GCC 4.7 Release Series
Changes, New Features, and Fixes

Caveats

• Support for a number of older systems and recently unmaintained or untested target ports of GCC has been declared obsolete in GCC 4.7. Unless there is activity to revive them, the next release of GCC will have their sources permanently removed.

  The following ports for individual systems on particular architectures have been obsoleted:

  • IRIX 6.5 (mips-sgi-irix6.5)
  • MIPS OpenBSD (mips*-*-openbsd*)
  • Solaris 8 (*-*-solaris2.8). Details can be found in the announcement.
  • Tru64 UNIX V5.1 (alpha*-dec-osf5.1*)
• Support on ARM for the legacy floating-point accelerator (FPA) and the mixed-endian floating-point format that it used has been obsoleted. The ports that still use this format have been obsoleted as well. Many legacy ARM ports already provide an alternative that uses the VFP floating-point format. The obsolete ports will be deleted in the next release.

  The obsolete ports with alternatives are:

  • arm*-*-rtems (use arm*-*-rtemseabi)
  • arm*-*-linux-gnu (use arm*-*-linux-gnueabi)
  • arm*-*-elf (use arm*-*-eabi)
  • arm*-*-uclinux* (use arm*-*-uclinux*eabi)

  Note, however, that these alternatives are not binary compatible with their legacy counterparts (although some can support running legacy applications).

  The obsolete ports that currently lack a modern alternative are:

  • arm*-*-ecos-elf
  • arm*-*-freebsd
  • arm*-wince-pe*
  New ports that support more recent versions of the architecture are welcome.
• Support for the Maverick co-processor on ARM has been obsoleted. Code to support it will be deleted in the next release.
• Support has been removed for Unix International threads on Solaris 2, so the --enable-threads=solaris configure option and the -threads compiler option don't work any longer.
• Support has been removed for the Solaris BSD Compatibility Package, which lives in /usr/ucbinclude and /usr/ucblib. It has been removed from Solaris 11, and was only intended as a migration aid from SunOS 4 to SunOS 5. The -compat-bsd compiler option is not recognized any longer.
• The AVR port's libgcc has been improved and its multilib structure has been enhanced. As a result, all objects contributing to an application must either be compiled with GCC versions up to 4.6.x or with GCC versions 4.7.0 or later.
• The ARM port's -mwords-little-endian option has been deprecated. It will be removed in a future release.
• Support has been removed for the NetWare x86 configuration obsoleted in GCC 4.6.
• It is no longer possible to use the "l" constraint in MIPS16 asm statements.
• More information on porting to GCC 4.7 from previous versions of GCC can be found in the porting guide for this release.

General Optimizer Improvements

• Support for a new parameter --param case-values-threshold=n was added to allow users to control the cutoff between doing switch statements as a series of if statements and using a jump table.
• Link-time optimization (LTO) improvements:
  • Improved scalability and reduced memory usage. Link time optimization of Firefox now requires 3GB of RAM on a 64-bit system, while over 8GB was needed previously. Linking time has been improved, too. The serial stage of linking Firefox has been sped up by about a factor of 10.
  • Reduced size of object files and temporary storage used during linking.
  • Streaming performance (both outbound and inbound) has been improved.
  • ld -r is now supported with LTO.
  • Several bug fixes, especially in symbol table handling and merging.
• Interprocedural optimization improvements:
  • Heuristics now take into account that after inlining code will be optimized out because of known values (or properties) of function parameters. For example:

    void foo(int a)
    {
      if (a > 10)
        ... huge code ...
    }
    void bar (void)
    {
      foo (0);
    }
          

    The call of foo will be inlined into bar even when optimizing for code size. Constructs based on __builtin_constant_p are now understood by the inliner and code size estimates are evaluated a lot more realistically.
  • The representation of C++ virtual thunks and aliases (both implicit and defined via the alias attribute) has been re-engineered. Aliases no longer pose optimization barriers and calls to an alias can be inlined and otherwise optimized.
  • The inter-procedural constant propagation pass has been rewritten. It now performs generic function specialization. For example when compiling the following:

    void foo(bool flag)
    {
      if (flag)
        ... do something ...
      else
        ... do something else ...
    }
    void bar (void)
    {
      foo (false);
      foo (true);
      foo (false);
      foo (true);
      foo (false);
      foo (true);
    }
          

