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GCC 8 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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<h1>GCC 8 Release Series<br/>Changes, New Features, and Fixes</h1>
<p>
This page is a "brief" summary of some of the huge number of improvements
in GCC 8.
You may also want to check out our
<a href="porting_to.html">Porting to GCC 8</a> page and the
<a href="../onlinedocs/index.html#current">full GCC documentation</a>.
</p>
<h2>Caveats</h2>
<ul>
<li>Support for the obsolete SDB/coff debug info format has been
<strong>removed</strong>. The option <code>-gcoff</code> no longer
does anything.</li>
<li>The Cilk+ extensions to the C and C++ languages have been removed.</li>
<li>
The MPX extensions to the C and C++ languages have been deprecated and
will be removed in a future release.
</li>
<li>
The extension allowing arithmetic on <code>std::atomic<void*></code>
and types like <code>std::atomic<R(*)()></code>
has been deprecated.</li>
<li>
The non-standard C++0x <code>std::copy_exception</code> function was
removed. <code>std::make_exception_ptr</code> should be used instead.
</li>
<li><p>Support for the <code>powerpc*-*-*spe*</code> target ports which have
been recently unmaintained and untested in GCC has been declared
obsolete in GCC 8 as announced
<a href="https://gcc.gnu.org/ml/gcc/2018-04/msg00102.html">here</a>.
Unless there is activity to revive them, the
next release of GCC will have their sources permanently
<strong>removed</strong>.</p>
</li>
</ul>
<!-- .................................................................. -->
<h2 id="general">General Improvements</h2>
<ul>
<li>Inter-procedural optimization improvements:
<ul>
<li>Reworked run-time estimation metrics leading to more realistic guesses
driving inliner and cloning heuristics.</li>
<li>The ipa-pure-const pass is extended to propagate the
<code>malloc</code> attribute, and the corresponding warning option
<code>-Wsuggest-attribute=malloc</code> emits a diagnostic for
functions which can be annotated with the <code>malloc</code>
attribute.</li>
</ul></li>
<li>Profile driven optimization improvements:
<ul>
<li>New infrastructure for representing profiles (both statically guessed
and profile feedback) which allows propagation of additional information
about the reliability of the profile.</li>
<li>A number of improvements in the profile updating code solving problems
found by new verification code.</li>
<li>Static detection of code which is not executed in a valid run of the
program. This includes paths which trigger undefined behavior
as well as calls to functions declared with the <code>cold</code> attribute.
Newly the <code>noreturn</code> attribute does not imply all effects of
<code>cold</code> to differentiate between <code>exit</code> (which
is <code>noreturn</code>) and <code>abort</code> (which is in addition
not executed in valid runs).</li>
<li><code>-freorder-blocks-and-partition</code>, a pass splitting function
bodies into hot and cold regions, is now enabled by default at <code>-O2</code>
and higher for x86 and x86-64.</li>
</ul></li>
<li>Link-time optimization improvements:
<ul>
<li>We have significantly improved debug information on ELF targets
using DWARF by properly preserving language-specific information.
This allows for example the libstdc++ pretty-printers to work with
LTO optimized executables.</li>
</ul></li>
<li>
A new option <code>-fcf-protection=[full|branch|return|none]</code> is
introduced to perform code instrumentation to increase program security by
checking that target addresses of control-flow transfer instructions (such as
indirect function call, function return, indirect jump) are valid. Currently
the instrumentation is supported on x86 GNU/Linux targets only. See the user
guide for further information about the option syntax and section "New Targets
and Target Specific Improvements" for IA-32/x86-64 for more details.
</li>
<li>The <code>-gcolumn-info</code> option is now enabled by default.
It includes column information in addition to just filenames and
line numbers in DWARF debugging information.</li>
<li>
The polyhedral-based loop nest optimization pass
<code>-floop-nest-optimize</code> has been overhauled. It's still
considered experimental and may not result in any runtime improvements.
</li>
<li>
Two new classical loop nest optimization passes have been added.
<code>-floop-unroll-and-jam</code> performs outer loop unrolling
and fusing of the inner loop copies. <code>-floop-interchange</code>
exchanges loops in a loop nest to improve data locality. Both passes
are enabled by default at <code>-O3</code> and above.
</li>
<li>
The classic loop nest optimization pass <code>-ftree-loop-distribution</code>
has been improved and enabled by default at <code>-O3</code> and above.
It supports loop nest distribution in some restricted scenarios; it also
supports cancellable innermost loop distribution with loop versioning
under run-time alias checks.
</li>
<li>
The new option <code>-fstack-clash-protection</code> causes the
compiler to insert probes whenever stack space is allocated
statically or dynamically to reliably detect stack overflows and
thus mitigate the attack vector that relies on jumping over
a stack guard page as provided by the operating system.
</li>
<li>
A new pragma <code>GCC unroll</code> has been implemented in the C
family of languages, as well as Fortran and Ada, so as to make it
possible for the user to have a finer-grained control over the loop
unrolling optimization.
</li>
<li>
GCC has been enhanced to detect more instances of meaningless or
mutually exclusive attribute specifications and handle such conflicts
more consistently. Mutually exclusive attribute specifications are
ignored with a warning regardless of whether they appear on the same
declaration or on distinct declarations of the same entity. For
example, because the <code>noreturn</code> attribute on the second
declaration below is mutually exclusive with the <code>malloc</code>
attribute on the first, it is ignored and a warning is issued.
<pre>
void* __attribute__ ((malloc)) f (unsigned);
void* __attribute__ ((noreturn)) f (unsigned);
<span class="boldmagenta">warning: </span>ignoring attribute '<b>noreturn</b>' because it conflicts with attribute '<b>malloc</b>' [<span class="boldmagenta">-Wattributes</span>]</pre></li>
<li>
The <code>gcov</code> tool can distinguish functions that begin
on a same line in a source file. This can be a different template
instantiation or a class constructor:
<blockquote><pre>
File 'ins.C'
Lines executed:100.00% of 8
Creating 'ins.C.gcov'
-: 0:Source:ins.C
-: 0:Graph:ins.gcno
-: 0:Data:ins.gcda
-: 0:Runs:1
-: 0:Programs:1
-: 1:template<class T>
-: 2:class Foo
-: 3:{
-: 4: public:
2: 5: Foo(): b (1000) {}
------------------
Foo<char>::Foo():
1: 5: Foo(): b (1000) {}
------------------
Foo<int>::Foo():
1: 5: Foo(): b (1000) {}
------------------
2: 6: void inc () { b++; }
------------------
Foo<char>::inc():
1: 6: void inc () { b++; }
------------------
Foo<int>::inc():
1: 6: void inc () { b++; }
------------------
-: 7:
-: 8: private:
-: 9: int b;
-: 10:};
-: 11:
1: 12:int main(int argc, char **argv)
-: 13:{
1: 14: Foo<int> a;
1: 15: Foo<char> b;
-: 16:
1: 17: a.inc ();
1: 18: b.inc ();
1: 19:}
</pre></blockquote>
</li>
<li>The <code>gcov</code> tool has more accurate numbers for execution of lines
in a source file.</li>
<li>The <code>gcov</code> tool can use TERM colors to provide more readable output.</li>
<li>AddressSanitizer gained a new pair of sanitization options,
<code>-fsanitize=pointer-compare</code> and <code>-fsanitize=pointer-subtract</code>, which
warn about subtraction (or comparison) of pointers that point to
a different memory object:
<blockquote><pre>
int
main ()
{
/* Heap allocated memory. */
char *heap1 = (char *)__builtin_malloc (42);
char *heap2 = (char *)__builtin_malloc (42);
if (heap1 > heap2)
return 1;
return 0;
}
<span class="boldred">==17465==ERROR: AddressSanitizer: invalid-pointer-pair: 0x604000000010 0x604000000050</span>
#0 0x40070f in main /tmp/pointer-compare.c:7
#1 0x7ffff6a72a86 in __libc_start_main (/lib64/libc.so.6+0x21a86)
#2 0x400629 in _start (/tmp/a.out+0x400629)
<span class="boldlime">0x604000000010 is located 0 bytes inside of 42-byte region [0x604000000010,0x60400000003a)</span>
allocated by thread T0 here:
#0 0x7ffff6efb390 in __interceptor_malloc ../../../../libsanitizer/asan/asan_malloc_linux.cc:86
#1 0x4006ea in main /tmp/pointer-compare.c:5
#2 0x7ffff6a72a86 in __libc_start_main (/lib64/libc.so.6+0x21a86)
<span class="boldlime">0x604000000050 is located 0 bytes inside of 42-byte region [0x604000000050,0x60400000007a)</span>
allocated by thread T0 here:
#0 0x7ffff6efb390 in __interceptor_malloc ../../../../libsanitizer/asan/asan_malloc_linux.cc:86
#1 0x4006f8 in main /tmp/pointer-compare.c:6
#2 0x7ffff6a72a86 in __libc_start_main (/lib64/libc.so.6+0x21a86)
SUMMARY: AddressSanitizer: invalid-pointer-pair /tmp/pointer-compare.c:7 in main
</pre></blockquote>
</li>
<li>
The store merging pass has been enhanced to handle bit-fields and not
just constant stores, but also data copying from adjacent memory
locations into other adjacent memory locations, including bitwise
logical operations on the data. The pass can also handle byte swapping
into memory locations.
