-std=gnu11 instead of
-std=gnu89.has_trivial_default_constructor,
has_trivial_copy_constructor and
has_trivial_copy_assign have been deprecated and will
be removed in a future version. The standard C++11 traits
is_trivially_default_constructible,
is_trivially_copy_constructible and
is_trivially_copy_assignable should be used instead.-fipa-icf) 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.-fno-semantic-interposition option can be used
to improve code quality of shared libraries where interposition of
exported symbols is not allowed.--param inline-insns-auto and --param
inline-insns-single limits for hot calls.-fipa-reference with
-fprofile-generate. This also solves a bottleneck
seen when building Chromium with link-time optimization.-flto-odr-type-merging.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
-ffast-math, -mavx, or
-finline).
Contrary to earlier GCC releases, the optimization and target
options passed on the link command line are ignored.
Note that this applies only to those command-line options
that can be passed to optimize and
target attributes.
Command-line options affecting global code generation
(such as -fpic), warnings
(such as -Wodr),
optimizations affecting the way static variables
are optimized (such as -fcommon), debug output (such as
-g),
and --param 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.
-fprofile-generate). SPEC2006
benchmarks on x86-64 improve by 4.7% with auto-FDO and by 7.3% with
traditional feedback directed optimization.gcov-tool utility allows manipulating
profiles.--param profile-func-internal-id).-flra-remat, 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.-fipa-ra-fsanitize=float-divide-by-zero: detect floating-point
division by zero;-fsanitize=float-cast-overflow: check that the result
of floating-point type to integer conversions do not overflow;-fsanitize=bounds: enable instrumentation of array
bounds and detect out-of-bounds accesses;-fsanitize=alignment: enable alignment checking, detect
various misaligned objects;-fsanitize=object-size: enable object size checking, detect
various out-of-bounds accesses.-fsanitize=vptr: 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.-fcheck-pointer-bounds. 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
Wiki page for more details.
-fdiagnostics-color=
command-line option is now
configurable
when building GCC using configuration option
--with-diagnostics-color=. The possible values
are: never, always, auto
and auto-if-env. The new
default auto uses color only when the
standard error is a terminal. The default in GCC 4.9
was auto-if-env, which is equivalent to
auto if there is a
non-empty GCC_COLORS environment variable,
and never otherwise. As in GCC 4.9, an empty
GCC_COLORS variable in the environment will
always disable colors, no matter what the default is or what
command-line options are used.-Wswitch-bool has been added for
the C and C++ compilers, which warns whenever a switch
statement has an index of boolean type.-Wlogical-not-parentheses has
been added for the C and C++ compilers, which warns about "logical not"
used on the left hand side operand of a comparison.-Wsizeof-array-argument has been
added for the C and C++ compilers, which warns when the
sizeof operator is applied to a parameter that has been
declared as an array in a function definition.-Wbool-compare has been added
for the C and C++ compilers, which warns about boolean expressions
compared with an integer value different from
true/false.no_reorder 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 -fno-toplevel-reorder that disables
several optimizations.__has_include
and __has_include_next, to test the availability of headers
have been added.<optional>
only if it is available:The header search paths for#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
__has_include
and __has_include_next are equivalent to those
of the standard directive #include
and the extension #include_next respectively.
__has_attribute, has been added.
The equivalent built-in macro __has_cpp_attribute was
added to C++ to support
Feature-testing recommendations for C++.
The macro __has_attribute is added to all C-like
languages as an extension:
If an attribute exists, a nonzero constant integer is returned. For standardized C++ attributes a date is returned, otherwise the constant returned is 1. Bothint #ifdef __has_attribute # if __has_attribute(__noinline__) __attribute__((__noinline__)) # endif #endif foo(int x);
__has_attribute and
__has_cpp_attribute will add underscores to an
attribute name if necessary to resolve the name.
For C++11 and onwards the attribute may be scoped.
__builtin_add_overflow,
__builtin_sub_overflow and __builtin_mul_overflow
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, +, - or *
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
false, otherwise true. 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
calloc function:
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;
}
mul
instruction followed by a jump on overflow.
-fextended-identifiers 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.-std=gnu11.-Wc90-c99-compat has been added
to warn about features not present in ISO C90, but present in ISO
C99.-Wc99-c11-compat has been added
to warn about features not present in ISO C99, but present in ISO
C11.-Wno-incompatible-pointer-types; warnings about implicit
incompatible integer to pointer and pointer to integer conversions via
a new warning option -Wno-int-conversion; and warnings
about qualifiers on pointers being discarded via a new warning option
-Wno-discarded-qualifiers.-pedantic). Instead, a new warning is
emitted only if the const qualifier is lost. This can be controlled with
a new warning option -Wno-discarded-array-qualifiers.-pg command-line option now only affects the current
file in an LTO build.-Wnon-virtual-dtor doesn't warn anymore
for final classes.typename in a template template parameter.
template<template<typename> typename X> struct D; // OK
struct A { int i, j = i; };
A a = { 42 }; // a.j is also 42constexpr.
