
BPF backend. CO-RE allows to compile portable BPF programs that are able
to run among different versions of the Linux kernel.
IA-32/x86-64
* New ISA extension support for Intel AVX512-FP16 was added. AVX512FP16
intrinsics are available via the -mavx512fp16 compiler switch.
* For both C and C++ the _Float16 type is supported on x86 systems with
SSE2 enabled. Without {-mavx512fp16}, all operations will be emulated in
software and float instructions.
* Mitigation against straight line speculation (SLS) for function return
and indirect jump is supported via -mharden-sls=
[none|all|return|indirect-jmp].
* Add CS prefix to call and jmp to indirect thunk with branch target in r8-
r15 registers via -mindirect-branch-cs-prefix.
* Always use global offset table (GOT) to access external data and function
symbols when the new -mno-direct-extern-access command-line option is
specified.
LoongArch
* Support for the LoongArch architecture instruction set has been added.
* The Loongson CPU codename LA464 and LoongArch 64-bit generic CPU codename
loongarch64 are supported through the -march= and -mtune= options (GCC
identifiers in parentheses).
o Loongson LA464 core (la464).
o LoongArch 64-bit generic core (loongarch64).
MIPS
* The ABI passing arguments containing zero-width fields (for example, C/
C++ zero-width bit-fields, GNU C/C++ zero-length arrays, and GNU C empty
structs) has changed. Now a zero-width field will not prevent an aligned
64-bit floating-point field next to it from being passed through FPR.
This is compatible with LLVM, but incompatible with previous GCC
releases. GCC 12 on MIPS will report such incompatibilities as an inform
unless -Wno-psabi is used.
* The ABI returning values containing C++17 empty bases has changed. Now an
empty base will not prevent an aggregate containing only one or two
floating-point fields from being returned through FPR. This is compatible
with GCC 6 and earlier, but incompatible with GCC 7 through 11. GCC 12 on
MIPS will report such incompatibilities as an inform unless -Wno-psabi is
used.
NVPTX
* The -march flag has been added. The -misa flag is now considered an alias
of the -march flag.
* Support for PTX ISA target architectures sm_53, sm_70, sm_75 and sm_80
has been added. These can be specified using the -march flag.
* The default PTX ISA target architecture has been set back to sm_30, to
fix support for sm_30 boards.
* The -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.
* The -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.
* An mptx-3.1 multilib was added. This allows using older drivers which do
not support PTX ISA version 6.0.
* The new __PTX_SM__ predefined macro allows code to check the PTX ISA
target architecture being targeted by the compiler.
* The new __PTX_ISA_VERSION_MAJOR__ and __PTX_ISA_VERSION_MINOR__
predefined macros allows code to check the PTX ISA version being targeted
by the compiler.
PowerPC / PowerPC64 / RS6000
* The internal implementation of Power's target-specific built-in functions
has been rewritten to be easier and less error-prone to maintain. Every
attempt has been made to ensure that the new behavior matches the old
behavior, but inevitably some bugs can be expected. Please report any
problems via GCC_Bugzilla.
* The built-in functions __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.
* The overloaded built-in functions 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.
PRU
* The __regio_symbol variable qualifier has been added. It allows easier
access in C programs to the __R30 and __R31 CPU I/O registers.
RISC-V
* Default ISA spec version was bump to 20191213, more detail see this
announcement
* New ISA extension support for zba, zbb, zbc, zbs was added.
* New ISA extension support for vector and scalar crypto was added, only
support architecture testing marco and -march= parsing.
* The option -mtune=thead-c906 is added to tune for T-HEAD c906 cores.
Operating Systems
Improvements to Static Analyzer
* The analyzer has gained a -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).
* The analyzer's "taint" mode, activated by -fanalyzer-checker=taint (in
addition to -fanalyzer), has gained four new taint-based warnings:
o -Wanalyzer-tainted-allocation-size for e.g. attacker-controlled
malloc and alloca,
o -Wanalyzer-tainted-divisor for detecting where an attacker can
inject a divide-by-zero,
o -Wanalyzer-tainted-offset for attacker-controlled pointer offsets,
o -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++
frontends, 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 attacked-
controlled, such as for annotating system calls in an operating system
kernel as being an "attack surface".
* The analyzer now respects __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).
* The analyzer is now able to split its analysis into multiple execution
paths in places where there isn't a split in the control flow graph. For
example, it now handles realloc calls by splitting the execution path
into three possible outcomes for the call:
o failure, returning NULL
o success, growing the buffer in-place without moving it
o success, allocating a new buffer, copying the content of the old
buffer to it, and freeing the old buffer
* The analyzer's interprocedural path exploration logic is now able to
track calls through function pointers.
* The analyzer now makes the assumption that if we know PTR is non-NULL,
then (PTR + OFFSET) is also non-NULL. This isn't strictly true, but
eliminates false positives in practice (PR101962).
* The analyzer has gained some initial support for inline assembler code.
This is extremely limited, and is purely to help suppress false positives
when analyzing the Linux kernel, which makes heavy use of inline
assembler (PR101570).
* The way the analyzer tracks the state of memory along an execution path
has been improved in various ways for GCC 12:
o An optimization for representing bulk updates to memory (e.g. zero
fills) has been removed as it never worked well. In GCC 12 it has
been replaced with a simpler and more accurate approach,
eliminating many false positives (PR95006).
o Various optimizations have been added, speeding up the analysis on
a particularly problematic source file from 4 minutes down to 17
seconds (PR104943, PR104954, and PR104955).
o The analyzer now tracks the sizes of dynamically-allocated regions,
both on the heap (via malloc etc) and stack (via alloca), though
none of the analyzer warnings make use of this yet in GCC 12.
* The analyzer's handling of switch statements has been rewritten, fixing
various bugs.
Other significant improvements
Eliminating uninitialized variables
* GCC can now initialize_all_stack_variables_implicitly, including padding.
This is intended to eliminate all classes of uninitialized stack variable
flaws. Lack of explicit initialization will still warn when -
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).
Debugging formats
* GCC can now generate debugging information in CTF, a lightweight
debugging format that provides information about C types and the
association between functions and data symbols and types. This format is
designed to be embedded in ELF files and to be very compact and simple. A
new command-line option -gctf enables the generation of CTF.
* GCC can now generate debugging information in BTF. This is a debugging
format mainly used in BPF programs and the Linux kernel. The compiler can
generate BTF for any target, when enabled with the command-line option -
gbtf
GCC 12.1
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).
https://gcc.gnu.org/bugzilla/buglist.cgi?bug_status=RESOLVED&resolution=FIXED&target_milestone=12.0
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