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GCC 5 Release Series — Changes, New Features, and Fixes
- GNU Project - Free Software Foundation (FSF)</title>
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<!-- GCC maintainers, please do not hesitate to update/contribute entries
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<body>
<h1>GCC 5 Release Series<br />Changes, New Features, and Fixes</h1>
<h2>Caveats</h2>
<ul>
<li>The default mode for C is now <code>-std=gnu11</code> instead of
<code>-std=gnu89</code>.</li>
<li>The C++ runtime library (libstdc++) uses a new ABI by default
(see <a href="#libstdcxx">below</a>).</li>
<li>The Graphite framework for loop optimizations no longer requires the
CLooG library, only ISL version 0.14 (recommended) or 0.12.2. The
installation manual contains more information about requirements to
build GCC.</li>
<li>The non-standard C++0x type traits
<code>has_trivial_default_constructor</code>,
<code>has_trivial_copy_constructor</code> and
<code>has_trivial_copy_assign</code> have been deprecated and will
be removed in a future version. The standard C++11 traits
<code>is_trivially_default_constructible</code>,
<code>is_trivially_copy_constructible</code> and
<code>is_trivially_copy_assignable</code> should be used instead.</li>
</ul>
<h2 id="general">General Optimizer Improvements</h2>
<ul>
<li>Inter-procedural optimization improvements:
<ul>
<li>An Identical Code Folding (ICF) pass (controlled via
<code>-fipa-icf</code>) has been added. Compared to the identical
code folding performed by the Gold linker this
pass does not require function sections. It also performs merging
before inlining, so inter-procedural optimizations are aware of the
code re-use. On the other hand not all unifications performed
by a linker are doable by GCC which must honor
aliasing information. During link-time optimization of Firefox,
this pass unifies about 31000 functions, that is 14% overall.</li>
<li>The devirtualization pass was significantly improved by adding
better support for speculative devirtualization and dynamic type
detection. About 50% of virtual calls in Firefox are now
speculatively devirtualized during link-time optimization.</li>
<li>A new comdat localization pass allows the linker to eliminate more
dead code in presence of C++ inline functions.</li>
<li>Virtual tables are now optimized. Local aliases are used to reduce
dynamic linking time of C++ virtual tables on ELF targets and
data alignment has been reduced to limit data segment bloat.</li>
<li>A new <code>-fno-semantic-interposition</code> option can be used
to improve code quality of shared libraries where interposition of
exported symbols is not allowed.</li>
<li>Write-only variables are now detected and optimized out.</li>
<li>With profile feedback the function inliner can now bypass
<code>--param inline-insns-auto</code> and <code>--param
inline-insns-single</code> limits for hot calls.</li>
<li>The IPA reference pass was significantly sped up making it feasible
to enable <code>-fipa-reference</code> with
<code>-fprofile-generate</code>. This also solves a bottleneck
seen when building Chromium with link-time optimization.</li>
<li>The symbol table and call-graph API was reworked to C++ and
simplified.</li>
<li>The interprocedural propagation of constants now also propagates
alignments of pointer parameters. This for example means that the
vectorizer often does not need to generate loop prologues and epilogues
to make up for potential misalignments.</li>
</ul></li>
<li>Link-time optimization improvements:
<ul>
<li>One Definition Rule based merging of C++ types has been implemented.
Type merging enables better devirtualization and alias analysis.
Streaming extra information needed to merge types adds about 2-6% of
memory size and object size increase. This can be controlled by
<code>-flto-odr-type-merging</code>.</li>
<li><p>Command-line optimization and target options are now streamed on
a per-function basis and honored by the link-time optimizer.
This change makes link-time optimization a more transparent
replacement of per-file optimizations.
It is now possible to build projects that require
different optimization
settings for different translation units (such as
<code>-ffast-math</code>, <code>-mavx</code>, or
<code>-finline</code>).
Contrary to earlier GCC releases, the optimization and target
options passed on the link command line are ignored.</p>
<p>Note that this applies only to those command-line options
that can be passed to <code>optimize</code> and
<code>target</code> attributes.
Command-line options affecting global code generation
(such as <code>-fpic</code>), warnings
(such as <code>-Wodr</code>),
optimizations affecting the way static variables
are optimized (such as <code>-fcommon</code>), debug output (such as
<code>-g</code>),
and <code>--param</code> parameters can be applied only
to the whole link-time optimization unit.
