This page is a brief summary of some of the huge number of improvements in GCC 6. For more information, see the Porting to GCC 6 page and the full GCC documentation.
-std=gnu++14 instead of
-std=gnu++98.Support for a number of older systems and recently unmaintained or untested target ports of GCC has been declared obsolete in GCC 6. Unless there is activity to revive them, the next release of GCC will have their sources permanently removed.
The following ports for individual systems on particular architectures have been obsoleted:
-fsanitize=bounds-strict, which enables strict checking
of array bounds. In particular, it enables
-fsanitize=bounds as well as instrumentation of
flexible array member-like arrays.-fno-strict-aliasing
to work correctly.weakref and
alias attributes. This makes it possible to access
both a variable and its alias in one translation unit which is common
with link-time optimization.this pointer
of C++ member functions is non-null. This eliminates
common null pointer checks
but also breaks some non-conforming code-bases (such as Qt-5, Chromium,
KDevelop). As a temporary work-around
-fno-delete-null-pointer-checks can be used. Wrong
code can be identified by using -fsanitize=undefined.warning and error attributes are now
correctly preserved by declaration linking and thus
-D_FORTIFY_SOURCE=2 is now supported with -flto.Type merging was fixed to handle C and Fortran interoperability rules as defined by the Fortran 2008 language standard.
As an exception, CHARACTER(KIND=C_CHAR) is not inter-operable
with char in all cases because it is an array while
char is scalar.
INTEGER(KIND=C_SIGNED_CHAR) should be used instead.
In general, this inter-operability cannot be implemented, for
example, on targets where function passing conventions of arrays
differs from scalars.
-Wodr-type-mismatch.-flto=n)
was significantly improved by decreasing the size of streamed
data when partitioning programs. The size of streamed
IL while compiling Firefox 46.0 was reduced by 66%.The linker plugin was extended to pass information about type of
binary produced to GCC back end (that can be also manually controlled
by -flinker-output). This makes it possible to
properly configure the code generator and support incremental
linking. Incremental linking of LTO objects by gcc -r is
now supported on plugin-enabled setups.
There are two ways to perform incremental linking:
ld -r will result in an object file
with all sections from individual object files mechanically merged.
This delays the actual link time optimization to final linking step
and thus permits whole program optimization. Linking final binary
with such object files is however slower.gcc -r will lead to link time optimization
and produce final binary into the object file. Linking such object
file is fast but avoids any benefits from whole program optimization.gcc -r.-fopenacc combined with -O2 or
higher.device_type clause is not supported.
The bind and nohost clauses are not
supported. The host_data directive is not supported in
Fortran.acc_on_device function has a
compile-time constant argument, the function call evaluates to a
compile-time constant value only for C and C++ but not for
Fortran.
enum {
newval,
oldval __attribute__ ((deprecated ("too old")))
};
In addition, there is now initial support for precise diagnostic locations within strings:test.cc: In function 'int test(int, int, foo, int, int)': test.cc:5:16: error: no match for 'operator*' (operand types are 'int' and 'foo') return p + q * r * s + t; ~~^~~
format-strings.c:3:14: warning: field width specifier '*' expects a matching 'int' argument [-Wformat=] printf("%*d"); ^
fixits.c: In function 'bad_deref': fixits.c:11:13: error: 'ptr' is a pointer; did you mean to use '->'? return ptr.x; ^ ->
spellcheck-fields.cc:52:13: error: 'struct s' has no member named 'colour'; did you mean 'color'? return ptr->colour; ^~~~~~
-Wshift-negative-value warns about left shifting a
negative value.-Wshift-overflow warns about left shift overflows.
This warning is enabled by default.
