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GCC 12 Release Series — Changes, New Features, and Fixes
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<h1>GCC 12 Release Series<br/>Changes, New Features, and Fixes</h1>
<p>
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
<a href="porting_to.html">Porting to GCC 12</a> page and the
<a href="../onlinedocs/index.html#current">full GCC documentation</a>.
</p>
<!-- .................................................................. -->
<h2>Caveats</h2>
<ul>
<li>
An <a name="zero_width_bitfields">ABI</a> 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 <a href="https://gcc.gnu.org/PR102024">PR102024</a>,
since the <a href="https://gcc.gnu.org/PR42217">PR42217</a> 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.
<code>struct S { float a; int : 0; float b; }</code> or
<code>struct T { float c; int : 0; }</code> 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
<a href="#mips_zero_width_fields">the MIPS specific entry</a>.
GCC 12 on the above targets will report such incompatibilities as
warnings or other diagnostics unless <code>-Wno-psabi</code> is used.
</li>
<li>
<strong>C:</strong>
Computed gotos require a pointer type now.
</li>
<li>
<strong>C++:</strong>
Two non-standard <code>std::pair</code> constructors have been deprecated.
These allowed the use of an rvalue and a literal <code>0</code> to
construct a pair containing a move-only type and a pointer.
The <code>nullptr</code> keyword should be used to initialize the pointer
member instead of a literal <code>0</code>, as this is portable to other
C++ implementations.
</li>
<li>The configuration option <code>--enable-libstdcxx-allocator</code>
no longer supports the <code>bitmap</code>, <code>mt</code>, and
<code>pool</code> arguments. Those configurations had been broken for
some time.
</li>
<li>
<strong>Fortran:</strong>
OpenMP code using the <code>omp_lib.h</code> include file can no longer be
compiled with <code>-std=f95</code> but now requires at least
<code>-std=f2003</code>. Alternatively, use the <code>omp_lib</code> module,
which still supports <code>-std=f95</code> and is recommended to be used
instead in general.
</li>
<li>
OpenMP offloading to Intel MIC has been deprecated and will be removed
in a future release.
</li>
<li>
The <code>cr16</code> target with the <code>cr16-*-*</code> configuration
has been obsoleted and will be removed in a future release.
</li>
<li>
The <code>hppa[12]*-*-hpux10*</code> and <code>hppa[12]*-*-hpux11*</code>
configurations targeting 32-bit PA-RISC with HP-UX have been obsoleted and
will be removed in a future release.
</li>
<li>
The <code>m32c*-*-rtems*</code> configuration has been obsoleted and will
be removed in a future release.
<li>
The support for the <code>m32r-*-linux*</code>, <code>m32rle-*-linux*</code>,
<code>m68k*-*-openbsd*</code> and <code>vax-*-openbsd*</code> configurations
has been removed.
</li>
<li>
<strong>STABS:</strong>
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.
</li>
<li>The optimization level <code>-Ofast</code> now implies
<code>-fno-semantic-interposition</code>.
</li>
</ul>
<!-- .................................................................. -->
<h2 id="general">General Improvements</h2>
<ul>
<li>Vectorization is enabled at <code>-O2</code> which is now equivalent to the
original <code>-O2 -ftree-vectorize -fvect-cost-model=very-cheap</code>.
Note that default vectorizer cost model has been changed which used to behave
as <code>-fvect-cost-model=cheap</code> were specified.
</li>
<li>
GCC now supports the
<a href="https://clang.llvm.org/docs/ShadowCallStack.html">
ShadowCallStack</a> sanitizer, which can be enabled using the
command-line option
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Instrumentation-Options.html#index-fsanitize_003dshadow-call-stack">
<code>-fsanitize=shadow-call-stack</code></a>. This sanitizer currently
only works on AArch64 targets and it requires an environment in
which all code has been compiled with <code>-ffixed-r18</code>.
Its primary initial user is the Linux kernel.
</li>
</ul>
<!-- .................................................................. -->
<h2 id="languages">New Languages and Language specific improvements</h2>
<ul>
<li>OpenMP
<ul>
<li>OpenMP 5.0 support has been extended: The <code>close</code> map
modifier and the <code>affinity</code> clause are now supported.
In addition Fortran gained additionally the following features which were
available in C and C++ before: <code>declare variant</code> is now
available, <code>depobj</code>, <code>mutexinoutset</code> and
<code>iterator</code> can now also be used with the <code>depend</code>
clause, <code>defaultmap</code> has been updated for OpenMP 5.0, and the
<code>loop</code> directive and combined directives involving the
<code>master</code> directive have been added.</li>
<li>The following OpenMP 5.1 features have been added: support for
expressing OpenMP directives as C++ 11 attributes, the <code>masked</code>
and <code>scope</code> construct, the <code>nothing</code> and
<code>error</code> directives, and using <code>primary</code> with the
<code>proc_bind</code> clause and <code>OMP_PROC_BIND</code> environment
variable, the <code>reproducible</code> and <code>unconstrained</code>
modifiers to the <code>order</code> clause, and, for C/C++ only, the
<code>align</code> and <code>allocator</code> modifiers to the
<code>allocate</code> clause and the <code>atomic</code> extensions are
now available. The <code>OMP_PLACE</code> environment variable supports
the OpenMP 5.1 features. In addition the <code>OMP_NUM_TEAMS</code> and
<code>OMP_TEAMS_THREAD_LIMIT</code> 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.</li>
<li>The <a
href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/libgomp/OpenMP-Implementation-Status.html"
>OpenMP Implementation Status</a> can be found in the libgomp manual.</li>
</ul>
</li>
<li id="openacc">
Version 2.6 of the <a href="https://www.openacc.org/">OpenACC</a>
specification continues to be maintained and improved in the C, C++ and
Fortran compilers.
