This page is a "brief" summary of some of the huge number of improvements in GCC 9. You may also want to check out our Porting to GCC 9 page and the full GCC documentation.
arm*-*-*),
a bug
in the implementation of the procedure call standard (AAPCS)
in the GCC 6, 7 and 8 releases has been fixed: a structure
containing a bit-field based on a 64-bit integral type and where
no other element in a structure required 64-bit alignment could be
passed incorrectly to functions. This is an ABI change. If the
option -Wpsabi is enabled (on by default) the
compiler will emit a diagnostic note for code that might be
affected.
Support for a number of older systems and recently unmaintained or untested target ports of GCC has been declared obsolete in GCC 9. 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:
*-*-solaris2.10). Details can be found
in the
announcement.spu*-*-*). Details can be found
in the
announcement.std::rotate algorithm in GCC 9.1.0 can
cause ABI incompatibilities with object files compiled with other versions
of GCC. If the std::rotate algorithm is called with
an empty range then it might cause a divide-by-zero error
(as a SIGFPE signal) and crash. The change has been reverted
for GCC 9.2.0 and future releases. For more details see
Bug 90920.
The problem can be avoided by recompiling any objects that might call
std::rotate with an empty range, so that the GCC 9.1.0
definition of std::rotate is not used.
-frepo)
has been deprecated and will be removed in a future release.
--with-default-libstdcxx-abi=gcc4-compatible configure
option is broken in the 9.1 and 9.2 releases, producing a shared library
with missing symbols
(see Bug 90361).
As a workaround, configure without that option and build GCC as normal,
then edit the installed <bits/c++config.h> headers
to define the _GLIBCXX_USE_CXX11_ABI macro to 0.
The following GCC command line options have been introduced or improved.
kb and KiB, MB and
MiB, or GB and GiB denoting
the corresponding multiples of bytes. See
Invoking GCC for more.
-flive-patching=[inline-only-static|inline-clone]
generates code suitable for live patching. At the same
time it provides multiple-level control over IPA optimizations.
See the user guide for more details.
--completion, has been added to provide more fine
option completion in a shell. It is intended to be used by Bash-completion.
GCC's diagnostics now print source code with a left margin showing line numbers, configurable with -fno-diagnostics-show-line-numbers.
GCC's diagnostics can also now label regions of the source code to show pertinent information, such as the types within an expression.
$ g++ t.cc t.cc: In function 'int test(const shape&, const shape&)': t.cc:15:4: error: no match for 'operator+' (operand types are 'boxed_value<double>' and 'boxed_value<double>') 14 | return (width(s1) * height(s1) | ~~~~~~~~~~~~~~~~~~~~~~ | | | boxed_value<[...]> 15 | + width(s2) * height(s2)); | ^ ~~~~~~~~~~~~~~~~~~~~~~ | | | boxed_value<[...]>
These labels can be disabled via -fno-diagnostics-show-labels.
-falign-functions,
-falign-labels,
-falign-loops,
and -falign-jumps received support for a secondary
alignment (e.g. -falign-loops=n:m:n2:m2).
-fprofile-filter-files
and -fprofile-exclude-files) has been added.
The options help to filter which source files are instrumented.
Numerous improvements have been made to the output of -fopt-info.
Messages are now prefixed with optimized,
missed, or note, rather than the old
behavior of all being prefixed with note.
The output from -fopt-info can now contain information
on inlining decisions:
$ g++ -c inline.cc -O2 -fopt-info-inline-all inline.cc:24:11: note: Considering inline candidate void foreach(T, T, void (*)(E)) [with T = char**; E = char*]/2. inline.cc:24:11: optimized: Inlining void foreach(T, T, void (*)(E)) [with T = char**; E = char*]/2 into int main(int, char**)/1. inline.cc:19:12: missed: not inlinable: void inline_me(char*)/0 -> int std::puts(const char*)/3, function body not available inline.cc:13:8: optimized: Inlined void inline_me(char*)/4 into int main(int, char**)/1 which now has time 127.363637 and size 11, net change of +0. Unit growth for small function inlining: 16->16 (0%) Inlined 2 calls, eliminated 1 functions
The output from the vectorizer has been rationalized so that failed attempts to vectorize a loop are displayed in the form
[LOOP-LOCATION]: couldn't vectorize this loop
[PROBLEM-LOCATION]: because of [REASON]
rather than an exhaustive log of all decisions made by the vectorizer. For example:
$ gcc -c v.c -O3 -fopt-info-all-vec
v.c:7:3: missed: couldn't vectorize loop
v.c:10:7: missed: statement clobbers memory: __asm__ __volatile__("" : : : "memory");
v.c:3:6: note: vectorized 0 loops in function.