</li>
<li>
The undefined behavior sanitizer gained two new options included in
<code>-fsanitize=undefined</code>: <code>-fsanitize=builtin</code> which
diagnoses at run time invalid arguments to <code>__builtin_clz</code> or
<code>__builtin_ctz</code> prefixed builtins, and
<code>-fsanitize=pointer-overflow</code> which performs cheap run time
tests for pointer wrapping.
</li>
</ul>
<!-- .................................................................. -->
<h2 id="languages">New Languages and Language specific improvements</h2>
<h3 id="ada">Ada</h3>
<ul>
<li>For its internal exception handling used on the host for error
recovery in the front-end, the compiler now relies on the native
exception handling mechanism of the host platform, which should
be more efficient than the former mechanism.
</li>
</ul>
<h3 id="brig">BRIG (HSAIL)</h3>
<p>In this release cycle, the focus for the BRIGFE was on stabilization and
performance improvements. Also a couple of completely new features were
added.</p>
<ul>
<li>Improved support for function and module scope group
segment variables. PRM specs define function and module scope group
segment variables as an experimental feature. However, PRM test
suite uses them. Now group segment is handled by separate book
keeping of module scope and function (kernel) offsets. Each function
has a "frame" in the group segment offset to which is given as an
argument, similar to traditional call stack frame handling.</li>
<li>Reduce the number of type conversions due to
the untyped HSAIL registers. Instead of always representing the HSAIL's
untyped registers as unsigned int, the gccbrig now pre-analyzes
the BRIG code and builds the register variables as a type used
the most when storing or reading data to/from each register.
This reduces the number of total casts which cannot be always
optimized away.</li>
<li>Support for BRIG_KIND_NONE directives.</li>
<li>Made -O3 the default optimization level for BRIGFE.</li>
<li>Fixed illegal addresses generated from address expressions
which refer only to offset 0.</li>
<li>Fixed a bug with reg+offset addressing on 32b segments.
In 'large' mode, the offset is treated as 32bits unless it's
in global, read-only or kernarg address space.</li>
<li>Fixed a crash caused sometimes by calls with more
than 4 arguments.</li>
<li>Fixed a mis-execution issue with kernels that have
both unexpanded ID functions and calls to subfunctions.</li>
<li>Treat HSAIL barrier builtins as setjmp/longjump style
functions to avoid illegal optimizations.</li>
<li>Ensure per WI copies of private variables are aligned correctly.</li>
<li>libhsail-rt: Assume the host runtime allocates the work group
memory.</li>
</ul>
<h3 id="c-family">C family</h3>
<ul>
<li>New command-line options have been added for the C and C++ compilers:
<ul>
<li><code><a href="https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html#index-Wmultistatement-macros">-Wmultistatement-macros</a></code>
warns about unsafe macros expanding to multiple statements used
as a body of a statement such as <code>if</code>, <code>else</code>,
<code>while</code>, <code>switch</code>, or <code>for</code>.</li>
<li><code><a href="https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html#index-Wstringop-truncation">-Wstringop-truncation</a></code>
warns for calls to bounded string manipulation functions such as
<code>strncat</code>, <code>strncpy</code>, and <code>stpncpy</code>
that might either truncate the copied string or leave the destination
unchanged. For example, the following call to <code>strncat</code>
is diagnosed because it appends just three of the four characters
from the source string.<pre>
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}
<span class="boldmagenta">warning: '</span><b>strncat</b>' output truncated copying 3 bytes from a string of length 4 [<span class="boldmagenta">-Wstringop-truncation</span>]</pre>
Similarly, in the following example, the call to <code>strncpy</code>
specifies the size of the destination buffer as the bound. If the
length of the source string is equal to or greater than this size
the result of the copy will not be NUL-terminated. Therefore,
the call is also diagnosed. To avoid the warning, specify
<code>sizeof buf - 1</code> as the bound and set the last element of
the buffer to NUL.<pre>
void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
…
}
<span class="boldmagenta">warning: '</span><b>strncpy</b>' specified bound 80 equals destination size [<span class="boldmagenta">-Wstringop-truncation</span>]</pre>
The <code>-Wstringop-truncation</code> option is included in
<code>-Wall</code>.<br/>
Note that due to GCC bug <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82944" title="missing -Wstringop-truncation on strncpy due to system header macro">82944</a>, defining <code>strncat</code>, <code>strncpy</code>,
or <code>stpncpy</code> as a macro in a system header as some
implementations do, suppresses the warning.</li>
<li><code><a href="https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html#index-Wif-not-aligned">-Wif-not-aligned</a></code> controls warnings issued in response
to invalid uses of objects declared with attribute
<code><a href="https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Common-Variable-Attributes.html#index-warn_005fif_005fnot_005faligned-variable-attribute">warn_if_not_aligned</a></code>.<br/>
The <code>-Wif-not-aligned</code> option is included in
<code>-Wall</code>.</li>
<li><code><a href="https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html#index-Wmissing-attributes">-Wmissing-attributes</a></code> warns
when a declaration of a function is missing one or more attributes
that a related function is declared with and whose absence may
adversely affect the correctness or efficiency of generated code.
For example, in C++, the warning is issued when an explicit
specialization of a primary template declared with attribute
<code>alloc_align</code>, <code>alloc_size</code>,
<code>assume_aligned</code>, <code>format</code>,
<code>format_arg</code>, <code>malloc</code>, or <code>nonnull</code>
is declared without it. Attributes <code>deprecated</code>,
<code>error</code>, and <code>warning</code> suppress the warning.