See the <a href="https://gcc.gnu.org/wiki/OpenACC/Implementation%20Status#status-12">implementation
status</a> section on the OpenACC wiki page and the
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/libgomp/Enabling-OpenACC.html">
run-time library documentation</a> for further information.
In addition to general performance tuning and bug fixing, new features
include:
<ul>
<li>
OpenACC worker parallelism for <a href="#amdgcn">AMD GPUs</a>
(already for a long time supported for <a href="#nvptx">Nvidia
GPUs</a>).
</li>
<li>
Data privatization/sharing at the OpenACC gang level.
</li>
<li>
Considerable improvements for the experimental OpenACC 'kernels'
decomposition
(<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Optimize-Options.html#index-param"><code>--param
openacc-kernels=decompose</code></a>).
</li>
<li>
A new warning
flag <a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Warning-Options.html#index-Wopenacc-parallelism"><code>-Wopenacc-parallelism</code></a>
to warn about potentially suboptimal choices related to OpenACC
parallelism.
</li>
</ul>
</li>
<li>The offload target code generation for OpenMP and OpenACC can now
be better adjusted using the new <a
href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/C-Dialect-Options.html#index-foffload-options"
><code>-foffload-options=</code></a> flag and the pre-existing but now
documented <a
href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/C-Dialect-Options.html#index-foffload"
><code>-foffload=</code></a> flag.
</li>
</ul>
<h3 id="ada">Ada</h3>
<ul>
<li>Ada 2022
<ul>
<li>Added the <code>-gnat2022</code> flag to indicate strict Ada
2022 compliance. The old <code>-gnat2020</code> flag is now
deprecated.</li>
<li>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.</li>
<li>Continuous improvements to the Ada 2022 standard since GCC 11.</li>
<li>Greatly improved compile time support. More functions can now
have the <code>with Static</code> aspect and can be used in more
contexts.</li>
</ul>
</li>
<li>Ada 2022 extensions. The use of the <code>-gnatX</code> flag is
necessary to access these features as they are not considered
stable or standard.
<ul>
<li>Fixed lower bound for unconstrained arrays.
<ul>
<li><code>type Matrix is array (Natural range 0 .. <>,
Natural range 0 .. <>) of Integer;</code> is now valid.</li>
<li>Subtypes can also specify a lower bound: <code>subtype
String_1 is String (1 .. <>);</code>. Boundaries from slices
will "slide" to the correct lower bound of the subtype.</li>
</ul>
</li>
<li>Generalized <code>Object.Operand</code> notation. The follwing
code is now valid <code>V.Add_Element(42);</code>,
with <code>V</code> being a vector, for example.</li>
<li>Additional <code>when</code> constructs. Keywords
<code>return</code>, <code>goto</code> and <code>raise</code>
can now use <code>when</code> in addition to the existing
<code>exit when</code>. The following expression is therefore
now valid <code>raise Constraint_Error with "Element is null"
when Element = null;</code></li>
<li>Pattern matching
<ul>
<li>The <code>case</code> 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 <pre>
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);
</pre></li>
</ul>
</li>
</ul>
</li>
<li><code>gnatfind</code> and <code>gnatxref</code>, which were
already deprecated, have been removed.</li>
<li>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.</li>
<li>Enable return-slot optimization for <code>Pure</code>
functions.</li>
<li>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.</li>
<li>Improvements to embedded-RTOS targets such as RTEMS, VxWorks and
QNX. Older targets were removed or cleaned.</li>
<li>Added some <a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gnat_rm/Security-Hardening-Features.html#Security-Hardening-Features">hardening features</a>.</li>
</ul>
<h3 id="c-family">C family</h3>
<ul>
<li>Support for <code>__builtin_shufflevector</code> compatible with
the clang language extension was added.</li>
<li>Support for attribute <code>unavailable</code> was added.</li>
<li>A new built-in function, <code>__builtin_assoc_barrier</code>, was added.
