GCC 8 Release Series — Changes, New Features, and Fixes - GNU Project - Free Software Foundation (FSF)

# GCC 8 Release Series
Changes, New Features, and Fixes

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 Porting to GCC 8 page and the full GCC documentation.

## Caveats

- Support for the obsolete SDB/coff debug info format has been removed. The option `-gcoff` no longer does anything.
- The Cilk+ extensions to the C and C++ languages have been removed.
- The MPX extensions to the C and C++ languages have been deprecated and will be removed in a future release.
- The extension allowing arithmetic on `std::atomic<void*>` and types like `std::atomic<R(*)()>` has been deprecated.
- The non-standard C++0x `std::copy_exception` function was removed. `std::make_exception_ptr` should be used instead.

- Support for the `powerpc*-*-*spe*` target ports which have been recently unmaintained and untested in GCC has been declared obsolete in GCC 8 as announced here. Unless there is activity to revive them, the next release of GCC will have their sources permanently removed.

## General Improvements

- Inter-procedural optimization improvements:
  - Reworked run-time estimation metrics leading to more realistic guesses driving inliner and cloning heuristics.
  - The ipa-pure-const pass is extended to propagate the `malloc` attribute, and the corresponding warning option `-Wsuggest-attribute=malloc` emits a diagnostic for functions which can be annotated with the `malloc` attribute.
- Profile driven optimization improvements:
  - New infrastructure for representing profiles (both statically guessed and profile feedback) which allows propagation of additional information about the reliability of the profile.
  - A number of improvements in the profile updating code solving problems found by new verification code.
  - 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 `cold` attribute. Newly the `noreturn` attribute does not imply all effects of `cold` to differentiate between `exit` (which is `noreturn`) and `abort` (which is in addition not executed in valid runs).
  - `-freorder-blocks-and-partition`, a pass splitting function bodies into hot and cold regions, is now enabled by default at `-O2` and higher for x86 and x86-64.
- Link-time optimization improvements:
  - 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.
- A new option `-fcf-protection=[full|branch|return|none]` 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.
- The `-gcolumn-info` option is now enabled by default. It includes column information in addition to just filenames and line numbers in DWARF debugging information.
- The polyhedral-based loop nest optimization pass `-floop-nest-optimize` has been overhauled. It's still considered experimental and may not result in any runtime improvements.
- Two new classical loop nest optimization passes have been added. `-floop-unroll-and-jam` performs outer loop unrolling and fusing of the inner loop copies. `-floop-interchange` exchanges loops in a loop nest to improve data locality. Both passes are enabled by default at `-O3` and above.
- The classic loop nest optimization pass `-ftree-loop-distribution` has been improved and enabled by default at `-O3` 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.
- The new option `-fstack-clash-protection` 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.
- A new pragma `GCC unroll` 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.
- 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 `noreturn` attribute on the second declaration below is mutually exclusive with the `malloc` attribute on the first, it is ignored and a warning is issued.

            void* __attribute__ ((malloc)) f (unsigned);
            void* __attribute__ ((noreturn)) f (unsigned);

            warning: ignoring attribute 'noreturn' because it conflicts with attribute 'malloc' [-Wattributes]

- The `gcov` 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:

  >     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:}

- The `gcov` tool has more accurate numbers for execution of lines in a source file.
- The `gcov` tool can use TERM colors to provide more readable output.
- AddressSanitizer gained a new pair of sanitization options, `-fsanitize=pointer-compare` and `-fsanitize=pointer-subtract`, which warn about subtraction (or comparison) of pointers that point to a different memory object:

  >     int
  >     main ()
  >     {
  >       /* Heap allocated memory.  */
  >       char *heap1 = (char *)__builtin_malloc (42);
  >       char *heap2 = (char *)__builtin_malloc (42);
  >       if (heap1 > heap2)
  >           return 1;

  >       return 0;
  >     }

  >     ==17465==ERROR: AddressSanitizer: invalid-pointer-pair: 0x604000000010 0x604000000050
  >         #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)

  >     0x604000000010 is located 0 bytes inside of 42-byte region [0x604000000010,0x60400000003a)
  >     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)

  >     0x604000000050 is located 0 bytes inside of 42-byte region [0x604000000050,0x60400000007a)
  >     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

- 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.
- The undefined behavior sanitizer gained two new options included in `-fsanitize=undefined`: `-fsanitize=builtin` which diagnoses at run time invalid arguments to `__builtin_clz` or `__builtin_ctz` prefixed builtins, and `-fsanitize=pointer-overflow` which performs cheap run time tests for pointer wrapping.