<br/>
The <code>-Wmissing-attributes</code> option is included in
<code>-Wall</code>.</li>
<li><code><a href="https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html#index-Wpacked-not-aligned">-Wpacked-not-aligned</a></code> warns
when a <code>struct</code> or <code>union</code> declared with
attribute <code>packed</code> defines a member with an explicitly
specified alignment greater than 1. Such a member will wind up
under-aligned. For example, a warning will be issued for
the definition of <code>struct A</code> in the following:
<pre>
struct __attribute__ ((aligned (8)))
S8 { char a[8]; };
struct __attribute__ ((packed)) A
{
struct S8 s8;
};
<span class="boldmagenta">warning: </span>alignment 1 of '<b>struct S</b>' is less than 8 [<span class="boldmagenta">-Wpacked-not-aligned</span>]</pre>
The <code>-Wpacked-not-aligned</code> option is included in
<code>-Wall</code>.</li>
</ul>
<ul>
<li><code>-Wcast-function-type</code> warns when a function pointer
is cast to an incompatible function pointer. This warning is enabled
by <code>-Wextra</code>.</li>
</ul>
<ul>
<li><code>-Wsizeof-pointer-div</code> warns for suspicious divisions
of the size of a pointer by the size of the elements it points to,
which looks like the usual way to compute the array size but
won't work out correctly with pointers.
This warning is enabled by <code>-Wall</code>.</li>
</ul>
<ul>
<li><code>-Wcast-align=strict</code> warns whenever a pointer is cast
such that the required alignment of the target is increased. For
example, warn if a <code>char *</code> is cast to an <code>int *</code>
regardless of the target machine.</li>
</ul>
<ul>
<li><code>-fprofile-abs-path</code> creates absolute path names in the
<code>.gcno</code> files. This allows <code>gcov</code> to find the
correct sources in projects where compilations occur with different
working directories.</li>
</ul>
</li>
<li><code>-fno-strict-overflow</code> is now mapped to
<code>-fwrapv -fwrapv-pointer</code> and signed integer overflow
is now undefined by default at all optimization levels. Using
<code>-fsanitize=signed-integer-overflow</code> is now the preferred
way to audit code, <code>-Wstrict-overflow</code> is deprecated.</li>
<li>The <code><a href="https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html#index-Warray-bounds">-Warray-bounds</a></code> option has been
improved to detect more instances of out-of-bounds array indices and
pointer offsets. For example, negative or excessive indices into
flexible array members and string literals are detected.</li>
<li>The <code><a href="https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html#index-Wrestrict">-Wrestrict</a></code> option introduced in
GCC 7 has been enhanced to detect many more instances of overlapping
accesses to objects via <code>restrict</code>-qualified arguments to
standard memory and string manipulation functions such as
<code>memcpy</code> and <code>strcpy</code>. For example,
the <code>strcpy</code> call in the function below attempts to truncate
the string by replacing its initial characters with the last four.
However, because the function writes the terminating NUL into
<code>a[4]</code>, the copies overlap and the call is diagnosed.<pre>
void f (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
…
}</pre>
The <code>-Wrestrict</code> option is included in <code>-Wall</code>.
</li>
<li>Several optimizer enhancements have enabled improvements to
the <code><a href="https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html#index-Wformat-overflow">-Wformat-overflow</a></code> and
<code><a href="https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html#index-Wformat-truncation">-Wformat-truncation</a></code> options.
The warnings detect more instances of buffer overflow and truncation
than in GCC 7 and are better at avoiding certain kinds of false
positives.</li>
<li>When reporting mismatching argument types at a function call, the
C and C++ compilers now underline both the argument and the pertinent
parameter in the declaration.
<pre class="blackbg">
$ gcc arg-type-mismatch.cc
<span class="bold">arg-type-mismatch.cc:</span> In function '<span class="bold">int caller(int, int, float)</span>':
<span class="bold">arg-type-mismatch.cc:5:24:</span> <span class="boldred">error: </span>invalid conversion from '<span class="bold">int</span>' to '<span class="bold">const char*</span>' [<span class="boldred">-fpermissive</span>]
return callee(first, <span class="boldred">second</span>, third);
<span class="boldred">^~~~~~</span>
<span class="bold">arg-type-mismatch.cc:1:40:</span> <span class="boldcyan">note: </span> initializing argument 2 of '<span class="bold">int callee(int, const char*, float)</span>'
extern int callee(int one, <span class="boldcyan">const char *two</span>, float three);
<span class="boldcyan">~~~~~~~~~~~~^~~</span>
</pre>
</li>
<li>When reporting on unrecognized identifiers, the C and C++ compilers
will now emit fix-it hints suggesting <code>#include</code> directives
for various headers in the C and C++ standard libraries.
<pre class="blackbg">
$ gcc incomplete.c
<span class="bold">incomplete.c:</span> In function '<span class="bold">test</span>':
<span class="bold">incomplete.c:3:10:</span> <span class="boldred">error: </span>'<span class="bold">NULL</span>' undeclared (first use in this function)
return <span class="boldred">NULL</span>;
<span class="boldred">^~~~</span>
<span class="bold">incomplete.c:3:10:</span> <span class="boldcyan">note: </span>'<span class="bold">NULL</span>' is defined in header '<span class="bold"><stddef.h></span>'; did you forget to '<span class="bold">#include <stddef.h></span>'?
<span class="bold">incomplete.c:1:1:</span>
+<span class="green">#include <stddef.h></span>
const char *test(void)
<span class="bold">incomplete.c:3:10:</span>
return <span class="boldcyan">NULL</span>;
<span class="boldcyan">^~~~</span>
<span class="bold">incomplete.c:3:10:</span> <span class="boldcyan">note: </span>each undeclared identifier is reported only once for each function it appears in
</pre>
<pre class="blackbg">
$ gcc incomplete.cc
<span class="bold">incomplete.cc:1:6:</span> <span class="boldred">error: </span>'<span class="bold">string</span>' in namespace '<span class="bold">std</span>' does not name a type
std::<span class="boldred">string</span> s("hello world");
<span class="boldred">^~~~~~</span>
<span class="bold">incomplete.cc:1:1:</span> <span class="boldcyan">note: </span>'<span class="bold">std::string</span>' is defined in header '<span class="bold"><string></span>'; did you forget to '<span class="bold">#include <string></span>'?