    GCC will now produce two copies of foo. One with flag being true, while other with flag being false. This leads to performance improvements previously possible only by inlining all calls. Cloning causes a lot less code size growth.
• A string length optimization pass has been added. It attempts to track string lengths and optimize various standard C string functions like strlen, strchr, strcpy, strcat, stpcpy and their _FORTIFY_SOURCE counterparts into faster alternatives. This pass is enabled by default at -O2 or above, unless optimizing for size, and can be disabled by the -fno-optimize-strlen option. The pass can e.g. optimize

  char *bar (const char *a)
  {
    size_t l = strlen (a) + 2;
    char *p = malloc (l); if (p == NULL) return p;
    strcpy (p, a); strcat (p, "/"); return p;
  }
        

  into:

  char *bar (const char *a)
  {
    size_t tmp = strlen (a);
    char *p = malloc (tmp + 2); if (p == NULL) return p;
    memcpy (p, a, tmp); memcpy (p + tmp, "/", 2); return p;
  }
        

  or for hosted compilations where stpcpy is available in the runtime and headers provide its prototype, e.g.

  void foo (char *a, const char *b, const char *c, const char *d)
  {
    strcpy (a, b); strcat (a, c); strcat (a, d);
  }
        

  can be optimized into:

  void foo (char *a, const char *b, const char *c, const char *d)
  {
    strcpy (stpcpy (stpcpy (a, b), c), d);
  }
        

New Languages and Language specific improvements

• Version 3.1 of the OpenMP specification is now supported for the C, C++, and Fortran compilers.

Ada

• The command-line option -feliminate-unused-debug-types has been re-enabled by default, as it is for the other languages, leading to a reduction in debug info size of 12.5% and more for relevant cases, as well as to a small compilation speedup.

C family

• A new built-in, __builtin_assume_aligned, has been added, through which the compiler can be hinted about pointer alignment and can use it to improve generated code.

• Experimental support for transactional memory has been added. It includes support in the compiler, as well as a supporting runtime library called libitm. To compile code with transactional memory constructs, use the -fgnu-tm option.

  Support is currently available for Alpha, ARM, PowerPC, SH, SPARC, and 32-bit/64-bit x86 platforms.

  For more details on transactional memory see the GCC WiKi.

• Support for atomic operations specifying the C++11/C11 memory model has been added. These new __atomic routines replace the existing __sync built-in routines.

  Atomic support is also available for memory blocks. Lock-free instructions will be used if a memory block is the same size and alignment as a supported integer type. Atomic operations which do not have lock-free support are left as function calls. A set of library functions is available on the GCC atomic wiki in the "External Atomics Library" section.

  For more details on the memory models and features, see the atomic wiki.

• When a binary operation is performed on vector types and one of the operands is a uniform vector, it is possible to replace the vector with the generating element. For example:

  typedef int v4si __attribute__ ((vector_size (16)));
  v4si res, a = {1,2,3,4};
  int x;
  
  res = 2 + a;  /* means {2,2,2,2} + a  */
  res = a - x;  /* means a - {x,x,x,x}  */
        

C

• There is support for some more features from the C11 revision of the ISO C standard. GCC now accepts the options -std=c11 and -std=gnu11, in addition to the previous -std=c1x and -std=gnu1x.
  • Unicode strings (previously supported only with options such as -std=gnu11, now supported with -std=c11), and the predefined macros __STDC_UTF_16__ and __STDC_UTF_32__.
  • Nonreturning functions (_Noreturn and <stdnoreturn.h>).
  • Alignment support (_Alignas, _Alignof, max_align_t, <stdalign.h>).
  • A built-in function __builtin_complex is provided to support C library implementation of the CMPLX family of macros.

• G++ now accepts the -std=c++11, -std=gnu++11, and -Wc++11-compat options, which are equivalent to -std=c++0x, -std=gnu++0x, and -Wc++0x-compat, respectively.
• G++ now implements C++11 extended friend syntax:

  template<class W>
  class Q
  {
    static const int I = 2;
  public:
    friend W;
  };
  
  struct B
  {
    int ar[Q<B>::I];
  };

• Thanks to Ville Voutilainen, G++ now implements C++11 explicit override control.

  struct B {
    virtual void f() const final;
    virtual void f(int);
  };
  
  struct D : B {
    void f() const;            // error: D::f attempts to override final B::f
    void f(long) override;     // error: doesn't override anything
    void f(int) override;      // ok
  };
  
  struct E final { };
  struct F: E { }; // error: deriving from final class

• G++ now implements C++11 non-static data member initializers.

  struct A {
    int i = 42;
  } a; // initializes a.i to 42

• Thanks to Ed Smith-Rowland, G++ now implements C++11 user-defined literals.