In these cases, it is recommended to consistently use the same
options at both compile time and link time.</p></li>
<li>GCC bootstrap now uses slim LTO object files.</li>
<li>Memory usage and link times were improved. Tree merging was sped up,
memory usage of GIMPLE declarations and types was reduced, and,
support for on-demand streaming of variable constructors was added.</li>
</ul></li>
<li>Feedback directed optimization improvements:
<ul>
<li>A new auto-FDO mode uses profiles collected by low overhead
profiling tools (perf) instead of more expensive program
instrumentation (via <code>-fprofile-generate</code>). SPEC2006
benchmarks on x86-64 improve by 4.7% with auto-FDO and by 7.3% with
traditional feedback directed optimization.</li>
<li>Profile precision was improved in presence of C++ inline and extern
inline functions.</li>
<li>The new <code>gcov-tool</code> utility allows manipulating
profiles.</li>
<li>Profiles are now more tolerant to source file changes (this can be
controlled by <code>--param profile-func-internal-id</code>).</li>
</ul></li>
<li>Register allocation improvements:
<ul>
<li>A new local register allocator (LRA) sub-pass, controlled by
<code>-flra-remat</code>, implements control-flow sensitive
global register rematerialization. Instead of spilling and
restoring a register value, it is recalculated if it is
profitable. The sub-pass improved SPEC2000 generated code
by 1% and 0.5% correspondingly on ARM and x86-64.</li>
<li>Reuse of the PIC hard register, instead of using a fixed
register, was implemented on x86/x86-64 targets. This
improves generated PIC code performance as more hard
registers can be used. Shared libraries can significantly
benefit from this optimization. Currently it is switched on
only for x86/x86-64 targets. As RA infrastructure is
already implemented for PIC register reuse, other targets
might follow this in the future.</li>
<li>A simple form of inter-procedural RA was implemented. When
it is known that a called function does not use caller-saved
registers, save/restore code is not generated around the
call for such registers. This optimization can be controlled
by <code>-fipa-ra</code></li>
<li>LRA is now much more effective at generating spills of
general registers into vector registers instead of memory on
architectures (e.g., modern Intel processors) where this is
profitable.</li>
</ul></li>
<li>UndefinedBehaviorSanitizer gained a few new sanitization options:
<ul>
<li><code>-fsanitize=float-divide-by-zero</code>: detect floating-point
division by zero;</li>
<li><code>-fsanitize=float-cast-overflow</code>: check that the result
of floating-point type to integer conversions do not overflow;</li>
<li><code>-fsanitize=bounds</code>: enable instrumentation of array
bounds and detect out-of-bounds accesses;</li>
<li><code>-fsanitize=alignment</code>: enable alignment checking, detect
various misaligned objects;</li>
<li><code>-fsanitize=object-size</code>: enable object size checking, detect
various out-of-bounds accesses.</li>
<li><code>-fsanitize=vptr</code>: enable checking of C++ member function
calls, member accesses and some conversions between pointers to base
and derived classes, detect if the referenced object does not have
the correct dynamic type.</li>
</ul>
</li>
<li>Pointer Bounds Checker, a bounds violation detector, has been added and
can be enabled via <code>-fcheck-pointer-bounds</code>. Memory accesses are
instrumented with run-time checks of used pointers against their bounds to
detect pointer bounds violations (overflows). The Pointer Bounds Checker
is available on x86/x86-64 GNU/Linux targets with a new ISA extension
Intel MPX support. See the Pointer Bounds Checker
<a href="https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler">
Wiki page</a> for more details.
</li>
</ul>
<h2 id="languages">New Languages and Language specific improvements</h2>
<ul>
<li id="offload"><a href="http://www.openmp.org/mp-documents/OpenMP4.0.0.pdf">
OpenMP 4.0 specification</a> offloading features are now supported by the C, C++,
and Fortran compilers. Generic changes:
<ul>
<li>Infrastructure (suitable for any vendor).</li>
<li>Testsuite which covers offloading from the
<a href="http://openmp.org/mp-documents/OpenMP4.0.0.Examples.pdf">
OpenMP 4.0 Examples</a> document.</li>
</ul>
Specific for upcoming Intel Xeon Phi products:
<ul>
<li>Run-time library.</li>
<li>Card emulator.</li>
</ul>
</li>
<li id="openacc">
GCC 5 includes a preliminary implementation of the OpenACC 2.0a
specification. OpenACC is intended for programming accelerator devices
such as GPUs. See <a href="https://gcc.gnu.org/wiki/OpenACC">the OpenACC
wiki page</a> for more information.
</li>
</ul>
<!-- h3 id="ada">Ada</h3 -->
<h3 id="c-family">C family</h3>
<ul>
<li>The default setting of the <code>-fdiagnostics-color=</code>
command-line option is now
<a href="https://gcc.gnu.org/install/configure.html">configurable
when building GCC</a> using configuration option
<code>--with-diagnostics-color=</code>. The possible values
are: <code>never</code>, <code>always</code>, <code>auto</code>
and <code>auto-if-env</code>. The new
default <code>auto</code> uses color only when the
standard error is a terminal. The default in GCC 4.9
was <code>auto-if-env</code>, which is equivalent to
<code>auto</code> if there is a
non-empty <code>GCC_COLORS</code> environment variable,
and <code>never</code> otherwise. As in GCC 4.9, an empty
<code>GCC_COLORS</code> variable in the environment will
always disable colors, no matter what the default is or what
command-line options are used.</li>
<li>A new command-line option <code>-Wswitch-bool</code> has been added for
the C and C++ compilers, which warns whenever a <code>switch</code>
statement has an index of boolean type.</li>
<li>A new command-line option <code>-Wlogical-not-parentheses</code> has
been added for the C and C++ compilers, which warns about "logical not"
used on the left hand side operand of a comparison.</li>
<li>A new command-line option <code>-Wsizeof-array-argument</code> has been
added for the C and C++ compilers, which warns when the
<code>sizeof</code> operator is applied to a parameter that has been
declared as an array in a function definition.</li>
<li>A new command-line option <code>-Wbool-compare</code> has been added
for the C and C++ compilers, which warns about boolean expressions
compared with an integer value different from
<code>true</code>/<code>false</code>.</li>
<li>Full support for <a href="https://www.cilkplus.org/">Cilk Plus</a>
has been added to the GCC compiler. Cilk Plus is an extension to
the C and C++ languages to support data and task parallelism.</li>
<li>A new attribute <code>no_reorder</code> prevents reordering of
selected symbols against other such symbols or inline assembler.