v.c:10:7: missed: statement clobbers memory: __asm__ __volatile__("" : : : "memory");
The old behavior can be obtained via a new -internals
suboption of -fopt-info.
SRCFILE.opt-record.json.gz
file describing the optimization decisions made by GCC. This is
similar to the output of -fopt-info, but with additional
metadata such as the inlining chain, and profile information (if
available).
-fipa-stack-alignment.
-fipa-reference-addressable.
The following built-in functions have been introduced.
__builtin_expect_with_probability to provide branch prediction probability hints to
the optimizer.
__builtin_has_attribute determines whether a function, type, or variable has been declared with some
attribute.
__builtin_speculation_safe_value can be used to help mitigate against unsafe speculative
execution.
The following attributes have been introduced.
copy function attribute has been
added. The attribute can also be applied to type definitions and to
variable declarations.
A large number of improvements to code generation have been made, including but not limited to the following.
-ftree-switch-conversion. For example:
int
foo (int how)
{
switch (how) {
case 2: how = 205; break;
case 3: how = 305; break;
case 4: how = 405; break;
case 5: how = 505; break;
case 6: how = 605; break;
}
return how;
}
100 * how + 5 (for values defined
in the switch statement).
max-inline-insns-small,
max-inline-insns-size,
uninlined-function-insns,
uninlined-function-time, uninlined-thunk-insns,
and uninlined-thunk-time were added.-fprofile-use now enables
-fversion-loops-for-strides,
-floop-interchange,
-floop-unroll-and-jam,
-ftree-loop-distribution.hot-bb-count-ws-permille was reduced
from 999 to 990 to account for more precise histograms.--param lto-partitions) was
increased from 32 to 128 enabling effective use of CPUs with more than
32 hyperthreads. --param lto-max-streaming-parallelism
can now be used to control number of streaming processes.-Wodr) are
now more informative and produce fewer redundant results.The following improvements to the gcov command-line utility
have been made.
--use-hotness-colors
(-q) that can provide perf-like coloring of hot functions.
__builtin_convertvector built-in for vector conversions
has been added. -Waddress-of-packed-member, enabled by default,
warns about an unaligned pointer value from the address of a packed
member of a struct or union.
-Warray-bounds detects more instances
of out-of-bounds indices.
-Wattribute-alias also detects attribute
mismatches between alias declarations and their targets, in addition
to mismatches between their types.
-Wformat-overflow and
-Wformat-truncation have been extended
to all formatted input/output functions (where applicable) and
enhanced to detect a subset of instances of reading past the end
of unterminated constant character arrays in %s
directives.
-Wmissing-attributes detects instances
of missing function attributes on declarations of aliases and weak
references.
-Wstringop-truncation also detects
a subset of instances of reading past the end of unterminated
constant character arrays,
note.
-std=c2x, to
select support for the upcoming C2X revision of the ISO C
standard. This standard is in the early stages of development and
the only feature supported in GCC 9 is _Static_assert
with a single argument (support for _Static_assert with two
arguments was added in C11 and GCC 4.6). There are also new
options -std=gnu2x, for C2X with GNU extensions,
and -Wc11-c2x-compat, to warn for uses of features
added in C2X (such warnings are also enabled by use
of -Wpedantic if not using -std=c2x
or -std=gnu2x).-Wabsolute-value
warns for calls to standard functions that compute the absolute
value of an argument when a more appropriate standard function
is available. For example, calling abs(3.14)
triggers the warning because the appropriate function to call to
compute the absolute value of a double argument
is fabs. The option also triggers warnings when
the argument in a call to such a function has an unsigned type.
This warning can be suppressed with an explicit type cast and it
is also enabled by -Wextra.