## New Languages and Language specific improvements

### Ada

- 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.

### BRIG (HSAIL)

In this release cycle, the focus for the BRIGFE was on stabilization and performance improvements. Also a couple of completely new features were added.

- 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.
- 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.
- Support for BRIG_KIND_NONE directives.
- Made -O3 the default optimization level for BRIGFE.
- Fixed illegal addresses generated from address expressions which refer only to offset 0.
- 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.
- Fixed a crash caused sometimes by calls with more than 4 arguments.
- Fixed a mis-execution issue with kernels that have both unexpanded ID functions and calls to subfunctions.
- Treat HSAIL barrier builtins as setjmp/longjump style functions to avoid illegal optimizations.
- Ensure per WI copies of private variables are aligned correctly.
- libhsail-rt: Assume the host runtime allocates the work group memory.

### C family

- New command-line options have been added for the C and C++ compilers:
  - `-Wmultistatement-macros` warns about unsafe macros expanding to multiple statements used as a body of a statement such as `if`, `else`, `while`, `switch`, or `for`.
  - `-Wstringop-truncation` warns for calls to bounded string manipulation functions such as `strncat`, `strncpy`, and `stpncpy` that might either truncate the copied string or leave the destination unchanged. For example, the following call to `strncat` is diagnosed because it appends just three of the four characters from the source string.

        	    void append (char *buf, size_t bufsize)
        	    {
        	        strncat (buf, ".txt", 3);
        	    }
        	    warning: 'strncat' output truncated copying 3 bytes from a string of length 4 [-Wstringop-truncation]

    Similarly, in the following example, the call to `strncpy` 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 `sizeof buf - 1` as the bound and set the last element of the buffer to NUL.

        	    void copy (const char *s)
        	    {
        	        char buf[80];
        	        strncpy (buf, s, sizeof buf);
        	        …
        	    }
        	    warning: 'strncpy' specified bound 80 equals destination size [-Wstringop-truncation]

    The `-Wstringop-truncation` option is included in `-Wall`.
    Note that due to GCC bug 82944, defining `strncat`, `strncpy`, or `stpncpy` as a macro in a system header as some implementations do, suppresses the warning.
  - `-Wif-not-aligned` controls warnings issued in response to invalid uses of objects declared with attribute `warn_if_not_aligned`.
    The `-Wif-not-aligned` option is included in `-Wall`.
  - `-Wmissing-attributes` 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 `alloc_align`, `alloc_size`, `assume_aligned`, `format`, `format_arg`, `malloc`, or `nonnull` is declared without it. Attributes `deprecated`, `error`, and `warning` suppress the warning.
    The `-Wmissing-attributes` option is included in `-Wall`.
  - `-Wpacked-not-aligned` warns when a `struct` or `union` declared with attribute `packed` 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 `struct A` in the following:

        	    struct __attribute__ ((aligned (8)))
        	    S8 { char a[8]; };

        	    struct __attribute__ ((packed)) A
        	    {
        	        struct S8 s8;
        	    };
        	    warning: alignment 1 of 'struct S' is less than 8 [-Wpacked-not-aligned]

    The `-Wpacked-not-aligned` option is included in `-Wall`.
  - `-Wcast-function-type` warns when a function pointer is cast to an incompatible function pointer. This warning is enabled by `-Wextra`.
  - `-Wsizeof-pointer-div` 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 `-Wall`.
  - `-Wcast-align=strict` warns whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a `char *` is cast to an `int *` regardless of the target machine.
  - `-fprofile-abs-path` creates absolute path names in the `.gcno` files. This allows `gcov` to find the correct sources in projects where compilations occur with different working directories.
- `-fno-strict-overflow` is now mapped to `-fwrapv -fwrapv-pointer` and signed integer overflow is now undefined by default at all optimization levels. Using `-fsanitize=signed-integer-overflow` is now the preferred way to audit code, `-Wstrict-overflow` is deprecated.
- The `-Warray-bounds` 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.
- The `-Wrestrict` option introduced in GCC 7 has been enhanced to detect many more instances of overlapping accesses to objects via `restrict`-qualified arguments to standard memory and string manipulation functions such as `memcpy` and `strcpy`. For example, the `strcpy` 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 `a[4]`, the copies overlap and the call is diagnosed.

      	void f (void)
      	{
      	    char a[] = "abcd1234";
      	    strcpy (a, a + 4);
      	    …
      	}

  The `-Wrestrict` option is included in `-Wall`.
- Several optimizer enhancements have enabled improvements to the `-Wformat-overflow` and `-Wformat-truncation` 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.
- 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.