This enables to link-time optimize the Linux kernel without having
to resort to <code>-fno-toplevel-reorder</code> that disables
several optimizations.</li>
<li>New preprocessor constructs, <code>__has_include</code>
and <code>__has_include_next</code>, to test the availability of headers
have been added.<br/>
This demonstrates a way to include the header <code><optional></code>
only if it is available:<br/>
<blockquote><pre>
#ifdef __has_include
# if __has_include(<optional>)
# include <optional>
# define have_optional 1
# elif __has_include(<experimental/optional>)
# include <experimental/optional>
# define have_optional 1
# define experimental_optional
# else
# define have_optional 0
# endif
#endif
</pre></blockquote>
The header search paths for <code>__has_include</code>
and <code>__has_include_next</code> are equivalent to those
of the standard directive <code>#include</code>
and the extension <code>#include_next</code> respectively.
</li>
<li>A new built-in function-like macro to determine the existence of an
attribute, <code>__has_attribute</code>, has been added.
The equivalent built-in macro <code>__has_cpp_attribute</code> was
added to C++ to support
<a href="http://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations">
Feature-testing recommendations for C++</a>.
The macro <code>__has_attribute</code> is added to all C-like
languages as an extension:
<blockquote><pre>
int
#ifdef __has_attribute
# if __has_attribute(__noinline__)
__attribute__((__noinline__))
# endif
#endif
foo(int x);
</pre></blockquote>
If an attribute exists, a nonzero constant integer is returned.
For standardized C++ attributes a date is returned, otherwise the
constant returned is 1.
Both <code>__has_attribute</code> and
<code>__has_cpp_attribute</code> will add underscores to an
attribute name if necessary to resolve the name.
For C++11 and onwards the attribute may be scoped.
</li>
<li>A new set of built-in functions for arithmetics with overflow checking
has been added: <code>__builtin_add_overflow</code>,
<code>__builtin_sub_overflow</code> and <code>__builtin_mul_overflow</code>
and for compatibility with clang also other variants.
These builtins have two integral arguments (which don't need to have
the same type), the arguments are extended to infinite precision
signed type, <code>+</code>, <code>-</code> or <code>*</code>
is performed on those, and the result is stored in an integer
variable pointed to by the last argument. If the stored value is
equal to the infinite precision result, the built-in functions return
<code>false</code>, otherwise <code>true</code>. The type of
the integer variable that will hold the result can be different from
the types of the first two arguments. The following snippet
demonstrates how this can be used in computing the size for the
<code>calloc</code> function:
<blockquote><pre>
void *
calloc (size_t x, size_t y)
{
size_t sz;
if (__builtin_mul_overflow (x, y, &sz))
return NULL;
void *ret = malloc (sz);
if (ret) memset (res, 0, sz);
return ret;
}
</pre></blockquote>
On e.g. i?86 or x86-64 the above will result in a <code>mul</code>
instruction followed by a jump on overflow.
</li>
<li>The option <code>-fextended-identifiers</code> is now enabled
by default for C++, and for C99 and later C versions. Various
bugs in the implementation of extended identifiers have been
fixed.</li>
</ul>
<h3 id="c">C</h3>
<ul>
<li>The default mode has been changed to <code>-std=gnu11</code>.</li>
<li>A new command-line option <code>-Wc90-c99-compat</code> has been added
to warn about features not present in ISO C90, but present in ISO
C99.</li>
<li>A new command-line option <code>-Wc99-c11-compat</code> has been added
to warn about features not present in ISO C99, but present in ISO
C11.</li>
<li>It is possible to disable warnings about conversions between pointers
that have incompatible types via a new warning option
<code>-Wno-incompatible-pointer-types</code>; warnings about implicit
incompatible integer to pointer and pointer to integer conversions via
a new warning option <code>-Wno-int-conversion</code>; and warnings
about qualifiers on pointers being discarded via a new warning option
<code>-Wno-discarded-qualifiers</code>.</li>
<li>To allow proper use of const qualifiers with multidimensional arrays,
GCC will not warn about incompatible pointer types anymore for
conversions between pointers to arrays with and without const qualifier
(except when using <code>-pedantic</code>). Instead, a new warning is
emitted only if the const qualifier is lost. This can be controlled with
a new warning option <code>-Wno-discarded-array-qualifiers</code>.</li>
<li>The C front end now generates more precise caret diagnostics.</li>
<li>The <code>-pg</code> command-line option now only affects the current
file in an LTO build.</li>
</ul>
<h3 id="cxx">C++</h3>
<ul>
<li>G++ now supports <a href="../projects/cxx1y.html">C++14</a> variable
templates.</li>
<li><code>-Wnon-virtual-dtor</code> doesn't warn anymore
for <code>final</code> classes.</li>
<li>Excessive template instantiation depth is now a fatal error. This
prevents excessive diagnostics that usually do not help to identify the
problem.</li>
<li>G++ and libstdc++ now implement the feature-testing macros from
<a href="http://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations">Feature-testing
recommendations for C++</a>.</li>
<li>G++ now allows <code>typename</code> in a template template parameter.