It can be used to inhibit re-association of floating-point
expressions.</li>
<li>Support for <code>__builtin_dynamic_object_size</code> compatible with
the clang language extension was added.</li>
<li>New warnings:
<ul>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Warning-Options.html#index-Wbidi-chars"><code>-Wbidi-chars</code></a>
warns about potentially misleading UTF-8
bidirectional control characters. The default is
<code>-Wbidi-chars=unpaired</code>
(<a href="https://gcc.gnu.org/PR103026">PR103026</a>)</li>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Warning-Options.html#index-Warray-compare"><code>-Warray-compare</code></a>
warns about comparisons between two operands of
array type (<a href="https://gcc.gnu.org/PR97573">PR97573</a>)</li>
</ul>
</li>
<li>Enhancements to existing warnings:
<ul>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Warning-Options.html#index-Wattributes"><code>-Wattributes</code></a>
has been extended so that it's
possible to use <code>-Wno-attributes=ns::attr</code> or
<code>-Wno-attributes=ns::</code> to suppress warnings about unknown scoped
attributes (in C++11 and C2X). Similarly,
<code>#pragma GCC diagnostic ignored_attributes "vendor::attr"</code> can
be used to achieve the same effect
(<a href="https://gcc.gnu.org/PR101940">PR101940</a>)</li>
</ul>
</li>
</ul>
<h3 id="c">C</h3>
<ul>
<li>Some new features from the upcoming C2X revision of the ISO C
standard are supported with <code>-std=c2x</code>
and <code>-std=gnu2x</code>. 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 <code>-std=c2x
-Wpedantic</code>.
<ul>
<li>Digit separators (as in C++) are supported for C2X.</li>
<li>The <code>#elifdef</code> and <code>#elifndef</code>
preprocessing directives are now supported.</li>
<li>The <code>printf</code> and <code>scanf</code> format checking
with <a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Warning-Options.html#index-Wformat"><code>-Wformat</code></a>
now supports the <code>%b</code> format
specified by C2X for binary integers, and the <code>%B</code>
format recommended by C2X for <code>printf</code>.
</ul></li>
</ul>
<h3 id="cxx">C++</h3>
<ul>
<li>Several C++23 features have been implemented:
<ul>
<li><a href="https://wg21.link/p1938">P1938R3</a>, <code>if consteval</code>
(<a href="https://gcc.gnu.org/PR100974">PR100974</a>)</li>
<li><a href="https://wg21.link/p0849">P0849R8</a>, <code>auto(x)</code>:
decay-copy in the language
(<a href="https://gcc.gnu.org/PR103049">PR103049</a>)</li>
<li><a href="https://wg21.link/p2242">P2242R3</a>, Non-literal variables (and
labels and gotos) in constexpr functions
(<a href="https://gcc.gnu.org/PR102612">PR102612</a>)</li>
<li><a href="https://wg21.link/p2334">P2334R1</a>, Support for preprocessing
directives <code>elifdef</code> and <code>elifndef</code>
(<a href="https://gcc.gnu.org/PR102616">PR102616</a>)</li>
<li><a href="https://wg21.link/p2360">P2360R0</a>, Extend <em>init-statement</em>
to allow <em>alias-declaration</em>
(<a href="https://gcc.gnu.org/PR102617">PR102617</a>)</li>
<li><a href="https://wg21.link/p2128">P2128R6</a>, Multidimensional subscript
operator</li>
<li><a href="https://wg21.link/cwg2397">DR 2397</a>, <code>auto</code> specifier
for pointers and references to arrays
(<a href="https://gcc.gnu.org/PR100975">PR100975</a>)</li>
</ul>
</li>
<li>Several C++ Defect Reports have been resolved, e.g.:
<ul>
<li><a href="https://wg21.link/cwg960">DR 960</a>, Covariant functions and
lvalue/rvalue references</li>
<li><a href="https://wg21.link/cwg1227">DR 1227</a>, Mixing immediate and
non-immediate contexts in deduction failure</li>
<li><a href="https://wg21.link/cwg1315">DR 1315</a>, Restrictions on non-type
template arguments in partial specializations</li>
<li><a href="https://wg21.link/cwg2082">DR 2082</a>, Referring to parameters
in unevaluated operands of default arguments</li>
<li><a href="https://wg21.link/cwg2351">DR 2351</a>, <code>void{}</code></li>
<li><a href="https://wg21.link/cwg2374">DR 2374</a>, Overly permissive
specification of <code>enum</code> direct-list-initialization</li>
<li><a href="https://wg21.link/cwg2397">DR 2397</a>, <code>auto</code> specifier
for pointers and references to arrays</li>
<li><a href="https://wg21.link/cwg2446">DR 2446</a>, Questionable type-dependency
of <em>concept-ids</em></li>
</ul>
</li>
<li>New command-line option <code>-fimplicit-constexpr</code> can be used to
make inline functions implicitly constexpr
(<a href="https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=87c2080b">git</a>)</li>
<li>New command-line option <code>-ffold-simple-inlines</code> can be used
to fold calls to certain trivial inline functions (currently
<code>std::move</code>, <code>std::forward</code>,
<code>std::addressof</code> and <code>std::as_const</code>). 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. <code>std::move</code> versus
<code>static_cast</code>). This flag is enabled by default when
<code>-fno-inline</code> is not active.</li>
<li>Deduction guides can be declared at class scope
(<a href="https://gcc.gnu.org/PR79501">PR79501</a>)</li>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Warning-Options.html#index-Wuninitialized"><code>-Wuninitialized</code></a>
warns about using uninitialized variables in
member initializer lists (<a href="https://gcc.gnu.org/PR19808">PR19808</a>)
</li>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Warning-Options.html#index-Wint-in-bool-context"><code>-Wint-in-bool-context</code></a>
is now disabled when instantiating
a template (<a href="https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=3a2b12bc">git</a>)</li>
<li>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
<em>decl-specifier-seq</em>.
