GCC now uses C++ as its implementation language. This means that to build GCC from sources, you will need a C++ compiler that understands C++ 2003. For more details on the rationale and specific changes, please refer to the C++ conversion page.
To enable the Graphite framework for loop optimizations you now need CLooG version 0.18.0 and ISL version 0.11.1. Both can be obtained from the GCC infrastructure directory. The installation manual contains more information about requirements to build GCC.
GCC now uses a more aggressive analysis to derive an upper bound for
the number of iterations of loops using constraints imposed by language
standards. This may cause non-conforming programs to no longer work as
expected, such as SPEC CPU 2006 464.h264ref and 416.gamess. A new
option, -fno-aggressive-loop-optimizations, was added
to disable this aggressive analysis. In some loops that have known
constant number of iterations, but undefined behavior is known to occur
in the loop before reaching or during the last iteration, GCC will warn
about the undefined behavior in the loop instead of deriving lower upper
bound of the number of iterations for the loop. The warning
can be disabled with -Wno-aggressive-loop-optimizations.
On ARM, a bug has been fixed in GCC's implementation of the AAPCS rules for the layout of vectors that could lead to wrong code being generated. Vectors larger than 8 bytes in size are now by default aligned to an 8-byte boundary. This is an ABI change: code that makes explicit use of vector types may be incompatible with binary objects built with older versions of GCC. Auto-vectorized code is not affected by this change.
On AVR, support has been removed for the command-line
option -mshort-calls deprecated in GCC 4.7.
On AVR, the configure option --with-avrlibc supported since
GCC 4.7.2 is turned on per default for all non-RTEMS configurations.
This option arranges for a better integration of
AVR Libc with avr-gcc.
For technical details, see PR54461.
To turn off the option in non-RTEMS configurations, use
--with-avrlibc=no. If the compiler is configured for
RTEMS, the option is always turned off.
More information on porting to GCC 4.8 from previous versions of GCC can be found in the porting guide for this release.
-g is used on a platform that uses DWARF debugging
information, GCC will now default to
-gdwarf-4 -fno-debug-types-section.-g together with -gdwarf-2 or
-gdwarf-3.
The default for Darwin and VxWorks is still
-gdwarf-2 -gstrict-dwarf.
-Og, has been
introduced. It addresses the need for fast compilation and a
superior debugging experience while providing a reasonable level
of run-time performance. Overall experience for development should
be better than the default optimization level -O0.
-ftree-partial-pre was added to control
the partial redundancy elimination (PRE) optimization.
This option is enabled by default at the -O3 optimization
level, and it makes PRE more aggressive.
-fconserve-space has been removed; it
was no longer useful on most targets since GCC supports putting
variables into BSS without making them common.-fipa-struct-reorg and
-fipa-matrix-reorg) have been removed. They did not
always work correctly, nor did they work with link-time optimization
(LTO), hence were only applicable to programs consisting of a single
translation unit.
flatten attribute in the
"Eigen" C++ linear algebra templates library, is significantly
faster than previous releases of GCC.
-fsanitize=address. Memory access
instructions will be instrumented to detect heap-, stack-, and
global-buffer overflow as well as use-after-free bugs. To get
nicer stacktraces, use -fno-omit-frame-pointer. The
AddressSanitizer is available on IA-32/x86-64/x32/PowerPC/PowerPC64
GNU/Linux and on x86-64 Darwin.-fsanitize=thread. Instructions will be instrumented to
detect data races. The ThreadSanitizer is available on x86-64
GNU/Linux.-fno-diagnostics-show-caret
suppresses this information.-ftrack-macro-expansion=2 is now
enabled by default. This allows the compiler to display the
macro expansion stack in diagnostics. Combined with the caret
information, an example diagnostic showing these two features
is:
t.c:1:94: error: invalid operands to binary < (have ‘struct mystruct’ and ‘float’)
#define MYMAX(A,B) __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a < __b ? __b : __a; })
^
t.c:7:7: note: in expansion of macro 'MYMAX'
X = MYMAX(P, F);
^
-Wsizeof-pointer-memaccess warning has been added
(also enabled by -Wall) to warn about suspicious length parameters
to certain string and memory built-in functions if the argument uses
sizeof. This warning warns e.g.