<blockquote><pre>
template<template<typename> <b>typename</b> X> struct D; // OK</pre></blockquote>
</li>
<li>G++ now supports <a href="../projects/cxx1y.html">C++14</a> aggregates with non-static data member initializers.
<blockquote><pre>
struct A { int i, j = i; };
A a = { 42 }; // a.j is also 42</pre></blockquote>
</li>
<li>G++ now supports <a href="../projects/cxx1y.html">C++14</a> extended <code>constexpr</code>.
<blockquote><pre>
constexpr int f (int i)
{
int j = 0;
for (; i > 0; --i)
++j;
return j;
}
constexpr int i = f(42); // i is 42</pre></blockquote>
</li>
<li>G++ now supports the <a href="../projects/cxx1y.html">C++14</a> sized
deallocation functions.
<blockquote><pre>
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;</pre></blockquote>
</li>
<li>A new One Definition Rule violation warning (controlled by <code>-Wodr</code>)
detects mismatches in type definitions and virtual table contents
during link-time optimization.</li>
<li>New warnings <code>-Wsuggest-final-types</code> and
<code>-Wsuggest-final-methods</code> help developers
to annotate programs with <code>final</code> specifiers (or anonymous
namespaces) to improve code generation.
These warnings can be used at compile time, but they are more
useful in combination with link-time optimization.</li>
<li>G++ no longer supports
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3639.html">N3639</a>
variable length arrays, as they were removed from the C++14 working paper
prior to ratification. GNU VLAs are still supported, so VLA support is
now the same in C++14 mode as in C++98 and C++11 modes.</li>
<li>G++ now allows passing a non-trivially-copyable class via C varargs,
which is conditionally-supported with implementation-defined semantics in
the standard. This uses the same calling convention as a normal value
parameter.</li>
<li>G++ now defaults to <code>-fabi-version=0</code>
and <code>-fabi-compat-version=2</code>. So various mangling bugs are
fixed, but G++ will still emit aliases with the old, wrong mangling where
feasible. <code>-Wabi</code> continues to warn about differences.</li>
</ul>
<h4 id="libstdcxx">Runtime Library (libstdc++)</h4>
<ul>
<li>A <a href="/onlinedocs/gcc-5.1.0/libstdc++/manual/using_dual_abi.html">Dual
ABI</a> is provided by the library. A new ABI is enabled by default.
The old ABI is still supported and can be used by defining the macro
<code>_GLIBCXX_USE_CXX11_ABI</code> to <code>0</code> before
including any C++ standard library headers. </li>
<li>A new implementation of <code>std::string</code> is enabled by default,
using the <em>small string optimization</em> instead of
<em>copy-on-write</em> reference counting.</li>
<li> A new implementation of <code>std::list</code> is enabled by default,
with an O(1) <code>size()</code> function; </li>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/libstdc++/manual/status.html#status.iso.2011">
Full support for C++11</a>, including the following new features:
<ul>
<li> <code>std::deque</code> and <code>std::vector<bool></code>
meet the allocator-aware container requirements;</li>
<li> movable and swappable iostream classes;</li>
<li> support for <code>std::align</code> and
<code>std::aligned_union</code>;</li>
<li> type traits <code>std::is_trivially_copyable</code>,
<code>std::is_trivially_constructible</code>,
<code>std::is_trivially_assignable</code> etc.;
</li>
<li> I/O manipulators <code>std::put_time</code>, <code>std::get_time</code>,
<code>std::hexfloat</code> and <code>std::defaultfloat</code>;
</li>
<li> generic locale-aware <code>std::isblank</code>; </li>
<li> locale facets for Unicode conversion; </li>
<li> atomic operations for <code>std::shared_ptr</code>; </li>
<li> <code>std::notify_all_at_thread_exit()</code> and functions
for making futures ready at thread exit.</li>
</ul>
</li>
<li> Support for the C++11 <code>hexfloat</code> manipulator changes how
the <code>num_put</code> facet formats floating point types when
<code>ios_base::fixed|ios_base::scientific</code> is set in a stream's
<code>fmtflags</code>. This change affects all language modes, even
though the C++98 standard gave no special meaning to that combination
of flags. To prevent the use of hexadecimal notation for floating point
types use <code>str.unsetf(std::ios_base::floatfield)</code> to clear
the relevant bits in <code>str.flags()</code>.