constexpr int f (int i)
{
int j = 0;
for (; i > 0; --i)
++j;
return j;
}
constexpr int i = f(42); // i is 42void operator delete (void *, std::size_t) noexcept; void operator delete[] (void *, std::size_t) noexcept;
-Wodr)
detects mismatches in type definitions and virtual table contents
during link-time optimization.-Wsuggest-final-types and
-Wsuggest-final-methods help developers
to annotate programs with final 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.-fabi-version=0
and -fabi-compat-version=2. So various mangling bugs are
fixed, but G++ will still emit aliases with the old, wrong mangling where
feasible. -Wabi continues to warn about differences._GLIBCXX_USE_CXX11_ABI to 0 before
including any C++ standard library headers. std::string is enabled by default,
using the small string optimization instead of
copy-on-write reference counting.std::list is enabled by default,
with an O(1) size() function; std::deque and std::vector<bool>
meet the allocator-aware container requirements;std::align and
std::aligned_union;std::is_trivially_copyable,
std::is_trivially_constructible,
std::is_trivially_assignable etc.;
std::put_time, std::get_time,
std::hexfloat and std::defaultfloat;
std::isblank; std::shared_ptr; std::notify_all_at_thread_exit() and functions
for making futures ready at thread exit.hexfloat manipulator changes how
the num_put facet formats floating point types when
ios_base::fixed|ios_base::scientific is set in a stream's
fmtflags. 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 str.unsetf(std::ios_base::floatfield) to clear
the relevant bits in str.flags().
std::is_final type trait; cbegin, cend, rbegin,
rend, crbegin, and crend for
range access to containers, arrays and initializer lists. std::experimental::any; std::experimental::apply; std::experimental::sample; std::experimental::search and
related searcher types; std::experimental::not_fn.logistic_distribution and
uniform_on_sphere_distribution as extensions.std::unique_ptr.-Werror=line-truncation
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. -ffree-line-length-n and
-ffixed-line-length-n can be used to modify the
default line lengths of 132 and 72 columns, respectively.)-Wtabs option is now more sensible: with
-Wtabs the compiler warns if it encounters tabs and with
-Wno-tabs this warning is turned off. Before,
-Wno-tabs warned and -Wtabs turned the warning
off. As before, the warning is also enabled by -Wall,
-pedantic and the f95, f2003,
f2008 and f2008ts options of -std=.-fdiagnostics-color controls when color is used in
diagnostics. The default value of this option can
be configured
when building GCC. The GCC_COLORS environment
variable can be used to customize the colors or disable coloring
completely. Sample diagnostics output:
$ gfortran -fdiagnostics-color=always -Wuse-without-only test.f90
test.f90:6:1:
0 continue
1
Error: Zero is not a valid statement label at (1)
test.f90:9:6:
USE foo
1
Warning: USE statement at (1) has no ONLY qualifier [-Wuse-without-only]
-Wuse-without-only option has been added to warn when a
USE statement has no ONLY qualifier and, thus,
implicitly imports all public entities of the used module.IEEE_FEATURES,
IEEE_EXCEPTIONS and IEEE_ARITHMETIC) are
now supported.-fcoarray=lib except for allocatable/pointer
components of derived-type coarrays. GCC currently only ships with a
single-image library (libcaf_single), but
multi-image support based on MPI and GASNet is provided by the libraries
of the OpenCoarrays project.
CO_MAX,
CO_MIN, CO_SUM, CO_BROADCAST and
CO_REDUCE has been added, including
-fcoarray=lib support.-fcoarray=lib support.IMPLICIT NONE (external, type).ERROR STOP is now permitted in pure procedures.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.
The shared library has a C API and a C++ wrapper API providing some "syntactic sugar". There are also bindings available from 3rd parties for Python and for D.
For example, this library can be used by interpreters for compiling functions from bytecode to machine code.
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 here.
libgccjit is licensed under the GPLv3 (or at your option, any later version)
It should be regarded as experimental at this time.
-mcpu=cortex-a57 or -mtune=cortex-a57.
-mfix-cortex-a53-835769 option.
Alternatively it can be enabled by default by configuring GCC with the
--enable-fix-cortex-a53-835769 option.
-mcpu=cortex-a53, -mcpu=cortex-a57 or
-mcpu=cortex-a57.cortex-a53 options. To enable these
extensions add +crypto to the value of
-mcpu or -march e.g.
-mcpu=cortex-a53+crypto.
cortex-a72) and initial support for its big.LITTLE
combination with the ARM Cortex-A53 (cortex-a72.cortex-a53),
Cavium ThunderX (thunderx), Applied Micro X-Gene 1
(xgene1), and Samsung Exynos M1 (exynos-m1).
The GCC identifiers can be used
as arguments to the -mcpu or -mtune options,
for example: -mcpu=xgene1 or
-mtune=cortex-a72.cortex-a53.
Using -mcpu=cortex-a72 requires a version of GNU binutils
that has support for the Cortex-A72.