about memset (ptr, 0, sizeof (ptr)); if ptr is not an array,
but a pointer, and suggests a possible fix, or about
memcpy (&foo, ptr, sizeof (&foo));.-Wpedantic is an alias for
-pedantic, which is now deprecated. The forms
-Wno-pedantic, -Werror=pedantic, and
-Wno-error=pedantic work in the same way as for any other
-W option. One caveat is that
-Werror=pedantic is not equivalent to
-pedantic-errors, since the latter makes into errors
some warnings that are not controlled by -Wpedantic,
and the former only affects diagnostics that are disabled when using
-Wno-pedantic.-Wshadow no longer warns if a
declaration shadows a function declaration, unless the former
declares a function or pointer to function, because this is a common and valid case
in real-world code.G++ now implements the C++11
thread_local keyword; this differs from the
GNU __thread keyword primarily in that it allows dynamic
initialization and destruction semantics. Unfortunately, this support
requires a run-time penalty for references to non-function-local
thread_local variables defined in a different
translation unit even if they don't need dynamic
initialization, so users may want to continue to
use __thread for TLS variables with static initialization
semantics.
If the programmer can be sure that no use of the variable in a
non-defining TU needs to trigger dynamic initialization (either because
the variable is statically initialized, or a use of the variable in the
defining TU will be executed before any uses in another TU), they can
avoid this overhead with the -fno-extern-tls-init option.
OpenMP threadprivate variables now also support dynamic
initialization and destruction by the same mechanism.
and also the alignment specifier, e.g.[[noreturn]] void f();
alignas(double) int i;
struct A { A(int); };
struct B: A { using A::A; }; // defines B::B(int)
B b(42); // OK
decltype semantics from N3276.
struct A f(); decltype(f()) g(); // OK, return type of f() is not required to be complete.
struct A { int f() &; };
int i = A().f(); // error, f() requires an lvalue object
-std=c++1y option for experimentation
with features proposed for the next revision of the standard, expected
around 2014. Currently the only difference from -std=c++11
is support for return type deduction in normal functions, as proposed in
N3386.
Status of C++1y features in GCC 4.8 can be found here.
__attribute ((strong)),
has been deprecated. Inline namespaces should be used instead.-fext-numeric-literal option to control
whether GNU numeric literal suffixes are accepted as extensions or processed
as C++11 user-defined numeric literal suffixes. The flag is on
(use suffixes for GNU literals) by default for -std=gnu++*,
and -std=c++98. The flag is off (use suffixes for user-defined
literals) by default for -std=c++11 and later.forward_list meets the allocator-aware container requirements; this_thread::sleep_for(), this_thread::sleep_until() and
this_thread::yield() are defined without requiring the configure
option --enable-libstdcxx-time; <random>:
normal_distribution. random_device
on new x86 processors (requires the assembler to support the
instruction.) <ext/random>:
simd_fast_mersenne_twister_engine
with an optimized SSE implementation. beta_distribution,
normal_mv_distribution, rice_distribution,
nakagami_distribution, pareto_distribution,
k_distribution, arcsine_distribution,
hoyt_distribution. --disable-libstdcxx-verbose configure option
to disable diagnostic messages issued when a process terminates
abnormally. This may be useful for embedded systems to reduce the
size of executables that link statically to the library.
.mod)
has been incremented. Fortran MODULEs compiled by earlier
GCC versions have to be recompiled, when they are USEd by
files compiled with GCC 4.8. GCC 4.8 is not able to read
.mod files created by earlier versions; attempting to do so
gives an error message.type(c_funptr)) are not affected nor are
procedure-pointer components.BACKTRACE intrinsic subroutine has been added. It shows
a backtrace at an arbitrary place in user code; program execution
continues normally afterwards.