</li>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/libstdc++/manual/status.html#status.iso.2014">
Full experimental support for C++14</a>, including the following
new features:
<ul>
<li> <code>std::is_final</code> type trait; </li>
<li> heterogeneous comparison lookup in associative containers. </li>
<li> global functions <code>cbegin</code>, <code>cend</code>, <code>rbegin</code>,
<code>rend</code>, <code>crbegin</code>, and <code>crend</code> for
range access to containers, arrays and initializer lists. </li>
</ul>
</li>
<li><a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/libstdc++/manual/status.html#status.iso.2014">
Improved experimental support for the Library Fundamentals TS</a>, including:
<ul>
<li> class <code>std::experimental::any</code>; </li>
<li> function template <code>std::experimental::apply</code>; </li>
<li> function template <code>std::experimental::sample</code>; </li>
<li> function template <code>std::experimental::search</code> and
related searcher types; </li>
<li> variable templates for type traits; </li>
<li> function template <code>std::experimental::not_fn</code>.</li>
</ul>
</li>
<li>New random number distributions <code>logistic_distribution</code> and
<code>uniform_on_sphere_distribution</code> as extensions.</li>
<li><a href="https://sourceware.org/gdb/current/onlinedocs/gcc-5.1.0/gdb/Xmethods-In-Python.html">GDB
Xmethods</a> for containers and <code>std::unique_ptr</code>.</li>
</ul>
<h3 id="fortran">Fortran</h3>
<ul>
<li>Compatibility notice:<ul>
<li>The version of the module files (.mod) has been incremented.</li>
<li>For free-form source files,
<a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/gfortran/Error-and-Warning-Options.html"><code>-Werror=line-truncation</code></a>
is now enabled by default; note that comments exceeding the line length
are not diagnosed. (For fixed-form source code, the same warning is
available but turned off by default, such that excess characters are
ignored. <code>-ffree-line-length-<em>n</em></code> and
<code>-ffixed-line-length-<em>n</em></code> can be used to modify the
default line lengths of 132 and 72 columns, respectively.)</li>
<li>The <code>-Wtabs</code> option is now more sensible: with
<code>-Wtabs</code> the compiler warns if it encounters tabs and with
<code>-Wno-tabs</code> this warning is turned off. Before,
<code>-Wno-tabs</code> warned and <code>-Wtabs</code> turned the warning
off. As before, the warning is also enabled by <code>-Wall</code>,
<code>-pedantic</code> and the <code>f95</code>, <code>f2003</code>,
<code>f2008</code> and <code>f2008ts</code> options of <code>-std=</code>.</li>
</ul></li>
<li>Incomplete support for colorizing diagnostics emitted by
gfortran has been added. The
option <code><a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/gcc/Language-Independent-Options.html"
>-fdiagnostics-color</a></code> controls when color is used in
diagnostics. The default value of this option can
be <a href="https://gcc.gnu.org/install/configure.html">configured
when building GCC</a>. The <code>GCC_COLORS</code> environment
variable can be used to customize the colors or disable coloring
completely. Sample diagnostics output:<br/>
<pre>
$ gfortran -fdiagnostics-color=always -Wuse-without-only test.f90
<b>test.f90:6:1:</b>
0 continue
<b style='color:lime'>1</b>
<b style='color:red'>Error:</b> Zero is not a valid statement label at (1)
<b>test.f90:9:6:</b>
USE foo
<b style='color:lime'>1</b>
<b style='color:magenta'>Warning:</b> USE statement at (1) has no ONLY qualifier [-Wuse-without-only]
</pre></li>
<li>The <code>-Wuse-without-only</code> option has been added to warn when a
<code>USE</code> statement has no <code>ONLY</code> qualifier and, thus,
implicitly imports all public entities of the used module.</li>
<li>Formatted READ and WRITE statements now work correctly in locale-aware
programs. For more information and potential caveats, see
<a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/gfortran/Thread-safety-of-the-runtime-library.html">Section
5.3 Thread-safety of the runtime library in the manual</a>.</li>
<li><a href="https://gcc.gnu.org/wiki/Fortran2003Status">Fortran 2003</a>:
<ul>
<li>The intrinsic IEEE modules (<code>IEEE_FEATURES</code>,
<code>IEEE_EXCEPTIONS</code> and <code>IEEE_ARITHMETIC</code>) are
now supported.</li>
</ul></li>
<li><a href="https://gcc.gnu.org/wiki/Fortran2008Status">Fortran 2008</a>:
<ul>
<li><a href="https://gcc.gnu.org/wiki/Coarray">Coarrays</a>: Full
experimental support of Fortran 2008's coarrays with
<code>-fcoarray=lib</code> except for allocatable/pointer
components of derived-type coarrays. GCC currently only ships with a
single-image library (<code>libcaf_single</code>), but
multi-image support based on MPI and GASNet is provided by the libraries
of the <a href="http://www.opencoarrays.org/">OpenCoarrays project</a>.
</li>
</ul></li>
<li>TS18508 Additional Parallel Features in Fortran:
<ul>
<li>Support for the collective intrinsic subroutines <code>CO_MAX</code>,
<code>CO_MIN</code>, <code>CO_SUM</code>, <code>CO_BROADCAST</code> and
<code>CO_REDUCE</code> has been added, including
<code>-fcoarray=lib</code> support.</li>
<li>Support for the new atomic intrinsics has been added, including
<code>-fcoarray=lib</code> support.</li>
</ul></li>
<li>Fortran 2015:
<ul>
<li>Support for <code>IMPLICIT NONE (external, type)</code>.</li>
<li><code>ERROR STOP</code> is now permitted in pure procedures.</li>
</ul></li>
</ul>
<h3 id="go">Go</h3>
<ul>
<li>GCC 5 provides a complete implementation of the Go 1.4.2
release.</li>
<li>Building GCC 5 with Go enabled will install two new
programs: <a href="http://golang.org/cmd/go">go</a>
and <a href="http://golang.org/cmd/gofmt">gofmt</a>.</li>
</ul>
<!--h3 id="java">Java (GCJ)</h3-->
<h2 id="jit">libgccjit</h2>
<p>New in GCC 5 is the ability to build GCC as a shared library for embedding
in other processes (such as interpreters), suitable for Just-In-Time
compilation to machine code.</p>
<p>The shared library has a <a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/jit/intro/index.html">C API</a>
and a
<a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/jit/cp/index.html">C++ wrapper API</a>
providing some "syntactic sugar".