      $ gcc arg-type-mismatch.cc
      arg-type-mismatch.cc: In function 'int caller(int, int, float)':
      arg-type-mismatch.cc:5:24: error: invalid conversion from 'int' to 'const char*' [-fpermissive]
         return callee(first, second, third);
                              ^~~~~~
      arg-type-mismatch.cc:1:40: note:   initializing argument 2 of 'int callee(int, const char*, float)'
       extern int callee(int one, const char *two, float three);
                                  ~~~~~~~~~~~~^~~

- When reporting on unrecognized identifiers, the C and C++ compilers will now emit fix-it hints suggesting `#include` directives for various headers in the C and C++ standard libraries.

      $ gcc incomplete.c
      incomplete.c: In function 'test':
      incomplete.c:3:10: error: 'NULL' undeclared (first use in this function)
         return NULL;
                ^~~~
      incomplete.c:3:10: note: 'NULL' is defined in header '<stddef.h>'; did you forget to '#include <stddef.h>'?
      incomplete.c:1:1:
      +#include <stddef.h>
       const char *test(void)
      incomplete.c:3:10:
         return NULL;
                ^~~~
      incomplete.c:3:10: note: each undeclared identifier is reported only once for each function it appears in

      $ gcc incomplete.cc
      incomplete.cc:1:6: error: 'string' in namespace 'std' does not name a type
       std::string s("hello world");
            ^~~~~~
      incomplete.cc:1:1: note: 'std::string' is defined in header '<string>'; did you forget to '#include <string>'?
      +#include <string>
       std::string s("hello world");
       ^~~

- The C and C++ compilers now use more intuitive locations when reporting on missing semicolons, and offer fix-it hints:

      $ gcc t.c
      t.c: In function 'test':
      t.c:3:12: error: expected ';' before '}' token
         return 42
                  ^
                  ;
       }
       ~

- 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:

      $ gcc unclosed.c
      unclosed.c: In function 'log_when_out_of_range':
      unclosed.c:12:50: error: expected ')' before '{' token
             && (temperature < MIN || temperature > MAX) {
                                                        ^~
                                                        )
      unclosed.c:11:6: note: to match this '('
         if (logging_enabled && check_range ()
            ^

  or highlighting it directly if it's on the same line:

      $ gcc unclosed-2.c
      unclosed-2.c: In function 'test':
      unclosed-2.c:8:45: error: expected ')' before '{' token
         if (temperature < MIN || temperature > MAX {
            ~                                      ^~
                                                   )

  They will also emit fix-it hints.

### C++

- The value of the C++11 `alignof` operator has been corrected to match C `_Alignof` (minimum alignment) rather than GNU `__alignof__` (preferred alignment); on ia32 targets this means that `alignof(double)` is now 4 rather than 8. Code that wants the preferred alignment should use `__alignof__` instead.
- New command-line options have been added for the C++ compiler to control warnings:
  - `-Wclass-memaccess` warns when objects of non-trivial class types are manipulated in potentially unsafe ways by raw memory functions such as `memcpy`, or `realloc`. The warning helps detect calls that bypass user-defined constructors or copy-assignment operators, corrupt virtual table pointers, data members of `const`-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:

        	memcpy (&std::cout, &std::cerr, sizeof std::cout);

    results in

        	warning: 'void* memcpy(void*, const void*, long unsigned int)' writing to an object of type 'std::ostream' {aka 'class std::basic_ostream<char>'} with no trivial copy-assignment [-Wclass-memaccess]

    The `-Wclass-memaccess` option is included in `-Wall`.
- The C++ front end has experimental support for some of the upcoming C++2a draft features with the `-std=c++2a` or `-std=gnu++2a` flags, including designated initializers, default member initializers for bit-fields, `__VA_OPT__` (except that `#__VA_OPT__` is unsupported), lambda `[=, this]` captures, etc. For a full list of new features, see the C++ status page.
- 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.