+<span class="green">#include <string></span>
<span class="boldcyan">std</span>::string s("hello world");
<span class="boldcyan">^~~</span>
</pre>
</li>
<li>The C and C++ compilers now use more intuitive locations when
reporting on missing semicolons, and offer fix-it hints:
<pre class="blackbg">
$ gcc t.c
<span class="bold">t.c:</span> In function '<span class="bold">test</span>':
<span class="bold">t.c:3:12:</span> <span class="boldred">error: </span>expected '<span class="bold">;</span>' before '<span class="bold">}</span>' token
return 42
<span class="boldred">^</span>
<span class="green">;</span>
<span class="green">}</span>
<span class="green">~</span>
</pre>
</li>
<li>When reporting on missing '}' and ')' tokens, the C and C++
compilers will now highlight the corresponding '{' and '(' token,
issuing a 'note' if it's on a separate line:
<pre class="blackbg">
$ gcc unclosed.c
<span class="bold">unclosed.c:</span> In function '<span class="bold">log_when_out_of_range</span>':
<span class="bold">unclosed.c:12:50:</span> <span class="boldred">error: </span>expected '<span class="bold">)</span>' before '<span class="bold">{</span>' token
&& (temperature < MIN || temperature > MAX)<span class="boldred"> </span><span class="green">{</span>
<span class="boldred">^</span><span class="green">~</span>
<span class="green">)</span>
<span class="bold">unclosed.c:11:6:</span> <span class="boldcyan">note: </span>to match this '<span class="bold">(</span>'
if <span class="boldcyan">(</span>logging_enabled && check_range ()
<span class="boldcyan">^</span>
</pre>
or highlighting it directly if it's on the same line:
<pre class="blackbg">
$ gcc unclosed-2.c
<span class="bold">unclosed-2.c:</span> In function '<span class="bold">test</span>':
<span class="bold">unclosed-2.c:8:45:</span> <span class="boldred">error: </span>expected '<span class="bold">)</span>' before '<span class="bold">{</span>' token
if <span class="blue">(</span>temperature < MIN || temperature > MAX<span class="boldred"> </span><span class="green">{</span>
<span class="blue">~</span> <span class="boldred">^</span><span class="green">~</span>
<span class="green">)</span>
</pre>
They will also emit fix-it hints.
</li>
</ul>
<h3 id="cxx">C++</h3>
<ul>
<li>The value of the C++11 <code>alignof</code> operator has been corrected
to match C <code>_Alignof</code> (minimum alignment) rather than
GNU <code>__alignof__</code> (preferred alignment); on ia32 targets this
means that <code>alignof(double)</code> is now 4 rather than 8. Code that
wants the preferred alignment should use <code>__alignof__</code> instead.
</li>
<li>New command-line options have been added for the C++ compiler to
control warnings:
<ul>
<li><code><a href="https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Dialect-Options.html#index-Wclass-memaccess">-Wclass-memaccess</a></code> warns
when objects of non-trivial class types are manipulated in potentially
unsafe ways by raw memory functions such as <code>memcpy</code>, or
<code>realloc</code>. The warning helps detect calls that bypass
user-defined constructors or copy-assignment operators, corrupt
virtual table pointers, data members of <code>const</code>-qualified
types or references, or member pointers. The warning also detects
calls that would bypass access controls to data members. For example,
a call such as:
<pre>
memcpy (&std::cout, &std::cerr, sizeof std::cout);</pre>
results in
<pre>
<span class="boldmagenta">warning: </span>'<b>void* memcpy(void*, const void*, long unsigned int)</b>' writing to an object of type 'std::ostream' {aka 'class std::basic_ostream<char>'} with no trivial copy-assignment [<span class="boldmagenta">-Wclass-memaccess</span>]</pre>
The <code>-Wclass-memaccess</code> option is included in
<code>-Wall</code>.</li>
</ul>
</li>
<li>
The C++ front end has experimental support for some of the upcoming C++2a
draft features with the <code>-std=c++2a</code> or <code>-std=gnu++2a</code>
flags, including designated initializers, default member initializers for
bit-fields, <code>__VA_OPT__</code> (except that
<code>#__VA_OPT__</code> is unsupported), lambda <code>[=, this]</code>
captures, etc.
For a full list of new features,
see <a href="../projects/cxx-status.html#cxx2a">the C++
status page</a>.
</li>
<li>When reporting on attempts to access private fields of a class or
struct, the C++ compiler will now offer fix-it hints showing how to
use an accessor function to get at the field in question, if one exists.
<pre class="blackbg">
$ gcc accessor.cc
<span class="bold">accessor.cc:</span> In function '<span class="bold">void test(foo*)</span>':
<span class="bold">accessor.cc:12:12:</span> <span class="boldred">error: </span>'<span class="bold">double foo::m_ratio</span>' is private within this context
if (ptr-><span class="boldred">m_ratio</span> >= 0.5)
<span class="boldred">^~~~~~~</span>
<span class="bold">accessor.cc:7:10:</span> <span class="boldcyan">note: </span>declared private here
double <span class="boldcyan">m_ratio</span>;
<span class="boldcyan">^~~~~~~</span>
<span class="bold">accessor.cc:12:12:</span> <span class="boldcyan">note: </span>field '<span class="bold">double foo::m_ratio</span>' can be accessed via '<span class="bold">double foo::get_ratio() const</span>'
if (ptr-><span class="boldcyan">m_ratio</span> >= 0.5)
<span class="boldcyan">^~~~~~~</span>
<span class="green">get_ratio()</span>
</pre>
</li>
<li>The C++ compiler can now give you a hint if you use a macro before it
was defined (e.g. if you mess up the order of your <code>#include</code>
directives):
<pre class="blackbg">
$ gcc ordering.cc
<span class="bold">ordering.cc:2:24:</span> <span class="boldred">error: </span>expected '<span class="bold">;</span>' at end of member declaration
virtual void clone() <span class="boldred">const</span> OVERRIDE { }
<span class="boldred">^~~~~</span>
<span class="green">;</span>
<span class="bold">ordering.cc:2:30:</span> <span class="boldred">error: </span>'<span class="bold">OVERRIDE</span>' does not name a type
virtual void clone() const <span class="boldred">OVERRIDE</span> { }
<span class="boldred">^~~~~~~~</span>
<span class="bold">ordering.cc:2:30:</span> <span class="boldcyan">note: </span>the macro '<span class="bold">OVERRIDE</span>' had not yet been defined
In file included from <span class="bold">ordering.cc:5</span>:
<span class="bold">c++11-compat.h:2:</span> <span class="boldcyan">note: </span>it was later defined here
#define OVERRIDE override
</pre>
</li>
<li>The <code>-Wold-style-cast</code> diagnostic can now emit fix-it hints
telling you when you can use a <code>static_cast</code>,
<code>const_cast</code>, or <code>reinterpret_cast</code>.
<pre class="blackbg">
$ gcc -c old-style-cast-fixits.cc -Wold-style-cast
<span class="bold">old-style-cast-fixits.cc:</span> In function '<span class="bold">void test(void*)</span>':
<span class="bold">old-style-cast-fixits.cc:5:19:</span> <span class="boldmagenta">warning: </span>use of old-style cast to '<span class="bold">struct foo*</span>' [<span class="boldmagenta">-Wold-style-cast</span>]
foo *f = (foo *)<span class="boldmagenta">ptr</span>;
<span class="boldmagenta">^~~</span>
<span class="red">----------</span>
<span class="green">static_cast<foo *> (ptr)</span>
</pre>
</li>
<li>When reporting on problems within <code>extern "C"</code> linkage
specifications, the C++ compiler will now display the location of the
start of the <code>extern "C"</code>.