(<a href="https://gcc.gnu.org/PR99032">PR99032</a>)</li>
<li>New warning options for C++ language mismatches:
<code>-Wc++11-extensions</code>, <code>-Wc++14-extensions</code>,
<code>-Wc++17-extensions</code>, <code>-Wc++20-extensions</code>,
and <code>-Wc++23-extensions</code>. 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.</li>
<li>New warning
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Warning-Options.html#index-Wmissing-requires"><code>-Wmissing-requires</code></a>
warns about missing <code>requires</code>
(<a href="https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=e18e56c7">git</a>)</li>
<li>The existing <code>std::is_constant_evaluated</code> in <code>if</code>
warning was extended to warn in more cases
(<a href="https://gcc.gnu.org/PR100995">PR100995</a>)</li>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Warning-Options.html#index-Waddress"><code>-Waddress</code></a>
has been enhanced so that it now warns about, for
instance, comparing the address of a nonstatic member function to null
(<a href="https://gcc.gnu.org/PR102103">PR102103</a>)</li>
<li>Errors about narrowing are no longer hidden if they occur in system
headers</li>
<li>Ordered comparison of null pointers is now rejected
(<a href="https://gcc.gnu.org/PR99701">PR99701</a>)</li>
<li>Anonymous structs with bases are now rejected
(<a href="https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=3ead06c1">git</a>)</li>
<li>The compiler rejects taking the address of an immediate member function
(<a href="https://gcc.gnu.org/PR102753">PR102753</a>)</li>
<li>The compiler has support for C++20
<code>__cpp_lib_is_pointer_interconvertible</code> and
<code>__cpp_lib_is_layout_compatible</code> to help the C++
library implement <a href="https://wg21.link/p0466">P0466</a>,
Layout-compatibility and Pointer-interconvertibility Traits
(<a href="https://gcc.gnu.org/PR101539">PR101539</a>)</li>
<li>Memory usage of constraint subsumption has been improved
(<a href="https://gcc.gnu.org/PR100828">PR100828</a>)</li>
<li><code>constinit thread_local</code> variables are optimized better
(<a href="https://gcc.gnu.org/PR101786">PR101786</a>)</li>
<li>Support for C++17 <code>std::hardware_destructive_interference_size</code>
was added, along with the
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Warning-Options.html#index-Winterference-size"><code>-Winterference-size</code></a>
warning
(<a href="https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=76b75018">git</a>)</li>
<li>Many bugs in the CTAD handling have been fixed
(<a href="https://gcc.gnu.org/PR101344">PR101344</a>,
<a href="https://gcc.gnu.org/PR101883">PR101883</a>,
<a href="https://gcc.gnu.org/PR89062">PR89062</a>,
<a href="https://gcc.gnu.org/PR101233">PR101233</a>,
<a href="https://gcc.gnu.org/PR88252">PR88252</a>,
<a href="https://gcc.gnu.org/PR86439">PR86439</a>,
<a href="https://gcc.gnu.org/PR98832">PR98832</a>,
<a href="https://gcc.gnu.org/PR102933">PR102933</a> ...)</li>
<li>Two-stage name lookup for dependent operator expressions has been
corrected (<a href="https://gcc.gnu.org/PR51577">PR51577</a>)</li>
<li>Several issues with constrained variable templates have been fixed
(<a href="https://gcc.gnu.org/PR98486">PR98486</a>)</li>
<li>The compiler performs less instantiating when doing speculative constant
evaluation
(<a href="https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=1595fe44">git</a>)</li>
<li>Various diagnostic improvements; e.g., a more precise caret location for
pointer-to-member expressions</li>
<li>The new <code>-fconstexpr-fp-except</code> flag allows IEC559 floating point
exceptions in constant-expressions.</li>
</ul>
<h4 id="libstdcxx">Runtime Library (libstdc++)</h4>
<ul>
<li>Improved experimental C++20 support, including:
<ul>
<li><code>std::vector</code>, <code>std::basic_string</code>,
<code>std::optional</code>, and <code>std::variant</code>
can be used in <code>constexpr</code> functions.</li>
<li><code>std::make_shared</code> for arrays with default initialization,
and <code>std::atomic<std::shared_ptr<T>></code>.</li>
<li>Layout-compatibility and pointer-interconvertibility traits.</li>
</ul>
</li>
<li>Improved experimental C++23 support, including:
<ul>
<li>Monadic operations for <code>std::optional</code>.</li>
<li><code>std::move_only_function</code></li>
<li><code><spanstream></code></li>
<li><code>std::basic_string::resize_and_overwrite</code></li>
<li><code>std::unique_ptr</code>
can be used in <code>constexpr</code> functions.</li>
<li><code><stacktrace></code>
(not built by default, requires linking to an extra library).</li>
<li><code><stdatomic.h></code></li>
<li><code>std::invoke_r</code></li>
<li><code>constexpr std::type_info::operator==</code></li>
</ul>
</li>
</ul>
<!-- <h3 id="d">D</h3> -->
<h3 id="fortran">Fortran</h3>
<ul>
<li>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 <code>bind(c)</code>
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.