-Wdeprecated-copy, implied by -Wextra, warns
about the C++11 deprecation of implicitly declared copy constructor and
assignment operator if one of them is user-provided.
-Wdeprecated-copy-dtor also warns if the destructor is
user-provided, as specified in C++11.
-Winit-list-lifetime, on by default, warns about uses
of std::initializer_list that are likely to result in a
dangling pointer, such as returning or assigning from a temporary
list. -Wredundant-move, implied by -Wextra, warns about redundant calls to
std::move.-Wpessimizing-move, implied by -Wall, warns when a call to
std::move prevents copy elision.-Wclass-conversion, on by default, warns when a conversion function will never be
called due to the type it converts to.-std=c++2a
or -std=gnu++2a flags, including range-based for statements
with initializer, default constructible and assignable stateless lambdas,
lambdas in unevaluated contexts, language support for empty data members,
allowing pack expansion in lambda init-capture, likely and unlikely
attributes, class types in non-type template parameters, allowing virtual
function calls in constant expressions, explicit(bool),
std::is_constant_evaluated, nested inline namespaces, etc.
For a full list of new features,
see the C++
status page.
id-expression, and mem-initializer for longer.
For example it is now able to pin-point the pertinent locations for
bad initializations such as these
$ g++ -c bad-inits.cc bad-inits.cc:10:14: error: cannot convert 'json' to 'int' in initialization 10 | { 3, json::object }, | ~~~~~~^~~~~~ | | | json bad-inits.cc:14:31: error: initializer-string for array of chars is too long [-fpermissive] 14 | char buffers[3][5] = { "red", "green", "blue" }; | ^~~~~~~ bad-inits.cc: In constructor 'X::X()': bad-inits.cc:17:13: error: invalid conversion from 'int' to 'void*' [-fpermissive] 17 | X() : one(42), two(42), three(42) | ^~ | | | intrather than emitting the error at the final closing parenthesis or brace.
$ g++ param-type-mismatch.cc param-type-mismatch.cc: In function 'int test(int, const char*, float)': param-type-mismatch.cc:8:32: error: cannot convert 'const char*' to 'const char**' 8 | return foo::member_1 (first, second, third); | ^~~~~~ | | | const char* param-type-mismatch.cc:3:46: note: initializing argument 2 of 'static int foo::member_1(int, const char**, float)' 3 | static int member_1 (int one, const char **two, float three); | ~~~~~~~~~~~~~^~~highlights both the problematic argument, and the parameter that it can't be converted to.
$ g++ bad-conversion.cc bad-conversion.cc: In function 'void caller()': bad-conversion.cc:9:14: error: cannot convert 'bool' to 'void*' 9 | callee (0, false, 2); | ^~~~~ | | | bool bad-conversion.cc:3:19: note: initializing argument 2 of 'void callee(int, void*, int)' 3 | void callee (int, void *, int) | ^~~~~~
$ g++ -c bad-printf.cc -Wall bad-printf.cc: In function 'void print_field(const char*, float, long int, long int)': bad-printf.cc:6:17: warning: field width specifier '*' expects argument of type 'int', but argument 3 has type 'long int' [-Wformat=] 6 | printf ("%s: %*ld ", fieldname, column - width, value); | ~^~~ ~~~~~~~~~~~~~~ | | | | int long int bad-printf.cc:6:19: warning: format '%ld' expects argument of type 'long int', but argument 4 has type 'double' [-Wformat=] 6 | printf ("%s: %*ld ", fieldname, column - width, value); | ~~~^ ~~~~~ | | | | long int double | %*f
return *this; needed by various C++ operators:
$ g++ -c operator.cc operator.cc: In member function 'boxed_ptr& boxed_ptr::operator=(const boxed_ptr&)': operator.cc:7:3: warning: no return statement in function returning non-void [-Wreturn-type] 6 | m_ptr = other.m_ptr; +++ |+ return *this; 7 | } | ^for when the compiler needs a
typename:
$ g++ -c template.cc template.cc:3:3: error: need 'typename' before 'Traits::type' because 'Traits' is a dependent scope 3 | Traits::type type; | ^~~~~~ | typenamewhen trying to use an accessor member as if it were a data member:
$ g++ -c fncall.cc fncall.cc: In function 'void hangman(const mystring&)': fncall.cc:12:11: error: invalid use of member function 'int mystring::get_length() const' (did you forget the '()' ?) 12 | if (str.get_length > 0) | ~~~~^~~~~~~~~~ | ()for C++11's scoped enums:
$ g++ -c enums.cc enums.cc: In function 'void json::test(const json::value&)': enums.cc:12:26: error: 'STRING' was not declared in this scope; did you mean 'json::kind::STRING'? 12 | if (v.get_kind () == STRING) | ^~~~~~ | json::kind::STRING enums.cc:3:44: note: 'json::kind::STRING' declared here 3 | enum class kind { OBJECT, ARRAY, NUMBER, STRING, TRUE, FALSE, NULL_ }; | ^~~~~~and a tweak to integrate the suggestions about misspelled members with that for accessors:
$ g++ -c accessor-fixit.cc accessor-fixit.cc: In function 'int test(t*)': accessor-fixit.cc:17:15: error: 'class t' has no member named 'ratio'; did you mean 'int t::m_ratio'? (accessible via 'int t::get_ratio() const') 17 | return ptr->ratio; | ^~~~~ | get_ratio()In addition, various diagnostics in the C++ front-end have been streamlined by consolidating the suggestion into the initial error, rather than emitting a follow-up note:
$ g++ typo.cc typo.cc:5:13: error: 'BUFSIZE' was not declared in this scope; did you mean 'BUF_SIZE'? 5 | uint8_t buf[BUFSIZE]; | ^~~~~~~ | BUF_SIZE
<execution>
(requires Thread
Building Blocks 2018 or newer).
<memory_resource>. <filesystem>
does not require linking with -lstdc++fs now.
std::remove_cvref,
std::unwrap_reference, std::unwrap_decay_ref,
std::is_nothrow_convertible, and
std::type_identity.<bit> and
<version>.std::erase_if).contains member of maps and sets.starts_with,
ends_with).std::midpoint and std::lerp for
interpolation.std::bind_front.std::visit<R>.std::assume_aligned.std::pmr::polymorphic_allocator<std::byte>.char8_t type.delete.std::is_constant_evaluated() function.BACK argument for MINLOC and
MAXLOC has been implemented.
FINDLOC intrinsic function has been
implemented.
IS_CONTIGUOUS intrinsic function has been
implemented.
c%re
and c%im has been implemented.
str%len
and a%kind has been implemented.
ISO_Fortran_binding.h
source file have been implemented.
MAX and MIN intrinsics are no
longer guaranteed to return any particular value in case one of the
arguments is a NaN. Note that this conforms to the
Fortran standard and to what other Fortran compilers do. If there
is a need to handle that case in some specific way, one needs to
explicitly check for NaN's before
calling MAX or MIN, e.g. by using
the IEEE_IS_NAN function from the intrinsic
module IEEE_ARITHMETIC.
-fdec-include, set also
by the -fdec option,
has been added to increase compatibility with legacy code. With this
option, an INCLUDE directive is also parsed as a statement,
which allows the directive to be spread across multiple source lines
with line continuations.
BUILTIN directive, has been added.
The purpose of the directive is to provide an API between the GCC compiler and
the GNU C Library which would define vector implementations of math routines.
cortex-a76).cortex-a76.cortex-a55).neoverse-n1).-mcpu or -mtune options,
for example: -mcpu=cortex-a76 or
-mtune=cortex-a76.cortex-a55 or as arguments to the equivalent target
attributes and pragmas.
The Armv8.3-A complex number instructions are now supported via intrinsics
when the option -march=armv8.3-a or equivalent is specified.
For the half-precision floating-point variants of these instructions use the
architecture extension flag +fp16, e.g.
-march=armv8.3-a+fp16.
The intrinsics are defined by the ACLE specification.
-march=armv8.5-a option.
-march=armv8-a+sb option.-march=armv8-a+predres option.-march=armv8-a+ssbs option. This does not require a
compiler option for Arm and thus -march=armv8-a+ssbs
is an AArch64-specific option.-mcpu=neoverse-e1).
-fstack-clash-protection option. The probing interval/guard
size can be set by using --param stack-clash-protection-guard-size=12|16.
The value of this parameter must be in bytes represented as a power of two.