-Wshift-overflow=2 also warns about left-shifting 1 into
the sign bit.-Wtautological-compare warns if a self-comparison
always evaluates to true or false. This warning is enabled by
-Wall.-Wnull-dereference warns if the compiler detects paths
that trigger erroneous or undefined behavior due to dereferencing a
null pointer. This option is only active when
-fdelete-null-pointer-checks is active, which is enabled
by optimizations in most targets. The precision of the warnings
depends on the optimization options used.-Wduplicated-cond warns about duplicated conditions
in an if-else-if chain.-Wmisleading-indentation warns about places where the
indentation of the code gives a misleading idea of the block
structure of the code to a human reader. For example, given
CVE-2014-1266:
This warning is enabled bysslKeyExchange.c: In function 'SSLVerifySignedServerKeyExchange': sslKeyExchange.c:629:3: warning: this 'if' clause does not guard... [-Wmisleading-indentation] if ((err = SSLHashSHA1.update(&hashCtx, &signedParams)) != 0) ^~ sslKeyExchange.c:631:5: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the 'if' goto fail; ^~~~
-Wall.test.c:3:1: error: version control conflict marker in file <<<<<<< HEAD ^~~~~~~
-Woverride-init-side-effects.scalar_storage_order applying to
structures and unions has been introduced. It specifies the storage
order (aka endianness) in memory of scalar fields in structures
or unions.-std=gnu++14.-fconcepts.-flifetime-dse is more
aggressive in dead-store elimination in situations where
a memory store to a location precedes a constructor to the
memory location.u8 character literals,
extended static_assert, and nested namespace definitions.-fgnu-tm. std::uncaught_exceptions function (this is also
available for -std=gnu++NN modes); try_emplace and
insert_or_assign for unique_key maps;std::size,
std::empty, and std::data for
accessing containers and arrays;std::invoke;std::shared_mutex;std::void_t and std::bool_constant
metaprogramming utilities. shared_ptr, thanks to Fan You._GLIBCXX_ASSERTIONS. The subset of checks enabled by
the new macro have less run-time overhead than the full
_GLIBCXX_DEBUG checks and don't affect the library
ABI, so can be enabled per-translation unit.
std::locale support for DragonFly and FreeBSD,
thanks to John Marino and Andreas Tobler.
MATMUL intrinsic is now inlined for straightforward
cases if front-end optimization is active. The maximum size for
inlining can be set to n with the
-finline-matmul-limit=n option and turned off
with -finline-matmul-llimit=0.-Wconversion-extra option will warn about
REAL constants which have excess precision for
their kind.-Winteger-division option has been added, which
warns about divisions of integer constants which are truncated.
This option is included in -Wall by default.-march=native,
-mcpu=native and -mtune=native are now
available on native AArch64 GNU/Linux systems. Specifying
these options will cause GCC to auto-detect the host CPU and
rewrite these options to the optimal setting for that system.
If GCC is unable to detect the host CPU these options have no effect.
-fpic is now supported by the AArch64 target when generating
code for the small code model (-mcmodel=small). The size of
the global offset table (GOT) is limited to 28KiB under the LP64 SysV ABI
, and 15KiB under the ILP32 SysV ABI.
-mcpu and -mtune values that are
deprecated are:
arm2, arm250, arm3, arm6, arm60, arm600, arm610, arm620, arm7,
arm7d, arm7di, arm70, arm700, arm700i, arm710, arm720, arm710c,
arm7100, arm7500, arm7500fe, arm7m, arm7dm, arm7dmi, arm8, arm810,
strongarm, strongarm110, strongarm1100, strongarm1110, fa526,
fa626. The value
arm7tdmi is still supported.
The values of -march that are deprecated are:
armv2,armv2a,armv3,armv3m,armv4.
cortex-a32), ARM Cortex-A35 (cortex-a35).
The GCC identifiers can be used
as arguments to the -mcpu or -mtune options,
for example: -mcpu=cortex-a32 or
-mtune=cortex-a35.
GCC can now generate HSAIL (Heterogeneous System Architecture
Intermediate Language) for simple OpenMP device constructs if
configured with --enable-offload-targets=hsa. A new
libgomp plugin then runs the HSA GPU kernels implementing these
constructs on HSA capable GPUs via a standard HSA run time.
If the HSA compilation back end determines it cannot output HSAIL
for a particular input, it gives a warning by default. These
warnings can be suppressed with -Wno-hsa. To give a few
examples, the HSA back end does not implement compilation of code
using function pointers, automatic allocation of variable sized
arrays, functions with variadic arguments as well as a number of
other less common programming constructs.