      $ gcc accessor.cc
      accessor.cc: In function 'void test(foo*)':
      accessor.cc:12:12: error: 'double foo::m_ratio' is private within this context
         if (ptr->m_ratio >= 0.5)
                  ^~~~~~~
      accessor.cc:7:10: note: declared private here
         double m_ratio;
                ^~~~~~~
      accessor.cc:12:12: note: field 'double foo::m_ratio' can be accessed via 'double foo::get_ratio() const'
         if (ptr->m_ratio >= 0.5)
                  ^~~~~~~
                  get_ratio()

- 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 `#include` directives):

      $ gcc ordering.cc
      ordering.cc:2:24: error: expected ';' at end of member declaration
         virtual void clone() const OVERRIDE { }
                              ^~~~~
                                   ;
      ordering.cc:2:30: error: 'OVERRIDE' does not name a type
         virtual void clone() const OVERRIDE { }
                                    ^~~~~~~~
      ordering.cc:2:30: note: the macro 'OVERRIDE' had not yet been defined
      In file included from ordering.cc:5:
      c++11-compat.h:2: note: it was later defined here
       #define OVERRIDE override

- The `-Wold-style-cast` diagnostic can now emit fix-it hints telling you when you can use a `static_cast`, `const_cast`, or `reinterpret_cast`.

      $ gcc -c old-style-cast-fixits.cc -Wold-style-cast
      old-style-cast-fixits.cc: In function 'void test(void*)':
      old-style-cast-fixits.cc:5:19: warning: use of old-style cast to 'struct foo*' [-Wold-style-cast]
         foo *f = (foo *)ptr;
                         ^~~
                  ----------
                  static_cast<foo *> (ptr)

- When reporting on problems within `extern "C"` linkage specifications, the C++ compiler will now display the location of the start of the `extern "C"`.

      $ gcc -c extern-c.cc
      extern-c.cc:3:1: error: template with C linkage
       template <typename T> void test (void);
       ^~~~~~~~
      In file included from extern-c.cc:1:
      unclosed.h:1:1: note: 'extern "C"' linkage started here
       extern "C" {
       ^~~~~~~~~~
      extern-c.cc:3:39: error: expected '}' at end of input
       template <typename T> void test (void);
                                             ^
      In file included from extern-c.cc:1:
      unclosed.h:1:12: note: to match this '{'
       extern "C" {
                  ^

- 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 `[...]` instead:

      $ gcc templates.cc
      templates.cc: In function 'void test()':
      templates.cc:9:8: error: could not convert 'vector<double>()' from 'vector<double>' to 'vector<int>'
         fn_1(vector<double> ());
              ^~~~~~~~~~~~~~~~~
      templates.cc:10:8: error: could not convert 'map<int, double>()' from 'map<[...],double>' to 'map<[...],int>'
         fn_2(map<int, double>());
              ^~~~~~~~~~~~~~~~~~

  Those `[...]` elided parameters can be seen using `-fno-elide-type`:

      $ gcc templates.cc -fno-elide-type
      templates.cc: In function 'void test()':
      templates.cc:9:8: error: could not convert 'vector<double>()' from 'vector<double>' to 'vector<int>'
         fn_1(vector<double> ());
              ^~~~~~~~~~~~~~~~~
      templates.cc:10:8: error: could not convert 'map<int, double>()' from 'map<int,double>' to 'map<int,int>'
         fn_2(map<int, double>());
              ^~~~~~~~~~~~~~~~~~

  The C++ compiler has also gained an option `-fdiagnostics-show-template-tree` which visualizes such mismatching templates in a hierarchical form:

      $ gcc templates-2.cc -fdiagnostics-show-template-tree
      templates-2.cc: In function 'void test()':
      templates-2.cc:9:8: error: could not convert 'vector<double>()' from 'vector<double>' to 'vector<int>'
        vector<
          [double != int]>
         fn_1(vector<double> ());
              ^~~~~~~~~~~~~~~~~
      templates-2.cc:10:8: error: could not convert 'map<map<int, vector<double> >, vector<double> >()' from 'map<map<[...],vector<double>>,vector<double>>' to 'map<map<[...],vector<float>>,vector<float>>'
        map<
          map<
            [...],
            vector<
              [double != float]>>,
          vector<
            [double != float]>>
         fn_2(map<map<int, vector<double>>, vector<double>> ());
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  which again works with `-fno-elide-type`:

      $ gcc templates-2.cc -fdiagnostics-show-template-tree -fno-elide-type
      templates-2.cc: In function 'void test()':
      templates-2.cc:9:8: error: could not convert 'vector<double>()' from 'vector<double>' to 'vector<int>'
        vector<
          [double != int]>
         fn_1(vector<double> ());
              ^~~~~~~~~~~~~~~~~
      templates-2.cc:10:8: error: could not convert 'map<map<int, vector<double> >, vector<double> >()' from 'map<map<int,vector<double>>,vector<double>>' to 'map<map<int,vector<float>>,vector<float>>'
        map<
          map<
            int,
            vector<
              [double != float]>>,
          vector<
            [double != float]>>
         fn_2(map<map<int, vector<double>>, vector<double>> ());
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

- 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, `-Wreturn-type` warnings are enabled by default for C++.

#### Runtime Library (libstdc++)

- Improved experimental support for C++17, including the following features:
  - Deduction guides to support class template argument deduction.
  - `std::filesystem` implementation.
  - `std::char_traits<char>` and `std::char_traits<wchar_t>` are usable in constant expressions.
  - `std::to_chars` and `std::from_chars` (for integers only, not for floating point types).
- Experimental support for C++2a: `std::to_address` (thanks to Glen Fernandes) and `std::endian`.
- On GNU/Linux, `std::random_device::entropy()` accesses the kernel's entropy count for the random device, if known (thanks to Xi Ruoyao).
- Support for `std::experimental::source_location`.
- AddressSanitizer integration for `std::vector`, detecting out-of-range accesses to the unused capacity of a vector.
- Extensions `__gnu_cxx::airy_ai` and `__gnu_cxx::airy_bi` added to the Mathematical Special Functions.

### Fortran

- The main version of libfortran has been changed to 5.
- Parameterized derived types, a major feature of Fortran 2003, have been implemented.
- The maximum rank for arrays has been increased to 15, conforming to the Fortran 2008 standard.
- Transformational intrinsics are now fully supported in initialization expressions.
- New flag `-fc-prototypes` to write C prototypes for `BIND(C)` procedures and variables.
- If `-fmax-stack-var-size` is honored if given together with `-Ofast`, `-fstack-arrays` is no longer set in that case.
- New options `-fdefault-real-16` and `-fdefault-real-10` to control the default kind of `REAL` variables.
- A warning is now issued if an array subscript inside a DO loop could lead to an out-of-bounds-access. The new option `-Wdo-subscript`, enabled by `-Wextra`, warns about this even if the compiler can not prove that the code will be executed.
- The Fortran front end now attempts to interchange loops if it is deemed profitable. So far, this is restricted to `FORALL` and `DO CONCURRENT` statements with multiple indices. This behavior be controlled with the new flag `-ffrontend-loop-interchange`, which is enabled with optimization by default. The `-Wfrontend-loop-interchange` option warns about such occurrences.
- When an actual argument contains too few elements for a dummy argument, an error is now issued. The `-std=legacy` option can be used to still compile such code.
- The `RECL=` argument to `OPEN` and `INQUIRE` statements now allows 64-bit integers, making records larger than 2GiB possible.
- The `GFORTRAN_DEFAULT_RECL` 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 `RECL=` specifier.
- Character variables longer than `HUGE(0)` 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 `INTEGER(C_SIZE_T)`.

### Go

- GCC 8 provides a complete implementation of the Go 1.10.1 user packages.
- 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.
- 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.

## libgccjit

The libgccjit API gained four new entry points:

- gcc_jit_type_get_vector and
- gcc_jit_context_new_rvalue_from_vector for working with vectors,
- gcc_jit_type_get_aligned
- gcc_jit_function_get_address

The C code generated by gcc_jit_context_dump_reproducer_to_file is now easier-to-read.