-mlra and -mno-lra
have been removed. The AArch64 backend now uses the local register
allocator (LRA) only.
-masm-syntax-unified 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.
--with-cpu
configure option with either of --with-tune or
--with-arch.
-mcpu=cortex-a57 or -mtune=cortex-a57.
cortex-a17) and
initial support for its big.LITTLE combination with the ARM Cortex-A7
(cortex-a17.cortex-a7), ARM Cortex-A72
(cortex-a72) and initial support for its big.LITTLE
combination with the ARM Cortex-A53 (cortex-a72.cortex-a53),
ARM Cortex-M7 (cortex-m7), Applied Micro X-Gene 1
(xgene1), and Samsung Exynos M1 (exynos-m1).
The GCC identifiers can be used
as arguments to the -mcpu or -mtune options,
for example: -mcpu=xgene1 or
-mtune=cortex-a72.cortex-a53.
Using -mcpu=cortex-a72 requires a version of GNU binutils
that has support for the Cortex-A72.
-mwords-little-endian
has been removed.
-mapcs, -mapcs-frame,
-mtpcs-frame and -mtpcs-leaf-frame
which are only applicable to the old ABI have been deprecated.
-mlra and -mno-lra
have been removed. The ARM backend now uses the local register allocator
(LRA) only.
-mavx512vl, AVX-512 Byte and Word instructions:
-mavx512bw, AVX-512 Dword and Qword instructions:
-mavx512dq, AVX-512 FMA-52 instructions:
-mavx512ifma and for AVX-512 Vector Bit Manipulation
Instructions: -mavx512vbmi.-mmpx 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.-mrecord-mcount option for -pg
generates a Linux kernel style table of pointers to
mcount or __fentry__ calls at the beginning
of functions. The new -mnop-mcount option in addition
also generates nops in place of the __fentry__ or
mcount call, so that a call per function can be later
patched in. This can be used for low overhead tracing or hot code
patching.-malign-data option controls how
GCC aligns variables. -malign-data=compat uses
increased alignment compatible with GCC 4.8 and earlier,
-malign-data=abi uses alignment as specified by
the psABI, and -malign-data=cacheline uses increased
alignment to match the cache line size.
-malign-data=compat is the default.-mskip-rax-setup 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.-mips32r3, -mips64r3,
-mips32r5 and -mips64r5 to enable
code-generation for these processors.-march=p5600 command-line option.-march=octeon3 command-line option.-mips32r6
and -mips64r6 command-line options.-mfp64 command-line option has
changed. It is now used to enable the FP64A and FP64 ABI extensions.
-mabi=32
-mfpxx can be used to enable this extension. MIPS II is
the minimum processor required.-mabi=32 -mfp64 -mno-odd-spreg
can be used to enable this extension. MIPS32R2 is the
minimum processor required.-mabi=32 -mfp64 -modd-spreg can be used to
enable this extension. MIPS32R2 is the minimum processor required.
--with-fp-32=[32|xx|64] and
--with(out)-odd-sp-reg-32. 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..set directive to override the global assembler options
when compiling for soft-float targets.-mforce-fp-as-gp, -mforbid-fp-as-gp,
and -mex9 have been removed since they are not yet available
in the nds32 port of GNU binutils.-mcmodel=[small|medium|large] supports
varied code models on code generation. The -mgp-direct
option became meaningless and can be discarded.-mno-allow-string-insns can be
used to disable the generation of the SCMPU, SMOVU,
SMOVB, SMOVF, SUNTIL, SWHILE
and RMPA 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.--isa= option
to the assembler.GBR has been changed from
call clobbered to call preserved. The old behavior can be reinstated by
specifying the option -fcall-used-gbr.fpchg instruction has been added which
will be utilized when switching between single and double precision FPU
modes.__fpscr_values 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 FPSCR, so that other FPSCR bits are
preserved across mode switches. The __fpscr_values array that
is defined in libgcc is still present for backwards compatibility, but it
will not be referenced by compiler generated code anymore.__builtin_sh_get_fpscr and
__builtin_sh_set_fpscr have been added. The
__builtin_sh_set_fpscr function will mask the specified bits
in such a way that the SZ, PR and FR
mode bits will be preserved, while changing the other bits. These new
functions do not reference the __fpscr_values array. The old
functions __set_fpscr and __get_fpscr in libgcc
which access the __fpscr_values array are still present for
backwards compatibility, but their usage is highly discouraged.__atomic built-in
functions.bt and
bf. The old behavior can be reinstated (e.g. to work around a
hardware bug in the original SH7055) by specifying the new option
-mcbranch-force-delay-slot.arm*-*-freebsd* target triplets.gcc-ar, gcc-nm, gcc-ranlib
wrappers now understand a -B option to set the compiler
to use.-freport-bug is
used, GCC automatically generates a developer-friendly reproducer
whenever an internal compiler error is encountered.Copyright (C) Free Software Foundation, Inc. Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved.
These pages are maintained by the GCC team. Last modified 2015-04-22.