<pre class="blackbg">
$ gcc -c extern-c.cc
<span class="bold">extern-c.cc:3:1:</span> <span class="boldred">error: </span>template with C linkage
<span class="boldred">template</span> <typename T> void test (void);
<span class="boldred">^~~~~~~~</span>
In file included from <span class="bold">extern-c.cc:1</span>:
<span class="bold">unclosed.h:1:1:</span> <span class="boldcyan">note: </span>'<span class="bold">extern "C"</span>' linkage started here
<span class="boldcyan">extern "C"</span> {
<span class="boldcyan">^~~~~~~~~~</span>
<span class="bold">extern-c.cc:3:39:</span> <span class="boldred">error: </span>expected '<span class="bold">}</span>' at end of input
template <typename T> void test (void)<span class="boldred">;</span>
<span class="boldred">^</span>
In file included from <span class="bold">extern-c.cc:1</span>:
<span class="bold">unclosed.h:1:12:</span> <span class="boldcyan">note: </span>to match this '<span class="bold">{</span>'
extern "C" <span class="boldcyan">{</span>
<span class="boldcyan">^</span>
</pre>
</li>
<li>When reporting on mismatching template types, the C++ compiler will
now use color to highlight the mismatching parts of the template, and will
elide the parameters that are common between two mismatching templates,
printing <code>[...]</code> instead:
<pre class="blackbg">
$ gcc templates.cc
<span class="bold">templates.cc:</span> In function '<span class="bold">void test()</span>':
<span class="bold">templates.cc:9:8:</span> <span class="boldred">error: </span>could not convert '<span class="bold">vector<double>()</span>' from '<span class="bold">vector<<span class="boldgreen">double</span>></span>' to '<span class="bold">vector<<span class="boldgreen">int</span>></span>'
fn_1(<span class="boldred">vector<double> ()</span>);
<span class="boldred">^~~~~~~~~~~~~~~~~</span>
<span class="bold">templates.cc:10:8:</span> <span class="boldred">error: </span>could not convert '<span class="bold">map<int, double>()</span>' from '<span class="bold">map<[...],<span class="boldgreen">double</span>></span>' to '<span class="bold">map<[...],<span class="boldgreen">int</span>></span>'
fn_2(<span class="boldred">map<int, double>()</span>);
<span class="boldred">^~~~~~~~~~~~~~~~~~</span>
</pre>
Those <code>[...]</code> elided parameters can be seen using
<code>-fno-elide-type</code>:
<pre class="blackbg">
$ gcc templates.cc -fno-elide-type
<span class="bold">templates.cc:</span> In function '<span class="bold">void test()</span>':
<span class="bold">templates.cc:9:8:</span> <span class="boldred">error: </span>could not convert '<span class="bold">vector<double>()</span>' from '<span class="bold">vector<<span class="boldgreen">double</span>></span>' to '<span class="bold">vector<<span class="boldgreen">int</span>></span>'
fn_1(<span class="boldred">vector<double> ()</span>);
<span class="boldred">^~~~~~~~~~~~~~~~~</span>
<span class="bold">templates.cc:10:8:</span> <span class="boldred">error: </span>could not convert '<span class="bold">map<int, double>()</span>' from '<span class="bold">map<int,<span class="boldgreen">double</span>></span>' to '<span class="bold">map<int,<span class="boldgreen">int</span>></span>'
fn_2(<span class="boldred">map<int, double>()</span>);
<span class="boldred">^~~~~~~~~~~~~~~~~~</span>
</pre>
The C++ compiler has also gained an option
<code>-fdiagnostics-show-template-tree</code> which visualizes such
mismatching templates in a hierarchical form:
<pre class="blackbg">
$ gcc templates-2.cc -fdiagnostics-show-template-tree
<span class="bold">templates-2.cc:</span> In function '<span class="bold">void test()</span>':
<span class="bold">templates-2.cc:9:8:</span> <span class="boldred">error: </span>could not convert '<span class="bold">vector<double>()</span>' from '<span class="bold">vector<<span class="boldgreen">double</span>></span>' to '<span class="bold">vector<<span class="boldgreen">int</span>></span>'
vector<
[<span class="boldgreen">double</span> != <span class="boldgreen">int</span>]>
fn_1(<span class="boldred">vector<double> ()</span>);
<span class="boldred">^~~~~~~~~~~~~~~~~</span>
<span class="bold">templates-2.cc:10:8:</span> <span class="boldred">error: </span>could not convert '<span class="bold">map<map<int, vector<double> >, vector<double> >()</span>' from '<span class="bold">map<map<[...],vector<<span class="boldgreen">double</span>>>,vector<<span class="boldgreen">double</span>>></span>' to '<span class="bold">map<map<[...],vector<<span class="boldgreen">float</span>>>,vector<<span class="boldgreen">float</span>>></span>'
map<
map<
[...],
vector<
[<span class="boldgreen">double</span> != <span class="boldgreen">float</span>]>>,
vector<
[<span class="boldgreen">double</span> != <span class="boldgreen">float</span>]>>
fn_2(<span class="boldred">map<map<int, vector<double>>, vector<double>> ()</span>);
<span class="boldred">^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</span>
</pre>
which again works with <code>-fno-elide-type</code>:
<pre class="blackbg">
$ gcc templates-2.cc -fdiagnostics-show-template-tree -fno-elide-type
<span class="bold">templates-2.cc:</span> In function '<span class="bold">void test()</span>':
<span class="bold">templates-2.cc:9:8:</span> <span class="boldred">error: </span>could not convert '<span class="bold">vector<double>()</span>' from '<span class="bold">vector<<span class="boldgreen">double</span>></span>' to '<span class="bold">vector<<span class="boldgreen">int</span>></span>'
vector<
[<span class="boldgreen">double</span> != <span class="boldgreen">int</span>]>
fn_1(<span class="boldred">vector<double> ()</span>);
<span class="boldred">^~~~~~~~~~~~~~~~~</span>
<span class="bold">templates-2.cc:10:8:</span> <span class="boldred">error: </span>could not convert '<span class="bold">map<map<int, vector<double> >, vector<double> >()</span>' from '<span class="bold">map<map<int,vector<<span class="boldgreen">double</span>>>,vector<<span class="boldgreen">double</span>>></span>' to '<span class="bold">map<map<int,vector<<span class="boldgreen">float</span>>>,vector<<span class="boldgreen">float</span>>></span>'
map<
map<
int,
vector<
[<span class="boldgreen">double</span> != <span class="boldgreen">float</span>]>>,
vector<
[<span class="boldgreen">double</span> != <span class="boldgreen">float</span>]>>
fn_2(<span class="boldred">map<map<int, vector<double>>, vector<double>> ()</span>);
<span class="boldred">^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</span>
</pre>
</li>
<li>Flowing off the end of a non-void function
is considered unreachable and may be subject to optimization
on that basis. As a result of this change, <code>-Wreturn-type</code>
warnings are enabled by default for C++.</li>
</ul>
<h4 id="libstdcxx">Runtime Library (libstdc++)</h4>
<ul>
<li>Improved experimental support for C++17, including the following features:
<ul>
<li>Deduction guides to support class template argument deduction.</li>
<li><code>std::filesystem</code> implementation.</li>
<li><code>std::char_traits<char></code> and
<code>std::char_traits<wchar_t></code> are usable in constant
expressions.</li>
<li><code>std::to_chars</code> and <code>std::from_chars</code> (for
integers only, not for floating point types).</li>
</ul>
</li>
<li>Experimental support for C++2a:
<code>std::to_address</code> (thanks to Glen Fernandes)
and <code>std::endian</code>.</li>
<li>On GNU/Linux, <code>std::random_device::entropy()</code> accesses the
kernel's entropy count for the random device, if known
(thanks to Xi Ruoyao).</li>
<li>Support for <code>std::experimental::source_location</code>.</li>
<li>AddressSanitizer integration for <code>std::vector</code>, detecting
out-of-range accesses to the unused capacity of a vector.
</li>
<li>Extensions <code>__gnu_cxx::airy_ai</code> and
<code>__gnu_cxx::airy_bi</code> added to the Mathematical Special
Functions.