## New Targets and Target Specific Improvements

### AArch64

- The Armv8.4-A architecture is now supported. It can be used by specifying the `-march=armv8.4-a` option.
- 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 `+dotprod` architecture extension. E.g. `-march=armv8.2-a+dotprod`.
- The Armv8-A `+crypto` extension has now been split into two extensions for finer grained control:
  - `+aes` which contains the Armv8-A AES crytographic instructions.
  - `+sha2` which contains the Armv8-A SHA2 and SHA1 cryptographic instructions.
  Using `+crypto` will now enable these two extensions.
- 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 `+fp16fml` architectural extension on Armv8.2-A and Armv8.3-A. On Armv8.4-A the instructions can be enabled by specifying `+fp16`.
- New cryptographic instructions have been added as optional extensions to Armv8.2-A and newer. These instructions can be enabled with:
  - `+sha3` New SHA3 and SHA2 instructions from Armv8.4-A. This implies `+sha2`.
  - `+sm4` New SM3 and SM4 instructions from Armv8.4-A.
- 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 `+sve` architecture extension (for example, `-march=armv8.2-a+sve`). By default, the generated code works with all vector lengths, but it can be made specific to N-bit vectors using `-msve-vector-bits=N`.
- Support has been added for the following processors (GCC identifiers in parentheses):
  - Arm Cortex-A75 (`cortex-a75`).
  - Arm Cortex-A55 (`cortex-a55`).
  - Arm Cortex-A55/Cortex-A75 DynamIQ big.LITTLE (`cortex-a75.cortex-a55`).
  The GCC identifiers can be used as arguments to the `-mcpu` or `-mtune` options, for example: `-mcpu=cortex-a75` or `-mtune=cortex-a75` or as arguments to the equivalent target attributes and pragmas.

### ARC

- Added support for:
  - Fast interrupts.
  - Naked functions.
  - `aux` variable attributes.
  - `uncached` type qualifier.
  - Secure functions via `sjli` instruction.
- New exception handling implementation.
- Revamped trampoline implementation.
- Refactored small data feature implementation, controlled via `-G` command line option.
- New support for reduced register set ARC architecture configurations, controlled via `-mrf16` command line option.
- Refurbished and improved support for zero overhead loops. Introduced `-mlpc-width` command line option to control the width of `lp_count` register.

### ARM

- The `-mfpu` option now takes a new option setting of `-mfpu=auto`. When set to this the floating-point and SIMD settings are derived from the settings of the `-mcpu` or `-march` 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 `-march` option. `-mfpu=auto` is now the default setting unless the compiler has been configured with an explicit `--with-fpu` option.
- The `-march` and `-mcpu` 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 `-mcpu=cortex-a53+nofp` will generate code for the Cortex-A53 processor with no floating-point support. This, in combination with the new `-mfpu=auto` option, provides a straightforward way of specifying a valid build target through a single `-mcpu` or `-march` option. The `-mtune` option accepts the same arguments as `-mcpu` 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 documentation.
- The `-mstructure-size-boundary` option has been deprecated and will be removed in a future release.
- 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 `-mbe32` 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.
- The Armv8-R architecture is now supported. It can be used by specifying the `-march=armv8-r` option.
- The Armv8.3-A architecture is now supported. It can be used by specifying the `-march=armv8.3-a` option.
- The Armv8.4-A architecture is now supported. It can be used by specifying the `-march=armv8.4-a` option.
- 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 `+dotprod` architecture extension. E.g. `-march=armv8.2-a+dotprod`.
- Support for setting extensions and architectures using the GCC target pragma and attribute has been added. It can be used by specifying `#pragma GCC target ("arch=...")`, `#pragma GCC target ("+extension")`, `__attribute__((target("arch=...")))` or `__attribute__((target("+extension")))`.
- 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 `+fp16fml` architectural extension on Armv8.2-A and Armv8.3-A. On Armv8.4-A the instructions can be enabled by specifying `+fp16`.
- Support has been added for the following processors (GCC identifiers in parentheses):
  - Arm Cortex-A75 (`cortex-a75`).
  - Arm Cortex-A55 (`cortex-a55`).
  - Arm Cortex-A55/Cortex-A75 DynamIQ big.LITTLE (`cortex-a75.cortex-a55`).
  - Arm Cortex-R52 for Armv8-R (`cortex-r52`).
  The GCC identifiers can be used as arguments to the `-mcpu` or `-mtune` options, for example: `-mcpu=cortex-a75` or `-mtune=cortex-r52` or as arguments to the equivalent target attributes and pragmas.