There are also bindings available from 3rd parties for
<a href="https://github.com/davidmalcolm/pygccjit">Python</a> and for
<a href="https://github.com/ibuclaw/gccjitd">D</a>.</p>
<p>For example, this library can be used by interpreters for
<a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/jit/intro/tutorial04.html">compiling
functions from bytecode to machine code</a>.</p>
<p>The library can also be used for ahead-of-time compilation, enabling
GCC to be plugged into a pre-existing frontend. An example of using
this to build a compiler for an esoteric language we'll refer to as "brainf"
can be seen <a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/jit/intro/tutorial05.html">
here</a>.</p>
<p>libgccjit is licensed under the GPLv3 (or at your option, any later version)</p>
<p>It should be regarded as experimental at this time.</p>
<h2 id="targets">New Targets and Target Specific Improvements</h2>
<h3 id="aarch64">AArch64</h3>
<ul>
<li>Code generation for the ARM Cortex-A57 processor has been improved.
A more accurate instruction scheduling model for the processor is
now used, and a number of compiler tuning parameters have been set
to offer increased performance when compiling with
<code>-mcpu=cortex-a57</code> or <code>-mtune=cortex-a57</code>.
</li>
<li> A workaround for the ARM Cortex-A53 erratum 835769 has been
added and can be enabled by giving the
<code>-mfix-cortex-a53-835769</code> option.
Alternatively it can be enabled by default by configuring GCC with the
<code>--enable-fix-cortex-a53-835769</code> option.
</li>
<li> The optional cryptographic extensions to the ARMv8-A architecture
are no longer enabled by default when specifying the
<code>-mcpu=cortex-a53</code>, <code>-mcpu=cortex-a57</code> or
<code>-mcpu=cortex-a57.cortex-a53</code> options. To enable these
extensions add <code>+crypto</code> to the value of
<code>-mcpu</code> or <code>-march</code> e.g.
<code>-mcpu=cortex-a53+crypto</code>.
</li>
<li> Support has been added for the following processors
(GCC identifiers in parentheses): ARM Cortex-A72
(<code>cortex-a72</code>) and initial support for its big.LITTLE
combination with the ARM Cortex-A53 (<code>cortex-a72.cortex-a53</code>),
Cavium ThunderX (<code>thunderx</code>), Applied Micro X-Gene 1
(<code>xgene1</code>), and Samsung Exynos M1 (<code>exynos-m1</code>).
The GCC identifiers can be used
as arguments to the <code>-mcpu</code> or <code>-mtune</code> options,
for example: <code>-mcpu=xgene1</code> or
<code>-mtune=cortex-a72.cortex-a53</code>.
Using <code>-mcpu=cortex-a72</code> requires a version of GNU binutils
that has support for the Cortex-A72.
</li>
<li>The transitional options <code>-mlra</code> and <code>-mno-lra</code>
have been removed. The AArch64 backend now uses the local register
allocator (LRA) only.
</li>
</ul>
<h3 id="arm">ARM</h3>
<ul>
<li>Thumb-1 assembly code is now generated in unified syntax. The new option
<code>-masm-syntax-unified</code> specifies whether inline assembly
code is using unified syntax. By default the option is off which means
non-unified syntax is used. However this is subject to change in future releases.
Eventually the non-unified syntax will be deprecated.
</li>
<li> It is now a configure-time error to use the <code>--with-cpu</code>
configure option with either of <code>--with-tune</code> or
<code>--with-arch</code>.
</li>
<li>Code generation for the ARM Cortex-A57 processor has been improved.
A more accurate instruction scheduling model for the processor is
now used, and a number of compiler tuning parameters have been set
to offer increased performance when compiling with
<code>-mcpu=cortex-a57</code> or <code>-mtune=cortex-a57</code>.
</li>
<li> Support has been added for the following processors
(GCC identifiers in parentheses): ARM Cortex-A17 (<code>cortex-a17</code>) and
initial support for its big.LITTLE combination with the ARM Cortex-A7
(<code>cortex-a17.cortex-a7</code>), ARM Cortex-A72
(<code>cortex-a72</code>) and initial support for its big.LITTLE
combination with the ARM Cortex-A53 (<code>cortex-a72.cortex-a53</code>),
ARM Cortex-M7 (<code>cortex-m7</code>), Applied Micro X-Gene 1
(<code>xgene1</code>), and Samsung Exynos M1 (<code>exynos-m1</code>).
The GCC identifiers can be used
as arguments to the <code>-mcpu</code> or <code>-mtune</code> options,
for example: <code>-mcpu=xgene1</code> or
<code>-mtune=cortex-a72.cortex-a53</code>.
Using <code>-mcpu=cortex-a72</code> requires a version of GNU binutils
that has support for the Cortex-A72.
</li>
<li> The deprecated option <code>-mwords-little-endian</code>
has been removed.