</li>
</ul>
<h3 id="fortran">Fortran</h3>
<ul>
<li>
The main version of libfortran has been changed to 5.
</li>
<li>
Parameterized derived types, a major feature of Fortran 2003, have been
implemented.
</li>
<li>
The maximum rank for arrays has been increased to 15, conforming to the
Fortran 2008 standard.
</li>
<li>
Transformational intrinsics are now fully supported in initialization
expressions.
</li>
<li>
New flag <code>-fc-prototypes</code> to write C prototypes for
<code>BIND(C)</code> procedures and variables.
</li>
<li>
If <code>-fmax-stack-var-size</code> is honored if given together with
<code>-Ofast</code>, <code>-fstack-arrays</code> is no longer set in that
case.
</li>
<li>
New options <code>-fdefault-real-16</code> and <code>-fdefault-real-10</code>
to control the default kind of <code>REAL</code> variables.
</li>
<li>
A warning is now issued if an array subscript inside a DO loop could lead
to an out-of-bounds-access. The new option <code>-Wdo-subscript</code>,
enabled by <code>-Wextra</code>, warns about this even if the compiler can
not prove that the code will be executed.
</li>
<li>
The Fortran front end now attempts to interchange loops if it is deemed
profitable. So far, this is restricted to <code>FORALL</code> and <code>DO
CONCURRENT</code> statements with multiple indices. This behavior be
controlled with the new flag <code>-ffrontend-loop-interchange</code>,
which is enabled with optimization by default.
The <code>-Wfrontend-loop-interchange</code> option warns about such
occurrences.
</li>
<li>
When an actual argument contains too few elements for a dummy argument,
an error is now issued. The <code>-std=legacy</code> option can be
used to still compile such code.
</li>
<li>
The <code>RECL=</code> argument to <code>OPEN</code>
and <code>INQUIRE</code> statements now allows 64-bit
integers, making records larger than 2GiB possible.
</li>
<li>
The <code>GFORTRAN_DEFAULT_RECL</code> environment variable no
longer has any effect. The record length for preconnected units is
now larger than any practical limit, same as for sequential access
units opened without an explicit <code>RECL=</code> specifier.
</li>
<li>
Character variables longer than <code>HUGE(0)</code> elements are
now possible on 64-bit targets. Note that this changes the
procedure call ABI for all procedures with character arguments on
64-bit targets, as the type of the hidden character length
argument has changed. The hidden character length argument is now
of type <code>INTEGER(C_SIZE_T)</code>.
</li>
</ul>
<h3 id="go">Go</h3>
<ul>
<li>GCC 8 provides a complete implementation of the Go 1.10.1
user packages.</li>
<li>The garbage collector is now fully concurrent. As before,
values stored on the stack are scanned conservatively, but value
stored in the heap are scanned precisely.</li>
<li>Escape analysis is fully implemented and enabled by default in
the Go frontend. This significantly reduces the number of heap
allocations by allocating values on the stack instead.</li>
</ul>
<!-- .................................................................. -->
<h2 id="jit">libgccjit</h2>
<p>The libgccjit API gained four new entry points:</p>
<ul>
<li><a href="https://gcc.gnu.org/onlinedocs/jit/topics/types.html#gcc_jit_type_get_vector">gcc_jit_type_get_vector</a>
and
</li>
<li><a href="https://gcc.gnu.org/onlinedocs/jit/topics/expressions.html#gcc_jit_context_new_rvalue_from_vector">gcc_jit_context_new_rvalue_from_vector</a> for working with vectors,
</li>
<li><a href="https://gcc.gnu.org/onlinedocs/jit/topics/types.html#gcc_jit_type_get_aligned">gcc_jit_type_get_aligned</a></li>
<li><a href="https://gcc.gnu.org/onlinedocs/jit/topics/function-pointers.html#gcc_jit_function_get_address">gcc_jit_function_get_address</a></li>
</ul>
<p>The C code generated by
<a href="https://gcc.gnu.org/onlinedocs/jit/topics/contexts.html#gcc_jit_context_dump_reproducer_to_file">gcc_jit_context_dump_reproducer_to_file</a>
is now easier-to-read.</p>
<!-- .................................................................. -->
<h2 id="targets">New Targets and Target Specific Improvements</h2>
<h3 id="aarch64">AArch64</h3>
<ul>
<li>
The Armv8.4-A architecture is now supported. It can be used by
specifying the <code>-march=armv8.4-a</code> option.
</li>
<li>
The Dot Product instructions are now supported as an optional extension to the
Armv8.2-A architecture and newer and are mandatory on Armv8.4-A. The extension can be used by
specifying the <code>+dotprod</code> architecture extension. E.g. <code>-march=armv8.2-a+dotprod</code>.
</li>
<li>
The Armv8-A <code>+crypto</code> extension has now been split into two extensions for finer grained control:
<ul>
<li><code>+aes</code> which contains the Armv8-A AES crytographic instructions.</li>
<li><code>+sha2</code> which contains the Armv8-A SHA2 and SHA1 cryptographic instructions.</li>
</ul>
Using <code>+crypto</code> will now enable these two extensions.
</li>
<li>
New Armv8.4-A FP16 Floating Point Multiplication Variant instructions have been added. These instructions are
mandatory in Armv8.4-A but available as an optional extension to Armv8.2-A and Armv8.3-A. The new extension
can be used by specifying the <code>+fp16fml</code> architectural extension on Armv8.2-A and Armv8.3-A. On Armv8.4-A
the instructions can be enabled by specifying <code>+fp16</code>.
</li>
<li>
New cryptographic instructions have been added as optional extensions to Armv8.2-A and newer. These instructions can
be enabled with:
<ul>
<li><code>+sha3</code> New SHA3 and SHA2 instructions from Armv8.4-A. This implies <code>+sha2</code>.</li>
<li><code>+sm4</code> New SM3 and SM4 instructions from Armv8.4-A.</li>
</ul>
</li>
<li>
The Scalable Vector Extension (SVE) is now supported as an
optional extension to the Armv8.2-A architecture and newer.
This support includes automatic vectorization with SVE instructions,
but it does not yet include the SVE Arm C Language Extensions (ACLE).
It can be enabled by specifying the <code>+sve</code> architecture
extension (for example, <code>-march=armv8.2-a+sve</code>).
By default, the generated code works with all vector lengths,
but it can be made specific to <i>N</i>-bit vectors using
<code>-msve-vector-bits=<i>N</i></code>.
</li>
<li>
Support has been added for the following processors
(GCC identifiers in parentheses):
<ul>
<li>Arm Cortex-A75 (<code>cortex-a75</code>).</li>
<li>Arm Cortex-A55 (<code>cortex-a55</code>).</li>
<li>Arm Cortex-A55/Cortex-A75 DynamIQ big.LITTLE (<code>cortex-a75.cortex-a55</code>).</li>
</ul>
The GCC identifiers can be used
as arguments to the <code>-mcpu</code> or <code>-mtune</code> options,
for example: <code>-mcpu=cortex-a75</code> or
<code>-mtune=cortex-a75</code> or as arguments to the equivalent target
attributes and pragmas.
</li>
</ul>
<h3 id="arc">ARC</h3>
<ul>
<li>
Added support for:
<ul>
<li>Fast interrupts.</li>
<li>Naked functions.</li>
<li><code>aux</code> variable attributes.</li>
<li><code>uncached</code> type qualifier.</li>
<li>Secure functions via <code>sjli</code> instruction.</li>
</ul>
</li>
<li>
New exception handling implementation.