### AVR

- The AVR port now supports the following XMEGA-like devices:

  > ATtiny212, ATtiny214, ATtiny412, ATtiny414, ATtiny416, ATtiny417, ATtiny814, ATtiny816, ATtiny817, ATtiny1614, ATtiny1616, ATtiny1617, ATtiny3214, ATtiny3216, ATtiny3217

  The new devices are listed under `-mmcu=avrxmega3`.
  - These devices see flash memory in the RAM address space, so that features like `PROGMEM` and `__flash` 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 read-only data will be located in flash memory.
  - A new command-line option `-mshort-calls` is supported. This option is used internally for multilib selection of the `avrxmega3` variants. It is not an optimization option. Do not set it by hand.
- The compiler now generates efficient interrupt service routine (ISR) prologues and epilogues. This is achieved by using the new AVR pseudo instruction `__gcc_isr` which is supported and resolved by the GNU assembler.
  - As the `__gcc_isr` 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 `INC` 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.
  - A new command-line option `-mno-gas-isr-prologues` disables the generation of the `__gcc_isr` pseudo instruction. Any non-naked ISR will save and restore `SREG`, `tmp_reg` and `zero_reg`, no matter whether the respective register is clobbered or used.
  - The feature is turned on per default for all optimization levels except for `-O0` and `-Og`. It is explicitly enabled by means of option `-mgas-isr-prologues`.
  - Support has been added for a new AVR function attribute `no_gccisr`. It can be used to disable `__gcc_isr` pseudo instruction generation for individual ISRs.
  - 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 PR21683.
- The compiler no more saves / restores registers in `main`; the effect is the same as if attribute `OS_task` was specified for `main`. This optimization can be switched off by the new command-line option `-mno-main-is-OS_task`.

### IA-32/x86-64

- The x86 port now supports the `naked` function attribute.
- Better tuning for `znver1` and Intel Core based CPUs.
- Vectorization cost metrics has been reworked leading to significant improvements on some benchmarks.
- GCC now supports the Intel CPU named Cannonlake through `-march=cannonlake`. The switch enables the AVX512VBMI, AVX512IFMA and SHA ISA extensions.
- GCC now supports the Intel CPU named and Icelake through `-march=icelake`. The switch enables the AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ ISA extensions.
- GCC now supports the Intel Control-flow Enforcement Technology (CET) extension through `-mibt`, `-mshstk`, `-mcet` options. One of these options has to accompany the `-fcf-protection` option to enable code instrumentation for control-flow protection.

### NDS32

- New command-line options `-mext-perf`, `-mext-perf2`, and `-mext-string` have been added for performance extension instructions.

### Nios II

- 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.
- New command-line options `-mgprel-sec=` and `-mr0rel-sec=` have been added.
- The stack-smashing protection options are now enabled on Nios II.

### PA-RISC

- 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.
- Other PA-RISC targets are unchanged.

### PowerPC / PowerPC64 / RS6000

- The PowerPC SPE support is split off to a separate `powerpcspe` port. The separate port is deprecated and might be removed in a future release.
- The Paired Single support (as used on some PPC750 CPUs, `-mpaired`, `powerpc*-*-linux*paired*`) is deprecated and will be removed in a future release.
- The Xilinx floating point support (`-mxilinx-fpu`, `powerpc-xilinx-eabi*`) is deprecated and will be removed in a future release.
- Support for using big-endian AltiVec intrinsics on a little-endian target (`-maltivec=be`) is deprecated and will be removed in a future release.

### Tile

- The TILE-Gx port is deprecated and will be removed in a future release.

## Operating Systems

### Windows

- GCC on Microsoft Windows can now be configured via `--enable-mingw-wildcard` or `--disable-mingw-wildcard` 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 `--enable-mingw-wildcard=platform`.

## Improvements for plugin authors

- 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.
- The gdbinit support script for debugging GCC now has a `break-on-diagnostic` command, providing an easy way to trigger a breakpoint whenever a diagnostic is emitted.
- The API for creating fix-it hints now supports newlines, and for emitting mutually incompatible fix-it hints for one diagnostic.

## Other significant improvements

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 2018-04-27.