</li>
<li> The options <code>-mapcs</code>, <code>-mapcs-frame</code>,
<code>-mtpcs-frame</code> and <code>-mtpcs-leaf-frame</code>
which are only applicable to the old ABI have been deprecated.
</li>
<li>The transitional options <code>-mlra</code> and <code>-mno-lra</code>
have been removed. The ARM backend now uses the local register allocator
(LRA) only.
</li>
</ul>
<h3 id="x86">IA-32/x86-64</h3>
<ul>
<li>New ISA extensions support
<a href="https://software.intel.com/sites/default/files/managed/0d/53/319433-022.pdf">
AVX-512{BW,DQ,VL,IFMA,VBMI}</a> of Intel's CPU
codenamed Skylake Server was added to GCC. That includes inline
assembly support, new intrinsics, and basic autovectorization. These
new AVX-512 extensions are available via
the following GCC switches: AVX-512 Vector Length EVEX feature:
<code>-mavx512vl</code>, AVX-512 Byte and Word instructions:
<code>-mavx512bw</code>, AVX-512 Dword and Qword instructions:
<code>-mavx512dq</code>, AVX-512 FMA-52 instructions:
<code>-mavx512ifma</code> and for AVX-512 Vector Bit Manipulation
Instructions: <code>-mavx512vbmi</code>.</li>
<li>New ISA extensions support
<a href="https://software.intel.com/sites/default/files/managed/0d/53/319433-022.pdf">
Intel MPX</a> was added to GCC. This new extension is available via the
<code>-mmpx</code> compiler switch. Intel MPX is a set of processor features which,
with compiler, run-time library and OS support, brings increased robustness to
software by run-time checking pointer references against their bounds.
In GCC Intel MPX is supported by Pointer Bounds Checker and libmpx run-time
libraries.</li>
<li>The new <code>-mrecord-mcount</code> option for <code>-pg</code>
generates a Linux kernel style table of pointers to
<code>mcount</code> or <code>__fentry__</code> calls at the beginning
of functions. The new <code>-mnop-mcount</code> option in addition
also generates nops in place of the <code>__fentry__</code> or
<code>mcount</code> call, so that a call per function can be later
patched in. This can be used for low overhead tracing or hot code
patching.</li>
<li>The new <code>-malign-data</code> option controls how
GCC aligns variables. <code>-malign-data=compat</code> uses
increased alignment compatible with GCC 4.8 and earlier,
<code>-malign-data=abi</code> uses alignment as specified by
the psABI, and <code>-malign-data=cacheline</code> uses increased
alignment to match the cache line size.
<code>-malign-data=compat</code> is the default.</li>
<li>The new <code>-mskip-rax-setup</code> option skips setting
up the RAX register when SSE is disabled and there are no variable
arguments passed in vector registers. This can be used to
optimize the Linux kernel.</li>
</ul>
<h3 id="mips">MIPS</h3>
<ul>
<li>MIPS Releases 3 and 5 are now directly supported. Use the
command-line options <code>-mips32r3</code>, <code>-mips64r3</code>,
<code>-mips32r5</code> and <code>-mips64r5</code> to enable
code-generation for these processors.</li>
<li>The Imagination P5600 processor is now supported using the
<code>-march=p5600</code> command-line option.</li>
<li>The Cavium Octeon3 processor is now supported using the
<code>-march=octeon3</code> command-line option.</li>
<li>MIPS Release 6 is now supported using the <code>-mips32r6</code>
and <code>-mips64r6</code> command-line options.</li>
<li>The o32 ABI has been modified and extended. The o32 64-bit
floating-point register support is now obsolete and has been removed.
It has been replaced by three ABI extensions FPXX, FP64A, and FP64.
The meaning of the <code>-mfp64</code> command-line option has
changed. It is now used to enable the FP64A and FP64 ABI extensions.
<ul>
<li>The FPXX extension requires that code generated to access
double-precision values use even-numbered registers. Code that
adheres to this extension is link-compatible with all other o32
double-precision ABI variants and will execute correctly in all
hardware FPU modes. The command-line options <code>-mabi=32
-mfpxx</code> can be used to enable this extension. MIPS II is
the minimum processor required.</li>
<li>The o32 FP64A extension requires that floating-point registers be
64-bit and odd-numbered single-precision registers are not
allowed. Code that adheres to the o32 FP64A variant is
link-compatible with all other o32 double-precision ABI variants.
The command-line options <code>-mabi=32 -mfp64 -mno-odd-spreg
</code> can be used to enable this extension. MIPS32R2 is the
minimum processor required.</li>
<li>The o32 FP64 extension also requires that floating-point registers
be 64-bit, but permits the use of single-precision registers.
Code that adheres to the o32 FP64 variant is link-compatible with
o32 FPXX and o32 FP64A variants only, i.e. it is not compatible
with the original o32 double-precision ABI. The command-line
options <code>-mabi=32 -mfp64 -modd-spreg</code> can be used to
enable this extension. MIPS32R2 is the minimum processor required.