</li>
<li>
GCC 12 now uses <code>OPERATION</code> as the name of the function to
the <code>CO_REDUCE</code> intrinsic for the pairwise reduction, thus
conforming to the Fortran 2018 standard. Previous versions
used <code>OPERATOR</code> which conforms to TS 18508.
</li>
<li>
On POWER systems which support it, the <code>-mabi=ieeelongdouble</code>
option now selects the IEEE 128-bit floating point format
for <code>REAL(KIND=16)</code>.
<code>R16_IBM</code> and <code>R16_IEEE</code> have been added to the
<code>-fconvert</code> option, the <code>CONVERT</code> specifyer of
the <code>OPEN</code> statement and the <code>GFORTRAN_CONVERT_UNIT</code>
environment variable.
</li>
</ul>
<!-- <h3 id="go">Go</h3> -->
<!-- .................................................................. -->
<h2 id="jit">libgccjit</h2>
<ul>
<li>The libgccjit API gained 30 new entry points:
<ul>
<li>17 new "reflection" entrypoints for querying functions and types (<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/compatibility.html#libgccjit-abi-16"><code>LIBGCCJIT_ABI_16</code></a>)
</li>
<li>
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/expressions.html#c.gcc_jit_lvalue_set_tls_model"><code>gcc_jit_lvalue_set_tls_model</code></a>
for supporting thread-local variables
(<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/compatibility.html#libgccjit-abi-17"><code>LIBGCCJIT_ABI_17</code></a>)
</li>
<li>
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/expressions.html#c.gcc_jit_lvalue_set_link_section"><code>gcc_jit_lvalue_set_link_section</code></a>
for setting the link section of global variables, analogous to
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Common-Variable-Attributes.html#index-section-variable-attribute"><code>__attribute__((section(".section")))</code></a>
(<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/compatibility.html#libgccjit-abi-18"><code>LIBGCCJIT_ABI_18</code></a>)
</li>
<li>4 new entrypoints for initializing global variables and creating
constructors for rvalues
(<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/compatibility.html#libgccjit-abi-19"><code>LIBGCCJIT_ABI_19</code></a>)
</li>
<li>
Support for sized integer types, including 128-bit integers and helper functions for such types
(<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/compatibility.html#libgccjit-abi-20"><code>LIBGCCJIT_ABI_20</code></a>)
</li>
<li>
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/expressions.html#c.gcc_jit_context_new_bitcast"><code>gcc_jit_context_new_bitcast</code></a> for reinterpreting the bits of an rvalue as a different type
(<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/compatibility.html#libgccjit-abi-21"><code>LIBGCCJIT_ABI_21</code></a>)
</li>
<li>
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/expressions.html#c.gcc_jit_lvalue_set_register_name"><code>gcc_jit_lvalue_set_register_name</code></a> for setting a specific register for a variable
(<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/compatibility.html#libgccjit-abi-22"><code>LIBGCCJIT_ABI_22</code></a>)
</li>
<li>
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/contexts.html#c.gcc_jit_context_set_bool_print_errors_to_stderr"><code>gcc_jit_context_set_bool_print_errors_to_stderr</code></a>
(<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/compatibility.html#libgccjit-abi-23"><code>LIBGCCJIT_ABI_23</code></a>)
</li>
<li>
2 new entrypoints for setting the alignment of a variable
(<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/compatibility.html#libgccjit-abi-24"><code>LIBGCCJIT_ABI_24</code></a>)
</li>
</ul>
</li>
<li>libgccjit has gained support for the use of various atomic builtins
(<a href="https://gcc.gnu.org/PR96066">PR96066</a>,
<a href="https://gcc.gnu.org/PR96067">PR96067</a>)
</li>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/jit/topics/expressions.html#c.gcc_jit_context_new_cast">gcc_jit_context_new_cast</a>
is now able to handle truncation and extension between different
integer types
(<a href="https://gcc.gnu.org/PR95498">PR95498</a>)
</li>
</ul>
<!-- .................................................................. -->
<h2 id="targets">New Targets and Target Specific Improvements</h2>
<h3 id="arm-targets">AArch64 & arm</h3>
<ul>
<li>Newer revisions of the Arm Architecture are supported as arguments to the
<code>-march</code> option: <code>armv8.7-a, armv8.8-a, armv9-a</code>.</li>
<li>The Arm Cortex-A510 CPU is now supported through the <code>cortex-a510
</code> argument to the <code>-mcpu</code> and <code>-mtune</code> options.
</li>
<li>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.
</li>
</ul>
<h3 id="aarch64">AArch64</h3>
<ul>
<li>A number of new CPUs are supported through the <code>-mcpu</code> and
<code>-mtune</code> options (GCC identifiers in parentheses).