When compilation for HSA is enabled, the compiler attempts to compile composite OpenMP constructs
#pragma omp target teams distribute parallel for
into parallel HSA GPU kernels.
-march=skylake-avx512. The switch enables the following
ISA extensions: AVX-512F, AVX512VL, AVX-512CD, AVX-512BW, AVX-512DQ.
monitorx and
mwaitx has been added. This includes new intrinsic
and built-in support. It is enabled through option -mmwaitx.
The instructions monitorx and mwaitx
implement the same functionality as the old monitor
and mwait instructions. In addition mwaitx
adds a configurable timer. The timer value is received as third
argument and stored in register %ebx.
-mstackrealign or
__attribute__ ((force_align_arg_pointer)). This allows
functions compiled with a vector-aligned stack to be invoked from
objects that keep only word-alignment.
__seg_fs, __seg_gs,
and __seg_tls. These can be used to access data via the
%fs and %gs segments without having to
resort to inline assembly.
Please refer to the
documentation for usage instructions.
-march=znver1 and -mtune=znver1 options.
Support for the MeP (mep-elf) architecture has been deprecated and will be removed in a future GCC release.
The MSP430 compiler now has the ability to automatically distribute code and data between low memory (addresses below 64K) and high memory. This only applies to parts that actually have both memory regions and only if the linker script for the part has been specifically set up to support this feature.
A new attribute of either can be applied to both functions
and data, and this tells the compiler to place the object into low memory
if there is room and into high memory otherwise. Two other new attributes
- lower and upper - can be used to explicitly
state that an object should be placed in the specified memory region. If
there is not enough left in that region the compilation will fail.
Two new command-line options - -mcode-region=[lower|upper|either]
and -mdata-region=[lower|upper|either] - can be used to tell
the compiler what to do with objects that do not have one of these new
attributes.
PowerPC64 now supports IEEE 128-bit floating-point using the __float128 data type. In GCC 6, this is NOT enabled by default, but you can enable it with -mfloat128. The IEEE 128-bit floating-point support requires the use of the VSX instruction set. IEEE 128-bit floating-point values are passed and returned as a single vector value. The software emulator for IEEE 128-bit floating-point support is only built on PowerPC Linux systems where the default cpu is at least power7. On future ISA 3.0 systems (power9 and later), you will be able to use the -mfloat128-hardware option to use the ISA 3.0 instructions that support IEEE 128-bit floating-point. An additional type (__ibm128) has been added to refer to the IBM extended double type that normally implements long double. This will allow for a future transition to implementing long double with IEEE 128-bit floating-point.
Basic support has been added for POWER9 hardware that will use the recently published OpenPOWER ISA 3.0 instructions. The following new switches are available:
-mcpu=power9: Implement all of the ISA 3.0 instructions supported by the compiler.
-mtune=power9: In the future, apply tuning for POWER9 systems. Currently, POWER8 tunings are used.
-mmodulo: Generate code using the ISA 3.0 integer instructions (modulus, count trailing zeros, array index support, integer multiply/add).
-mpower9-fusion: Generate code to suitably fuse instruction sequences for a POWER9 system.
-mpower9-dform: Generate code to use the new D-form (register +offset) memory instructions for the vector registers.
-mpower9-vector: Generate code using the new ISA 3.0 vector (VSX or Altivec) instructions.
-mpower9-minmax: Reserved for future development.
-mtoc-fusion: Keep TOC entries together to provide more fusion opportunities.
New constraints have been added to support IEEE 128-bit floating-point and ISA 3.0 instructions:
wb: Altivec register if -mpower9-dform is enabled.
we: VSX register if -mpower9-vector is enabled for 64-bit code generation.
wo: VSX register if -mpower9-vector is enabled.
wp: Reserved for future use if long double is implemented with IEEE 128-bit floating-point instead of IBM extended double.
wq: VSX register if -mfloat128 is enabled.
wF: Memory operand suitable for POWER9 fusion load/store.
wG: Memory operand suitable for TOC fusion memory references.
wL: Integer constant identifying the element number mfvsrld accesses within a vector.