-Wc-binding-type warning option has been added (disabled
by default). It warns if the a variable might not be C interoperable; in
particular, if the variable has been declared using an intrinsic type with
default kind instead of using a kind parameter defined for C
interoperability in the intrinsic ISO_C_Binding module.
Before, this warning was always printed. The -Wc-binding-type
option is enabled by -Wall.-Wrealloc-lhs and -Wrealloc-lhs-all warning
command-line options have been added, which diagnose when code is
inserted for automatic (re)allocation of a variable during assignment.
This option can be used to decide whether it is safe to use
-fno-realloc-lhs. Additionally, it can be used to find automatic
(re)allocation in hot loops. (For arrays, replacing var=var(:)=-Wcompare-reals command-line option has been added. When
this is set, warnings are issued when comparing REAL or
COMPLEX types for equality and inequality; consider replacing
a == b by abs(a−b) < eps with a suitable
eps. -Wcompare-reals is enabled by
-Wextra.-Wtarget-lifetime command-line option has been added
(enabled with -Wall), which warns if the pointer in a
pointer assignment might outlive its target.Reading floating point numbers which use
for
the exponential (such as q4.0q0) is now supported as vendor
extension for better compatibility with old data files. It is strongly
recommended to use for I/O the equivalent but standard conforming
(such as e4.0e0).
(For Fortran
source code, consider replacing the
in
floating-point literals by a kind parameter (e.g. q4.0e0_qp
with a suitable qp). Note that – in Fortran
source code – replacing
by a simple
q
is not equivalent.)e
GFORTRAN_TMPDIR environment variable for specifying
a non-default directory for files opened with STATUS="SCRATCH",
is not used anymore. Instead gfortran checks the POSIX/GNU standard
TMPDIR environment variable. If TMPDIR is not
defined, gfortran falls back to other methods to determine the directory
for temporary files as documented in the
user
manual.CLASS(*))
has been added. Nonconstant character lengths are not yet
supported.TYPE(*)) are now supported.dimension(..)) has been added. Note that currently
gfortran's own array descriptor is used, which is different from the
one defined in TS29113, see gfortran's header file or use the Chasm Language
Interoperability Tools.-mcpu=cortex-a53 and -mcpu=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=marvell-pj4, has been added to
generate code for the Marvell PJ4 processor.VFMA,
VFMS, REVSH and REV16
instructions.-fno-sched-pressure.-mcpu=iwmmxt2 can be used to enable code generation for
the latter.arm-rtems) port has been updated to use the
EABI.arm*-*-linux-gnu (use
arm*-*-linux-gnueabi)arm*-*-elf (use arm*-*-eabi)arm*-*-uclinux* (use
arm*-*-uclinux*eabi)arm*-*-ecos-elf (no alternative)arm*-*-freebsd (no alternative)arm*-wince-pe* (no alternative).%r for register operands in inline
assembler is supported. It will print the raw register number without the
register prefix 'r':
/* Return the most significant byte of 'val', a 64-bit value. */
unsigned char msb (long long val)
{
unsigned char c;
__asm__ ("mov %0, %r1+7" : "=r" (c) : "r" (val));
return c;
}
The inline assembler in this example will generate code like
mov r24, 8+7
provided c is allocated to R24 and
val is allocated to
R8…R15. This works because
the GNU assembler accepts plain register numbers without register prefix.
extern const __memx char foo;
const __memx void *pfoo = &foo;
This requires at least Binutils 2.23.
-mpreferred-stack-boundary=3 for the x86-64
architecture with SSE extensions disabled. Since the x86-64 ABI
requires 16 byte stack alignment, this is ABI incompatible and
intended to be used in controlled environments where stack space
is an important limitation.
This option will lead to wrong code when functions compiled with 16 byte
stack alignment (such as functions from a standard library) are called
with misaligned stack. In this case, SSE instructions may lead to
misaligned memory access traps. In addition, variable arguments will
be handled incorrectly for 16 byte aligned objects (including x87
long double and __int128), leading to
wrong results. You must build all
modules with -mpreferred-stack-boundary=3, including any
libraries. This includes the system libraries and startup modules.RDSEED, ADCX, ADOX,
PREFETCHW is available through -madx,
-mprfchw, -mrdseed command-line options.-mrtm and
-mhle.-mfxsr, -mxsave and
-mxsaveopt respectively.-maddress-mode=[short|long] options for x32.