<ul>
<li>Ampere-1 (<code>ampere1</code>).</li>
<li>Arm Cortex-A710 (<code>cortex-a710</code>).</li>
<li>Arm Cortex-X2 (<code>cortex-x2</code>).</li>
</ul>
</li>
<li>The 64-byte atomic load/store intrinsics to accelerator memory from the
<a href="https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/arm-a-profile-architecture-developments-2020">
2020 Arm Architecture extensions</a> are supported through the
<code>+ls64</code> option extension.</li>
<li>Initial code generation support is supported for hardware instructions
used to accelerate the <code>memcpy</code>,<code>memmove</code> and
<code>memset</code> standard functions. These instructions can be generated
when compiling with the <code>+mops</code>option extension.</li>
<li>The ACLE Advanced SIMD intrinsics accessible through the
<code>arm_neon.h</code> header have been significantly reimplemented and
generate higher-performing code than previous GCC versions.</li>
<li>The option <code>-mtune=neoverse-512tvb</code> 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.</li>
</ul>
<h3 id="amdgcn">AMD Radeon (GCN)</h3>
<ul>
<li>Debug experience with ROCGDB has been improved.</li>
<li>Support for the type <code>__int128_t</code>/<code>integer(kind=16)</code>
was added.</li>
<li>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.</li>
</ul>
<!-- <h3 id="arc">ARC</h3> -->
<h3 id="arm">arm</h3>
<ul>
<li>Support is added for accessing the stack canary value via the TLS register
through the <code>-fstack-protector-guard=tls</code> and
<code>-mstack-protector-guard-offset=</code> options. This intended for use
in Linux kernel development. Please refer to the documentation for more
details.</li>
</ul>
<!-- <h3 id="avr">AVR</h3> -->
<h3 id="bpf">BPF</h3>
<ul>
<li>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.
</li>
</ul>
<h3 id="x86">IA-32/x86-64</h3>
<ul>
<li>New ISA extension support for Intel AVX512-FP16 was added.
AVX512FP16 intrinsics are available via the <code>-mavx512fp16</code>
compiler switch.
</li>
<li>For both C and C++ the <code>_Float16</code> type is supported on
x86 systems with SSE2 enabled. Without <code>{-mavx512fp16}</code>,
all operations will be emulated in software and <code>float</code>
instructions.
<li>Mitigation against straight line speculation (SLS) for function
return and indirect jump is supported via
<code>-mharden-sls=[none|all|return|indirect-jmp]</code>.
</li>
<li>Add CS prefix to call and jmp to indirect thunk with branch target
in r8-r15 registers via <code>-mindirect-branch-cs-prefix</code>.
</li>
<li>Always use global offset table (GOT) to access external data and
function symbols when the new <code>-mno-direct-extern-access</code>
command-line option is specified.
</li>
</ul>
<h3 id="loongarch">LoongArch</h3>
<ul>
<li>Support for the LoongArch architecture instruction set has been added.</li>
<li>The Loongson CPU codename LA464 and LoongArch 64-bit generic CPU codename loongarch64
are supported through the <code>-march=</code> and <code>-mtune=</code> options
(GCC identifiers in parentheses).</li>
<ul>
<li>Loongson LA464 core (<code>la464</code>).</li>
<li>LoongArch 64-bit generic core (<code>loongarch64</code>).</li>
</ul>
</ul>
<h3 id="mips">MIPS</h3>
<ul>
<li>The <a id="mips_zero_width_fields">ABI passing arguments
containing zero-width fields</a> (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 <code>-Wno-psabi</code> is used.
</li>
<li>The <a id="mips_cxx17_empty_bases">ABI returning values
containing C++17 empty bases</a> 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
<code>-Wno-psabi</code> is used.
</li>
</ul>
<!-- <h3 id="mep">MeP</h3> -->
<!-- <h3 id="msp430">MSP430</h3> -->
<!-- <h3 id="nds32">NDS32</h3> -->
<!-- <h3 id="nios2">Nios II</h3> -->
<h3 id="nvptx">NVPTX</h3>
<ul>
<li>The <code>-march</code> flag has been added. The <code>-misa</code>
flag is now considered an alias of the <code>-march</code> flag.</li>
<li>Support for PTX ISA target architectures <code>sm_53</code>,
<code>sm_70</code>, <code>sm_75</code> and <code>sm_80</code> has been
added. These can be specified using the <code>-march</code> flag.</li>
<li>The default PTX ISA target architecture has been set back
to <code>sm_30</code>, to fix support for <code>sm_30</code> boards.</li>
<li>The <code>-march-map</code> flag has been added. The
<code>-march-map</code> value will be mapped to an valid
<code>-march</code> flag value. For instance,
<code>-march-map=sm_50</code> maps to <code>-march=sm_35</code>.
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 <code>-march</code> flag.</li>
<li>The <code>-mptx</code> flag has been added to specify the PTX ISA version
for the generated code; permitted values are <code>3.1</code>
(matches previous GCC versions), <code>6.0</code>, <code>6.3</code>,
and <code>7.0</code>. If not specified, the used version is the minimal
version required for <code>-march</code> but at least <code>6.0</code>.
</li>
<li>An <code>mptx-3.1</code> multilib was added. This allows using older
drivers which do not support PTX ISA version 6.0.</li>
<li>The new <code>__PTX_SM__</code> predefined macro allows code to check the
PTX ISA target architecture being targeted by the compiler.</li>
<li>The new <code>__PTX_ISA_VERSION_MAJOR__</code>
and <code>__PTX_ISA_VERSION_MINOR__</code> predefined macros allows code
to check the PTX ISA version being targeted by the compiler.</li>
</ul>
<!-- <h3 id="hppa">PA-RISC</h3> -->
<h3 id="powerpc">PowerPC / PowerPC64 / RS6000</h3>
<ul>
<li>
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 <a href="https://gcc.gnu.org/bugzilla/">GCC Bugzilla</a>.