</li>
<li>
Revamped trampoline implementation.
</li>
<li>
Refactored small data feature implementation, controlled
via <code>-G</code> command line option.
</li>
<li>
New support for reduced register set ARC architecture
configurations, controlled via <code>-mrf16</code> command line
option.
</li>
<li>
Refurbished and improved support for zero overhead loops.
Introduced <code>-mlpc-width</code> command line option to control the
width of <code>lp_count</code> register.
</li>
</ul>
<h3 id="arm">ARM</h3>
<ul>
<li>
The <code>-mfpu</code> option now takes a new option setting of
<code>-mfpu=auto</code>. When set to this the floating-point and SIMD
settings are derived from the settings of the <code>-mcpu</code>
or <code>-march</code> options. The internal CPU configurations have been
updated with information about the permitted floating-point configurations
supported. See the user guide for further information about the extended
option syntax for controlling architectural extensions via the
<code>-march</code> option. <code>-mfpu=auto</code> is now the default
setting unless the compiler has been configured with an explicit
<code>--with-fpu</code> option.
</li>
<li>
The <code>-march</code> and <code>-mcpu</code> options now accept optional
extensions to the architecture or CPU option, allowing the user to enable
or disable any such extensions supported by that architecture or CPU
such as (but not limited to) floating-point and AdvancedSIMD.
For example: the option
<code>-mcpu=cortex-a53+nofp</code> will generate code for the Cortex-A53
processor with no floating-point support.
This, in combination with the new <code>-mfpu=auto</code> option,
provides a straightforward way of specifying a valid build target through
a single <code>-mcpu</code> or <code>-march</code> option.
The <code>-mtune</code> option accepts the same arguments as
<code>-mcpu</code> but only the CPU name has an effect on tuning.
The architecture extensions do not have any effect.
For details of what extensions a particular architecture or CPU option
supports please refer to the
<a href="https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html#ARM-Options">documentation</a>.
</li>
<li>
The <code>-mstructure-size-boundary</code> option has been deprecated and will be
removed in a future release.
</li>
<li>
The default link behavior for Armv6 and Armv7-R targets has been
changed to produce BE8 format when generating big-endian images. A new
flag <code>-mbe32</code> can be used to force the linker to produce
legacy BE32 format images. There is no change of behavior for
Armv6-M and other Armv7 or later targets: these already defaulted
to BE8 format. This change brings GCC into alignment with other
compilers for the ARM architecture.
</li>
<li>
The Armv8-R architecture is now supported. It can be used by specifying the
<code>-march=armv8-r</code> option.
</li>
<li>
The Armv8.3-A architecture is now supported. It can be used by
specifying the <code>-march=armv8.3-a</code> option.
</li>
<li>
The Armv8.4-A architecture is now supported. It can be used by
specifying the <code>-march=armv8.4-a</code> option.
</li>
<li>
The Dot Product instructions are now supported as an optional extension to the
Armv8.2-A architecture and newer and are mandatory on Armv8.4-A. The extension can be used by
specifying the <code>+dotprod</code> architecture extension. E.g. <code>-march=armv8.2-a+dotprod</code>.
</li>
<li>
Support for setting extensions and architectures using the GCC target pragma and attribute has been added.
It can be used by specifying <code>#pragma GCC target ("arch=...")</code>, <code>#pragma GCC target ("+extension")</code>,
<code>__attribute__((target("arch=...")))</code> or <code>__attribute__((target("+extension")))</code>.
</li>
<li>
New Armv8.4-A FP16 Floating Point Multiplication Variant instructions have been added. These instructions are
mandatory in Armv8.4-A but available as an optional extension to Armv8.2-A and Armv8.3-A. The new extension
can be used by specifying the <code>+fp16fml</code> architectural extension on Armv8.2-A and Armv8.3-A. On Armv8.4-A
the instructions can be enabled by specifying <code>+fp16</code>.
</li>
<li>
Support has been added for the following processors
(GCC identifiers in parentheses):
<ul>
<li>Arm Cortex-A75 (<code>cortex-a75</code>).</li>
<li>Arm Cortex-A55 (<code>cortex-a55</code>).</li>
<li>Arm Cortex-A55/Cortex-A75 DynamIQ big.LITTLE (<code>cortex-a75.cortex-a55</code>).</li>
<li>Arm Cortex-R52 for Armv8-R (<code>cortex-r52</code>).</li>
</ul>
The GCC identifiers can be used
as arguments to the <code>-mcpu</code> or <code>-mtune</code> options,
for example: <code>-mcpu=cortex-a75</code> or
<code>-mtune=cortex-r52</code> or as arguments to the equivalent target
attributes and pragmas.
</li>
</ul>
<h3 id="avr">AVR</h3>
<ul>
<li>
The AVR port now supports the following XMEGA-like devices:
<blockquote>
ATtiny212, ATtiny214, ATtiny412, ATtiny414, ATtiny416, ATtiny417,
ATtiny814, ATtiny816, ATtiny817, ATtiny1614, ATtiny1616, ATtiny1617,
ATtiny3214, ATtiny3216, ATtiny3217
</blockquote>
The new devices are listed under
<a href="https://gcc.gnu.org/onlinedocs/gcc/AVR-Options.html"><code>-mmcu=avrxmega3</code></a>.
<ul>
<li>These devices see flash memory in the RAM address space, so that
features like <code>PROGMEM</code> and <code>__flash</code>
are not needed any more (as opposed to other AVR families for which
read-only data will be located in RAM except special, non-standard
features are used to locate and access such data). This requires
that the compiler is used with Binutils 2.29 or newer so that
<a href="https://sourceware.org/PR21472">read-only data will be
located in flash memory</a>.</li>
<li>A new command-line option <code>-mshort-calls</code> is supported.
This option is used internally for multilib selection of the
<code>avrxmega3</code> variants. It is
<em>not an optimization option</em>. Do not set it by hand.</li>
</ul>
</li>
<li>
The compiler now generates
<a href="https://gcc.gnu.org/PR20296"> efficient interrupt service routine
(ISR) prologues and epilogues</a>. This is achieved by using the new
<a href="https://sourceware.org/binutils/docs-2.29/as/AVR-Pseudo-Instructions.html">
AVR pseudo instruction</a> <code>__gcc_isr</code> which is supported
and resolved by the GNU assembler.
<ul>
<li>As the <code>__gcc_isr</code> pseudo-instruction will be resolved by
the assembler, inline assembly is transparent to the process.
This means that when inline assembly uses an instruction like
<code>INC</code> that clobbers the condition code,
then the assembler will detect this and generate an appropriate
ISR prologue / epilogue chunk to save / restore SREG as needed.</li>
<li>A new command-line option <code>-mno-gas-isr-prologues</code>
disables the generation of the <code>__gcc_isr</code> pseudo
instruction. Any non-naked ISR will save and restore <code>SREG</code>,
<code>tmp_reg</code> and <code>zero_reg</code>, no matter
whether the respective register is clobbered or used.</li>
<li>The feature is turned on per default for all optimization levels
except for <code>-O0</code> and <code>-Og</code>. It is explicitly
enabled by means of option <code>-mgas-isr-prologues</code>.</li>
<li>Support has been added for a new
<a href="https://gcc.gnu.org/onlinedocs/gcc/AVR-Function-Attributes.html">
AVR function attribute</a> <code>no_gccisr</code>. It can be used
to disable <code>__gcc_isr</code> pseudo instruction generation
for individual ISRs.</li>
<li>This optimization is only available if GCC is configured with GNU
Binutils 2.29 or newer; or at least with a version of Binutils
that implements feature
<a href="https://sourceware.org/PR21683">PR21683</a>.</li>
</ul>
</li>
<li>
The compiler no more saves / restores registers in <code>main</code>;
the effect is the same as if attribute <code>OS_task</code> was
specified for <code>main</code>. This optimization can be switched
off by the new command-line option <code>-mno-main-is-OS_task</code>.