-maddress-mode=short overrides default 64-bit addresses to
32-bit by emitting the 0x67 address-size override prefix.
This is the default address mode for x32.__builtin_cpu_is has been added to
detect if the run-time CPU is of a particular type. It returns a
positive integer on a match and zero otherwise. It accepts one string
literal argument, the CPU name. For example,
__builtin_cpu_is("westmere") returns a positive integer if
the run-time CPU is an Intel Core i7 Westmere processor. Please refer
to the
user manual for the list of valid CPU names recognized.__builtin_cpu_supports has been
added to detect if the run-time CPU supports a particular ISA feature.
It returns a positive integer on a match and zero otherwise. It accepts
one string literal argument, the ISA feature. For example,
__builtin_cpu_supports("ssse3") returns a positive integer
if the run-time CPU supports SSSE3 instructions. Please refer to the
user manual for the list of valid ISA names recognized.Caveat: If these built-in functions are called before any static
constructors are invoked, like during IFUNC initialization, then the CPU
detection initialization must be explicitly run using this newly provided
built-in function, __builtin_cpu_init. The initialization
needs to be done only once. For example, this is how the invocation would
look like inside an IFUNC initializer:
static void (*some_ifunc_resolver(void))(void)
{
__builtin_cpu_init();
if (__builtin_cpu_is("amdfam10h") ...
if (__builtin_cpu_supports("popcnt") ...
}
It is now possible to create multiple function versions each targeting a specific processor and/or ISA. Function versions have the same signature but different target attributes. For example, here is a program with function versions:
__attribute__ ((target ("default")))
int foo(void)
{
return 1;
}
__attribute__ ((target ("sse4.2")))
int foo(void)
{
return 2;
}
int main (void)
{
int (*p) = &foo;
assert ((*p)() == foo());
return 0;
}
Please refer to this
wiki for more
information.
-fschedule-insns to work reliably.
This option can be used to schedule instructions better and leads to
improved performace in certain cases.*-w64-mingw*) require at least r5437 from the Mingw-w64 trunk. -march=bdver3 and
-mtune=bdver3 options.-march=btver2 and
-mtune=btver2 options.-fstack-usage
command-line option.-march options
are -march=r4700, -march=xlp
and -march=34kn respectively.-fstack-check
option.-mmcu and -mno-mcu
options to the assembler.-fpic
and -fPIC for -mno-abicalls targets
like mips*-elf. This combination was not intended
or supported, and did not generate position-independent code.
GCC 4.8 now reports an error when this combination is used.-mcmodel=large.-mno-warn-multiple-fast-interrupts
command-line option.-march=zEC12 option, the
compiler will generate code making use of the following new
instructions:
-mtune=zEC12 option enables zEC12 specific
instruction scheduling without making use of new
instructions.ifunc function attribute is enabled by default.memcpy and memcmp invokations on big
memory chunks or with run time lengths are not generated inline
anymore when tuning for z10 or higher. The purpose is to make
use of the IFUNC optimized versions in Glibc.-Os.__atomic built-in functions:
-matomic-model=model selects the
model for the generated atomic sequences. The following models are
supported:
soft-gusamovco.l and
movli.l instructions. This is the default when the target
is sh3*-*-linux* or sh4*-*-linux*.hard-llcsmovco.l / movli.l sequences
(SH4A only).soft-tcbsoft-imasksh1*-*-linux* or
sh2*-*-linux*.none__atomic
built-in functions. This is the default for SH64 targets or when
the target is not sh*-*-linux*.-msoft-atomic has been deprecated. It is
now an alias for -matomic-model=soft-gusa.-mtas makes the compiler generate
the tas.b instruction for the
__atomic_test_and_set built-in function regardless of the
selected atomic model.__sync functions in libgcc now reflect
the selected atomic model when building the toolchain.mov.b and mov.w
instructions with displacement addressing.movu.b and
movu.w.-mzdcbranch tells the compiler to favor
zero-displacement branches. This is enabled by default for SH4* targets.