The two supported values for this parameter are 12 (for a 4KiB size, 2^12)
and 16 (for a 64KiB size, 2^16).
The default value is 16 (64Kb) and can be changed at configure
time using the flag --with-stack-clash-protection-guard-size=12|16.
-msign-return-address= has been deprecated. This
has been replaced by the new -mbranch-protection= option. This
new option can now be used to enable the return address signing as well as
the new Branch Target Identification feature of Armv8.5-A architecture. For
more information on the arguments accepted by this option, please refer to
AArch64-Options.
-march=armv8.5-a+rng option.-march=armv8.5-a+memtag option.-march
values and the -mcpu options that used these architectures
have been removed.
-mcpu options.
fiji).gfx900).-mlra.-mptwrite compiler switch.
-march=cascadelake. The switch enables the following
ISA extensions: AVX512F, AVX512VL, AVX512CD, AVX512BW, AVX512DQ, AVX512VNNI.
loongson-mmi and loongson-ext
extensions have been split from loongson3a:
loongson-mmi contains
the Loongson MMI (MultiMedia extensions Instructions).loongson-ext contains
the Loongson EXT (EXTensions instructions).loongson-ext2 contains the Loongson EXT2 instructions.-m[no-]loongson-mmi,
-m[no-]loongson-ext, and -m[no-]loongson-ext2
enable or disable those extensions.
gs464)
which enables loongson-mmi, loongson-ext by default.gs464e)
which enables loongson-mmi, loongson-ext, loongson-ext2 by default.gs264e)
which enables loongson-ext, loongson-ext2, msa by default.-mcpu
and -mtune options (as in -mcpu=gs464 or
-mtune=gs464e) or as arguments to the equivalent target
attributes and pragmas.
-march=arch13 option, the compiler will generate
code making use of the new instructions introduced with the vector
enhancement facility 2 and the miscellaneous instruction extension
facility 2. The -mtune=arch13 option enables arch13
specific instruction scheduling without making use of new
instructions.-mzvector option.-march=z14 or higher GCC emits
alignments hints on the vector load/store instructions (8 or 16
byte).-mfentry is now supported. As well
as the mcount mechanism the __fentry__ is called
before the function prologue.
However, since just a single instruction is required to call
__fentry__ the call sequence imposes a smaller
overhead than mcount (4 instructions).
The produced code is compatible only with newer glibc versions,
which provide the __fentry__ symbol and do not
clobber r0 when resolving lazily bound functions.
-mfentry is only supported when generating 64 bit
code and does not work with nested C functions.-mnop-mcount option can be used to emit NOP
instructions instead of an mcount or fentry call stub.-mrecord-mcount option
a __mcount_loc section is generated containing
pointers to each profiling call stub. This is useful for
automatically patching in and out calls.g++ now unconditionally enables large file support when
compiling 32-bit code.
-mms-bitfields option is specified,
or __attribute__((ms_struct)) is used
-mhitachi option is
specified, or __attribute__((renesas)) is used
auto_diagnostic_group. Such
diagnostics will be nested by the output of
-fdiagnostics-format=json.
This is the list of problem reports (PRs) from GCC's bug tracking system that are known to be fixed in the 9.1 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 9.2 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 9.3 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 9.4 release. This list might not be complete (that is, it is possible that some PRs that have been fixed are not listed here).
-moutline-atomics has been added to aid
deployment of the Large System Extensions (LSE) on GNU/Linux systems built
with a baseline architecture targeting Armv8-A. When the option is
specified code is emitted to detect the presence of LSE instructions at
run time and use them for standard atomic operations.
For more information please refer to the documentation.
-mcpu
and -mtune options are -mcpu=a64fx and
-mtune=a64fx respectively. In particular,
-mcpu=a64fx generates code for Armv8.2-A with SVE and
tunes the code for the A64FX. This includes tuning the SVE code,
although by default the code is still length-agnostic and so works for
all SVE implementations. Adding -msve-vector-bits=512
makes the code specific to 512-bit SVE.This is the list of problem reports (PRs) from GCC's bug tracking system that are known to be fixed in the 9.5 release. This list might not be complete (that is, it is possible that some PRs that have been fixed are not listed here).
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 2023-02-22.