</li>
<li>
The built-in functions <code>__builtin_get_texasr</code>,
<code>__builtin_get_texasru</code>, <code>__builtin_get_tfhar</code>,
<code>__builtin_get_tfiar</code>, <code>__builtin_set_texasr</code>,
<code>__builtin_set_texasru</code>, <code>__builtin_set_tfhar</code>, and
<code>__builtin_set_tfiar</code> now behave as documented in all
supported configurations. On prior releases, the arguments and return
values of these functions were treated as <code>unsigned long long</code>
instead of as <code>unsigned long</code>, when the options <code>-m32
-mpowerpc64</code> were in effect.
</li>
<li>
The overloaded built-in functions <code>vec_cntlz_lsbb</code> and
<code>vec_cnttz_lsbb</code> now behave as documented. On prior releases,
these built-in functions had incorrect semantics on little-endian targets.
</li>
</ul>
<h3 id="pru">PRU</h3>
<ul>
<li>The <a
href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Named-Address-Spaces.html#PRU-Named-Address-Spaces"
><code>__regio_symbol</code></a> variable qualifier has been added.
It allows easier access in C programs to the <code>__R30</code> and
<code>__R31</code> CPU I/O registers.
</li>
</ul>
<!-- <h3 id="s390">S/390, System z, IBM z Systems</h3> -->
<h3 id="riscv">RISC-V</h3>
<ul>
<li>Default ISA spec version was bump to 20191213, more detail see this <a
href="https://groups.google.com/a/groups.riscv.org/g/sw-dev/c/aE1ZeHHCYf4">
announcement</a></li>
<li>New ISA extension support for zba, zbb, zbc, zbs was added.</li>
<li>New ISA extension support for vector and scalar crypto was added, only
support architecture testing marco and <code>-march=</code> parsing.</li>
<li>The option <code>-mtune=thead-c906</code> is added to tune for T-HEAD
c906 cores.</li>
</li>
</ul>
<!-- <h3 id="rx">RX</h3> -->
<!-- <h3 id="sh">SH</h3> -->
<!-- <h3 id="sparc">SPARC</h3> -->
<!-- <h3 id="Tile">Tile</h3> -->
<!-- .................................................................. -->
<h2 id="os">Operating Systems</h2>
<!-- <h3 id="aix">AIX</h3> -->
<!-- <h3 id="fuchsia">Fuchsia</h3> -->
<!-- <h3 id="dragonfly">DragonFly BSD</h3> -->
<!-- <h3 id="freebsd">FreeBSD</h3> -->
<!-- <h3 id="gnulinux">GNU/Linux</h3> -->
<!-- <h3 id="rtems">RTEMS</h3> -->
<!-- <h3 id="solaris">Solaris</h3> -->
<!-- <h3 id="vxmils">VxWorks MILS</h3> -->
<!-- <h3 id="windows">Windows</h3> -->
<!-- .................................................................. -->
<!-- <h2>Documentation improvements</h2> -->
<h2 id="analyzer">Improvements to Static Analyzer</h2>
<ul>
<li>The analyzer has gained a <a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Static-Analyzer-Options.html#index-Wanalyzer-use-of-uninitialized-value"><code>-Wanalyzer-use-of-uninitialized-value</code></a>
warning, similar to
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Warning-Options.html#index-Wuninitialized"><code>-Wuninitialized</code></a>
and
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Warning-Options.html#index-Wmaybe-uninitialized"><code>-Wmaybe-uninitialized</code></a>,
but based on an interprocedural path-sensitive analysis
(<a href="https://gcc.gnu.org/PR95006">PR95006</a>).
<p>Such warnings are not disabled by the new
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Optimize-Options.html#index-ftrivial-auto-var-init"><code>-ftrivial-auto-var-init</code></a>
(see below), as the latter is considered a mitigation option.</p>
</li>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Static-Analyzer-Options.html#index-Wanalyzer-write-to-const"><code>-Wanalyzer-write-to-const</code></a>
and
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Static-Analyzer-Options.html#index-Wanalyzer-write-to-string-literal"><code>-Wanalyzer-write-to-string-literal</code></a>
will now check for
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Common-Function-Attributes.html"><code>__attribute__ ((access, ....))</code></a>
on calls to externally-defined functions, and complain about read-only
regions pointed to by arguments marked with a <code>write_only</code>
or <code>read_write</code> attribute
(<a href="https://gcc.gnu.org/PR104793">PR104793</a>).