pref instruction will now be emitted by the
__builtin_prefetch built-in function for SH3* targets.fmac instruction will now be emitted by the
fmaf standard function and the __builtin_fmaf
built-in function.-mfused-madd option has been deprecated in favor of
the machine-independent -ffp-contract option. Notice that the
fmac instruction will now be generated by default for
expressions like a * b + c. This is due to the compiler
default setting -ffp-contract=fast.-mfsrra and -mfsca to allow
the compiler using the fsrra and fsca
instructions on targets other than SH4A (where they are already enabled by
default).__builtin_bswap32 built-in function.
It is now expanded as a sequence of swap.b and
swap.w instructions instead of a library function call.-mieee option has been fixed and the
negative form -mno-ieee has been added to control the IEEE
conformance of floating point comparisons. By default -mieee
is now enabled and the option -ffinite-math-only implicitly
sets -mno-ieee.__builtin_thread_pointer and
__builtin_set_thread_pointer. This assumes that
GBR is used to hold the thread pointer of the current thread.
Memory loads and stores relative to the address returned by
__builtin_thread_pointer will now also utilize GBR
based displacement address modes.
-mdiv= option for targets other than SHmedia has been
fixed and documented.-mcmodel=MODEL
command-line option. The models supported are small
and large.E3V5 architecture via the use
of the new -mv850e3v5 command-line option. It also has
experimental support for the e3v5 LOOP instruction which
can be enabled via the new -mloop command-line option.-fstack-usage
command-line option.-static or
static-libgcc on the
command line. However it is strongly advised against, as it
will cause problems for any application that makes use of DLLs
compiled by GCC. It should be alright for a monolithic stand-alone
application that only links against the Windows DLLs, but
offers little or no benefit.This is the list of problem reports (PRs) from GCC's bug tracking system that are known to be fixed in the 4.8.1 release. This list might not be complete (that is, it is possible that some PRs that have been fixed are not listed here).
The C++11 <chrono>
std::chrono::system_clock and
std::chrono::steady_clock classes have changed ABI in
GCC 4.8.1, they both are now separate (never typedefs of each other),
both use std::chrono::nanoseconds resolution, on most
GNU/Linux configurations std::chrono::steady_clock is now
finally monotonic, and both classes are mangled differently than in
the previous GCC releases.
std::chrono::system_clock::now() with
std::chrono::microseconds resp.
std::chrono::seconds resolution is still exported for backwards
compatibility with default configured libstdc++. Note that libstdc++
configured with --enable-libstdcxx-time= used to be ABI
incompatible with default configured libstdc++ for those two classes and
no ABI compatibility can be offered for those configurations, so any C++11
code that uses those classes and has been compiled and linked against
libstdc++ configured with the non-default
--enable-libstdcxx-time= configuration option needs to be
recompiled.
This is the list of problem reports (PRs) from GCC's bug tracking system that are known to be fixed in the 4.8.2 release. This list might not be complete (that is, it is possible that some PRs that have been fixed are not listed here).
This is the list of problem reports (PRs) from GCC's bug tracking system that are known to be fixed in the 4.8.3 release. This list might not be complete (that is, it is possible that some PRs that have been fixed are not listed here).
Support for the new powerpc64le-linux platform has been added. It defaults to generating code that conforms to the ELFV2 ABI.
This is the list of problem reports (PRs) from GCC's bug tracking system that are known to be fixed in the 4.8.4 release. This list might not be complete (that is, it is possible that some PRs that have been fixed are not listed here).
This is the list of problem reports (PRs) from GCC's bug tracking system that are known to be fixed in the 4.8.5 release. This list might not be complete (that is, it is possible that some PRs that have been fixed are not listed here).
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These pages are maintained by the GCC team. Last modified 2022-11-05.