Support has been added for __builtin_cpu_is () and __builtin_cpu_supports (), allowing for very fast access to AT_PLATFORM, AT_HWCAP, and AT_HWCAP2 values. This requires use of glibc 2.23 or later.
All hardware transactional memory builtins now correctly behave as memory barriers. Programmers can use #ifdef __TM_FENCE__ to determine whether their "old" compiler treats the builtins as barriers.
Split-stack support has been added for gccgo on PowerPC64 for both big- and little-endian (but NOT for 32-bit). The gold linker from at least binutils 2.25.1 must be available in the PATH when configuring and building gccgo to enable split stack. (The requirement for binutils 2.25.1 applies to PowerPC64 only.) The split-stack feature allows a small initial stack size to be allocated for each goroutine, which increases as needed.
GCC on PowerPC now supports the standard lround function.
A new configuration option ---with-advance-toolchain=at
was added for PowerPC 64-bit GNU/Linux systems to use the header files, library
files, and the dynamic linker from a specific Advance Toolchain release
instead of the default versions that are provided by the GNU/Linux
distribution. In general, this option is intended for the developers of
GCC, and it is not intended for general use.
The "q", "S", "T", and "t" asm-constraints have been removed.
The "b", "B", "m", "M", and "W" format modifiers have been removed.
-march=z13 option, the compiler will generate
code making use of the new instructions and registers introduced
with the vector extension facility. The -mtune=z13
option enables z13 specific instruction scheduling without
making use of new instructions.-march=z13 reduces the default
alignment of vector types bigger than 8 bytes to 8. This is an
ABI change and care must be taken when linking modules compiled
with different arch levels which interchange variables
containing vector type values. For newly compiled code the GNU
linker will emit a warning.-mzvector option enables a C/C++ language
extension. This extension provides a new
keyword vector which can be used to define vector
type variables. (Note: This is not available when
enforcing strict standard compliance
e.g. with -std=c99. Either enable GNU extensions
with e.g. -std=gnu99 or use
__vector instead of vector.)vecintrin.h
header file needs to be included.-march=native,
and -mtune=native are now available on native IBM
z Systems. Specifying these options will cause GCC to
auto-detect the host CPU and rewrite these options to the
optimal setting for that system. If GCC is unable to detect
the host CPU these options have no effect.-fsplit-stack is now supported as part of the IBM
z Systems port. This feature requires a recent gold linker to
be used.g5 and g6
-march=/-mtune= CPU level switches has been deprecated
and will be removed in a future GCC release. -m31
from now on defaults to -march=z900 if not
specified otherwise. -march=native on a g5/g6
machine will default to -march=z900.-mfdpic target option and --enable-fdpic
configure option.double or a 8-byte vector in the second
half is passed to a subprogram in slot #15, for example as 16th parameter
if the first 15 ones have at most 8 bytes. The double or
vector was wrongly passed in floating-point register %d32
in lieu of on the stack as per the SPARC calling conventions.-mmusl option in case musl is not the default libc. GCC
defaults to musl libc if it is built with a target triplet matching the
*-linux-musl* pattern.once function are directly defined via <pthread.h>.
Self-contained condition variables are provided via Newlib
<sys/lock.h>. The RTEMS thread model also supports C++11
threads.libgomp. It is possible to
configure thread pools for each scheduler instance via the environment
variable GOMP_RTEMS_THREAD_POOLS.crt1.o,
crti.o, crtn.o), which GCC now prefers over
its own ones.libvtv has been ported to Solaris 11 and up.-mstackrealign is now automatically activated
in 32-bit mode whenever the use of SSE instructions is requested.gcc and g++ driver programs will now
provide suggestions for misspelled command line options.
$ gcc -static-libfortran test.f95
gcc: error: unrecognized command line option '-static-libfortran'; did you mean '-static-libgfortran'?
--enable-default-pie configure option enables
generation of PIE by default.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 2016-04-27.