</li>
</ul>
<!-- <h3 id="hsa">Heterogeneous Systems Architecture</h3> -->
<h3 id="x86">IA-32/x86-64</h3>
<ul>
<li>
The x86 port now supports the <code>naked</code> function attribute.</li>
<li>
Better tuning for <code>znver1</code> and Intel Core based CPUs.</li>
<li>
Vectorization cost metrics has been reworked leading to significant improvements
on some benchmarks.</li>
<li>GCC now supports the Intel CPU named Cannonlake through
<code>-march=cannonlake</code>. The switch enables the AVX512VBMI,
AVX512IFMA and SHA ISA extensions.</li>
<li>GCC now supports the Intel CPU named and Icelake through
<code>-march=icelake</code>. The switch enables the AVX512VNNI, GFNI, VAES,
AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ ISA
extensions.</li>
<li>
GCC now supports the Intel Control-flow Enforcement Technology
(CET) extension through <code>-mibt</code>, <code>-mshstk</code>,
<code>-mcet</code> options. One of these options has to accompany the
<code>-fcf-protection</code> option to enable code instrumentation for
control-flow protection.
</li>
</ul>
<!-- <h3 id="mips">MIPS</h3> -->
<!-- <h3 id="mep">MeP</h3> -->
<!-- <h3 id="msp430">MSP430</h3> -->
<!-- <h3 id="nds32">NDS32</h3> -->
<h3 id="nds32">NDS32</h3>
<ul>
<li>
New command-line options <code>-mext-perf</code>, <code>-mext-perf2</code>, and
<code>-mext-string</code> have been added for performance extension instructions.
</li>
</ul>
<h3 id="nios2">Nios II</h3>
<ul>
<li>
The Nios II back end has been improved to generate better-optimized
code. Changes include switching to LRA, more accurate cost models,
and more compact code for addressing static variables.
</li>
<li>
New command-line options <code>-mgprel-sec=</code> and
<code>-mr0rel-sec=</code> have been added.
</li>
<li>
The stack-smashing protection options are now enabled on Nios II.
</li>
</ul>
<!-- <h3 id="nvptx">NVPTX</h3> -->
<h3 id="hppa">PA-RISC</h3>
<ul>
<li>
The default call ABI on 32-bit linux has been changed from callee
copies to caller copies. This affects objects larger than eight
bytes passed by value. The goal is to improve compatibility with
x86 and resolve issues with OpenMP.
</li>
<li>
Other PA-RISC targets are unchanged.
</li>
</ul>
<h3 id="powerpc">PowerPC / PowerPC64 / RS6000</h3>
<ul>
<li>
The PowerPC SPE support is split off to a separate <code>powerpcspe</code>
port. The separate port is deprecated and might be removed in a future
release.
</li>
<li>
The Paired Single support (as used on some PPC750 CPUs,
<code>-mpaired</code>, <code>powerpc*-*-linux*paired*</code>)
is deprecated and will be removed in a future release.
</li>
<li>
The Xilinx floating point support (<code>-mxilinx-fpu</code>,
<code>powerpc-xilinx-eabi*</code>)
is deprecated and will be removed in a future release.
</li>
<li>
Support for using big-endian AltiVec intrinsics on a little-endian target
(<code>-maltivec=be</code>) is deprecated and will be removed in a
future release.
</li>
</ul>
<!-- <h3 id="s390">S/390, System z, IBM z Systems</h3> -->
<!-- <h3 id="riscv">RISC-V</h3> -->
<!-- <h3 id="rx">RX</h3> -->
<!-- <h3 id="sh">SH</h3> -->
<!-- <h3 id="sparc">SPARC</h3> -->
<h3 id="Tile">Tile</h3>
<ul>
<li>
The TILE-Gx port is deprecated and will be removed in a future release.
</li>
</ul>
<!-- .................................................................. -->
<h2 id="os">Operating Systems</h2>
<!-- <h3 id="aix">AIX</h3> -->
<!-- <h3 id="fuchsia">Fuchsia</h3> -->
<!-- <h3 id="dragonfly">DragonFly BSD</h3> -->
<!-- <h3 id="freebsd">FreeBSD</h3> -->
<!-- <h3 id="gnulinux">GNU/Linux</h3> -->
<!-- <h3 id="rtems">RTEMS</h3> -->
<!-- <h3 id="solaris">Solaris</h3> -->
<!-- <h3 id="vxmils">VxWorks MILS</h3> -->
<h3 id="windows">Windows</h3>
<ul>
<li>GCC on Microsoft Windows can now be configured via
<code>--enable-mingw-wildcard</code> or
<code>--disable-mingw-wildcard</code> to force a specific behavior for
GCC itself with regards to supporting the wildcard character. Prior
versions of GCC would follow the configuration of the MinGW runtime.
This behavior can still be obtained by not using the above options or by
using <code>--enable-mingw-wildcard=platform</code>.</li>
</ul>
<!-- .................................................................. -->
<!-- <h2>Documentation improvements</h2> -->
<!-- .................................................................. -->
<h2 id="plugins">Improvements for plugin authors</h2>
<ul>
<li>Plugins can now register a callback hook for when comments are
encountered by the C and C++ compilers, e.g. allowing for plugins
to handle documentation markup in code comments.
</li>
<li>The gdbinit support script for debugging GCC now has a
<code>break-on-diagnostic</code> command, providing an easy way
to trigger a breakpoint whenever a diagnostic is emitted.
</li>
<li>The API for creating fix-it hints now supports newlines, and for
emitting mutually incompatible fix-it hints for one diagnostic.
</li>
</ul>
<!-- .................................................................. -->
<h2>Other significant improvements</h2>
<ul>
<li></li>
</ul>
<!-- .................................................................. -->
<!-- <h2><a name="8.2">GCC 8.2</a></h2>
<p>This is the <a href="https://gcc.gnu.org/bugzilla/buglist.cgi?bug_status=RESOLVED&resolution=FIXED&target_milestone=8.2">list
of problem reports (PRs)</a> from GCC's bug tracking system that are
known to be fixed in the 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).</p>
-->
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<address>For questions related to the use of GCC,
please consult these web pages and the
<a href="https://gcc.gnu.org/onlinedocs/">GCC manuals</a>. If that fails,
the <a href="mailto:gcc-help@gcc.gnu.org">gcc-help@gcc.gnu.org</a>
mailing list might help.
Comments on these web pages and the development of GCC are welcome on our
developer list at <a href="mailto:gcc@gcc.gnu.org">gcc@gcc.gnu.org</a>.
All of <a href="https://gcc.gnu.org/lists.html">our lists</a>
have public archives.
</address>
<p>Copyright (C)
<a href="https://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>These pages are
<a href="https://gcc.gnu.org/about.html">maintained by the GCC team</a>.
Last modified 2018-04-27<!-- IGNORE DIFF
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