</li>
<li>The analyzer's "taint" mode, activated by
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Static-Analyzer-Options.html#index-fanalyzer-checker"><code>-fanalyzer-checker=taint</code></a>
(in addition to <a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Static-Analyzer-Options.html#index-fanalyzer"><code>-fanalyzer</code></a>),
has gained four new taint-based warnings:
<ul>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Static-Analyzer-Options.html#index-Wanalyzer-tainted-allocation-size"><code>-Wanalyzer-tainted-allocation-size</code></a>
for e.g. attacker-controlled <code>malloc</code>
and <code>alloca</code>,
</li>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Static-Analyzer-Options.html#index-Wanalyzer-tainted-divisor"><code>-Wanalyzer-tainted-divisor</code></a>
for detecting where an attacker can inject a divide-by-zero,
</li>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Static-Analyzer-Options.html#index-Wanalyzer-tainted-offset"><code>-Wanalyzer-tainted-offset</code></a>
for attacker-controlled pointer offsets,
</li>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Static-Analyzer-Options.html#index-Wanalyzer-tainted-size"><code>-Wanalyzer-tainted-size</code></a>
for attacker-controlled values being used as a size parameter to
calls to <code>memset</code> or to functions marked with
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Common-Function-Attributes.html"><code>__attribute__ ((access, ....))</code></a>.
</li>
</ul>
<p>The existing
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Static-Analyzer-Options.html#index-Wanalyzer-tainted-array-index"><code>-Wanalyzer-tainted-array-index</code></a>
has been reworded to talk about "attacker-controlled" rather than
"tainted" values, for consistency with the new warnings.
</p>
<p>A new <a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Common-Function-Attributes.html#index-tainted_005fargs-function-attribute"><code>__attribute__ ((tainted_args))</code></a> 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".
</p>
</li>
<li>The analyzer now respects
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Common-Function-Attributes.html#index-const-function-attribute"><code>__attribute__((const))</code></a>:
it will treat such functions as returning the same value when given
the same inputs (<a href="https://gcc.gnu.org/PR104434">PR104434</a>),
and as having no side effects (<a href="https://gcc.gnu.org/PR104576">PR104576</a>).
</li>
<li>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 <code>realloc</code> calls by
splitting the execution path into three possible outcomes for the
call:
<ul>
<li>failure, returning <code>NULL</code></li>
<li>success, growing the buffer in-place without moving it</li>
<li>success, allocating a new buffer, copying the content of the old
buffer to it, and freeing the old buffer</li>
</ul>
</li>
<li>The analyzer's interprocedural path exploration logic is now able to
track calls through function pointers.
</li>
<li>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
(<a href="https://gcc.gnu.org/PR101962">PR101962</a>).
</li>
<li>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 (<a href="https://gcc.gnu.org/PR101570">PR101570</a>).
</li>
<li>The way the analyzer tracks the state of memory along an execution
path has been improved in various ways for GCC 12:
<ul>
<li>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
(<a href="https://gcc.gnu.org/PR95006">PR95006</a>).
</li>
<li>Various optimizations have been added, speeding up the analysis
on a particularly problematic source file from 4 minutes down to
17 seconds
(<a href="https://gcc.gnu.org/PR104943">PR104943</a>,
<a href="https://gcc.gnu.org/PR104954">PR104954</a>, and
<a href="https://gcc.gnu.org/PR104955">PR104955</a>).
</li>
<li>The analyzer now tracks the sizes of dynamically-allocated regions,
both on the heap (via <code>malloc</code> etc) and stack
(via <code>alloca</code>), though none of the analyzer warnings make
use of this yet in GCC 12.</li>
</ul>
</li>
<li>The analyzer's handling of switch statements has been rewritten,
fixing various bugs.
</li>
</ul>
<!-- .................................................................. -->
<!-- <h2 id="plugins">Improvements for plugin authors</h2> -->
<!-- .................................................................. -->
<h2>Other significant improvements</h2>
<h3 id="uninitialized">Eliminating uninitialized variables</h3>
<ul>
<li>GCC can now <a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Optimize-Options.html#index-ftrivial-auto-var-init">initialize all stack variables implicitly</a>, including
padding. This is intended to eliminate all classes of uninitialized
stack variable flaws. Lack of explicit initialization will still
warn when
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Warning-Options.html#index-Wuninitialized"><code>-Wuninitialized</code></a>
is active. For best debugging, use of the new command-line option
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Optimize-Options.html#index-ftrivial-auto-var-init"><code>-ftrivial-auto-var-init=pattern</code></a>
can be used to fill variables with a repeated <code>0xFE</code> 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
<a href="https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Optimize-Options.html#index-ftrivial-auto-var-init"><code>-ftrivial-auto-var-init=zero</code></a>
can be
used to fill variables with <code>0x00</code>, which tends to provide
a safer state for bugs (e.g. pointers are <code>NULL</code>, strings
are <code>NUL</code> filled, and sizes and indices are <code>0</code>).
</li>
</ul>
<h3 id="debug">Debugging formats</h3>
<ul>
<li>GCC can now generate debugging information
in <a href="https://ctfstd.org">CTF</a>, 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 <code>-gctf</code> enables the generation of CTF.
</li>
<li>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 <code>-gbtf</code>
</li>
</ul>
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<h2><a name="12.1">GCC 12.1</a></h2>
<p>This is the <a href="https://gcc.gnu.org/bugzilla/buglist.cgi?bug_status=RESOLVED&resolution=FIXED&target_milestone=12.0">list
of problem reports (PRs)</a> 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).</p>
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