</li>
</ul>
The new ABI variants can be enabled by default using the configure time
options <code>--with-fp-32=[32|xx|64]</code> and
<code>--with(out)-odd-sp-reg-32</code>. It is strongly recommended that
all vendors begin to set o32 FPXX as the default ABI. This will be
required to run the generated code on MIPSR5 cores in conjunction with
future MIPS SIMD (MSA) code and MIPSR6 cores.</li>
<li>GCC will now pass all floating-point options to the assembler if GNU
binutils 2.25 is used. As a result, any inline assembly code that
uses hard-float instructions should be amended to include a
<code>.set</code> directive to override the global assembler options
when compiling for soft-float targets.</li>
</ul>
<h3 id="nds32">NDS32</h3>
<ul>
<li>The variadic function ABI implementation is now compatible with
past Andes toolchains where the caller uses registers to pass arguments
and the callee is in charge of pushing them on stack.</li>
<li>The options <code>-mforce-fp-as-gp</code>, <code>-mforbid-fp-as-gp</code>,
and <code>-mex9</code> have been removed since they are not yet available
in the nds32 port of GNU binutils.</li>
<li>A new option <code>-mcmodel=[small|medium|large]</code> supports
varied code models on code generation. The <code>-mgp-direct</code>
option became meaningless and can be discarded.</li>
</ul>
<h3 id="rx">RX</h3>
<ul>
<li>A new command line option <code>-mno-allow-string-insns</code> can be
used to disable the generation of the <code>SCMPU</code>, <code>SMOVU</code>,
<code>SMOVB</code>, <code>SMOVF</code>, <code>SUNTIL</code>, <code>SWHILE</code>
and <code>RMPA</code> instructions. An erratum released by Renesas shows
that it is unsafe to use these instructions on addresses within the I/O
space of the processor. The new option can be used when the programmer is
concerned that the I/O space might be accessed. The default is still to
enable these instructions.</li>
</ul>
<h3 id="sh">SH</h3>
<ul>
<li>The compiler will now pass the appropriate <code>--isa=</code> option
to the assembler.</li>
<li>The default handling for the <code>GBR</code> has been changed from
call clobbered to call preserved. The old behavior can be reinstated by
specifying the option <code>-fcall-used-gbr</code>.</li>
<li>Support for the SH4A <code>fpchg</code> instruction has been added which
will be utilized when switching between single and double precision FPU
modes.</li>
<li>The compiler no longer uses the <code>__fpscr_values</code> array for
switching between single and double FPU precision modes on non-SH4A targets.
Instead mode switching will now be performed by storing, modifying and
reloading the <code>FPSCR</code>, so that other <code>FPSCR</code> bits are
preserved across mode switches. The <code>__fpscr_values</code> array that
is defined in libgcc is still present for backwards compatibility, but it
will not be referenced by compiler generated code anymore.</li>
<li>New builtin functions <code>__builtin_sh_get_fpscr</code> and
<code>__builtin_sh_set_fpscr</code> have been added. The
<code>__builtin_sh_set_fpscr</code> function will mask the specified bits
in such a way that the <code>SZ</code>, <code>PR</code> and <code>FR</code>
mode bits will be preserved, while changing the other bits. These new
functions do not reference the <code>__fpscr_values</code> array. The old
functions <code>__set_fpscr</code> and <code>__get_fpscr</code> in libgcc
which access the <code>__fpscr_values</code> array are still present for
backwards compatibility, but their usage is highly discouraged.</li>
<li>Some improvements to code generated for <code>__atomic</code> built-in
functions.</li>
<li>When compiling for SH2E the compiler will no longer force the usage of
delay slots for conditional branch instructions <code>bt</code> and
<code>bf</code>. The old behavior can be reinstated (e.g. to work around a
hardware bug in the original SH7055) by specifying the new option
<code>-mcbranch-force-delay-slot</code>.</li>
</ul>
<h2 id="os">Operating Systems</h2>
<h3 id="dragonfly">DragonFly BSD</h3>
<ul>
<li>GCC now supports the DragonFly BSD operating system.</li>
</ul>
<h3 id="freebsd">FreeBSD</h3>
<ul>
<li>GCC now supports the FreeBSD operating system for the arm port
through the <code>arm*-*-freebsd*</code> target triplets.</li>
</ul>
<h3 id="vxmils">VxWorks MILS</h3>
<ul>
<li>GCC now supports the MILS (Multiple Independent Levels
of Security) variant of WindRiver's VxWorks operating system
for PowerPC targets.</li>
</ul>
<!-- h2>Documentation improvements</h2-->
<h2>Other significant improvements</h2>
<h3 id="gcc-ar"></h3>
<ul>
<li>The <code>gcc-ar</code>, <code>gcc-nm</code>, <code>gcc-ranlib</code>
wrappers now understand a <code>-B</code> option to set the compiler
to use.</li>
</ul>
<h3 id="driver"></h3>
<ul>
<li>When the new command-line option <code>-freport-bug</code> is
used, GCC automatically generates a developer-friendly reproducer
whenever an internal compiler error is encountered.</li>
</ul>
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<a href="https://gcc.gnu.org/onlinedocs/">GCC manuals</a>. If that fails,
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mailing list might help.
Comments on these web pages and the development of GCC are welcome on our
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All of <a href="https://gcc.gnu.org/lists.html">our lists</a>
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</address>
<p>Copyright (C)
<a href="http://www.fsf.org">Free Software Foundation, Inc.</a>
Verbatim copying and distribution of this entire article is
permitted in any medium, provided this notice is preserved.</p>
<p style="margin-bottom:0;">These pages are
<a href="https://gcc.gnu.org/about.html">maintained by the GCC team</a>.
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