  // Not actually a good approximation.  :)
  constexpr long double operator"" _degrees (long double d) { return d * 0.0175; }
  long double pi = 180.0_degrees;

• G++ now implements C++11 alias-declarations.

  template <class T> using Ptr = T*;
  Ptr<int> ip;  // decltype(ip) is int*

• Thanks to Ville Voutilainen and Pedro Lamarão, G++ now implements C++11 delegating constructors.

  struct A {
    A(int);
    A(): A(42) { } // delegate to the A(int) constructor
  };

• G++ now fully implements C++11 atomic classes rather than just integer derived classes.

  class POD {
    int a;
    int b;
  };
  std::atomic<POD> my_atomic_POD;
  

• G++ now sets the predefined macro __cplusplus to the correct value, 199711L for C++98/03, and 201103L for C++11.
• G++ now correctly implements the two-phase lookup rules such that an unqualified name used in a template must have an appropriate declaration found either in scope at the point of definition of the template or by argument-dependent lookup at the point of instantiation. As a result, code that relies on a second unqualified lookup at the point of instantiation to find functions declared after the template or in dependent bases will be rejected. The compiler will suggest ways to fix affected code, and using the -fpermissive compiler flag will allow the code to compile with a warning.

  template <class T>
  void f() { g(T()); } // error, g(int) not found by argument-dependent lookup
  void g(int) { } // fix by moving this declaration before the declaration of f
  
  template <class T>
  struct A: T {
    // error, B::g(B) not found by argument-dependent lookup
    void f() { g(T()); } // fix by using this->g or A::g
  };
  
  struct B { void g(B); };
  
  int main()
  {
    f<int>();
    A<B>().f();
  }

• G++ now properly re-uses stack space allocated for temporary objects when their lifetime ends, which can significantly lower stack consumption for some C++ functions. As a result of this, some code with undefined behavior will now break:

  const int &f(const int &i) { return i; }
  ....
  const int &x = f(1);
  const int &y = f(2);

  Here, x refers to the temporary allocated to hold the 1 argument, which only lives until the end of the initialization; it immediately becomes a dangling reference. So the next statement re-uses the stack slot to hold the 2 argument, and users of x get that value instead.

  Note that this should not cause any change of behavior for temporaries of types with non-trivial destructors, as they are already destroyed at end of full-expression; the change is that now the storage is released as well.

• A new command-line option -Wdelete-non-virtual-dtor has been added to warn when delete is used to destroy an instance of a class which has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by -Wall.
• A new command-line option -Wzero-as-null-pointer-constant has been added to warn when a literal '0' is used as null pointer constant. It can be useful to facilitate the conversion to nullptr in C++11.
• As per C++98, access-declarations are now deprecated by G++. Using-declarations are to be used instead. Furthermore, some efforts have been made to improve the support of class scope using-declarations. In particular, using-declarations referring to a dependent type now work as expected (bug c++/14258).
• The ELF symbol visibility of a template instantiation is now properly constrained by the visibility of its template arguments (bug c++/35688).

Runtime Library (libstdc++)

• Improved experimental support for the new ISO C++ standard, C++11, including:
  • using noexcept in most of the library;
  • implementations of pointer_traits, allocator_traits and scoped_allocator_adaptor;
  • uses-allocator construction for tuple;
  • vector meets the allocator-aware container requirements;
  • replacing monotonic_clock with steady_clock;
  • enabling the thread support library on most POSIX targets;
  • many small improvements to conform to the FDIS.
• Added --enable-clocale=newlib configure option.
• Debug Mode iterators for unordered associative containers.
• Avoid polluting the global namespace and do not include <unistd.h>.

Fortran

• The compile flag -fstack-arrays has been added, which causes all local arrays to be put on stack memory. For some programs this will improve the performance significantly. If your program uses very large local arrays, it is possible that you will have to extend your runtime limits for stack memory.
• The -Ofast flag now also implies -fno-protect-parens and -fstack-arrays.
• Front-end optimizations can now be selected by the -ffrontend-optimize option and deselected by the -fno-frontend-optimize option.
• When front-end optimization removes a function call, -Wfunction-elimination warns about that.
• When performing front-end-optimization, the -faggressive-function-elimination option allows the removal of duplicate function calls even for impure functions.
• The flag -Wreal-q-constant has been added, which warns if floating-point literals have been specified using q (such as 1.0q0); the q marker is now supported as a vendor extension to denote quad precision (REAL(16) or, if not available, REAL(10)). Consider using a kind parameter (such as in 1.0_qp) instead, which can be obtained via SELECTED_REAL_KIND.
• The GFORTRAN_USE_STDERR environment variable has been removed. GNU Fortran now always prints error messages to standard error. If you wish to redirect standard error, please consult the manual for your OS, shell, batch environment etc. as appropriate.
• The -fdump-core option and GFORTRAN_ERROR_DUMPCORE environment variable have been removed. When encountering a serious error, gfortran will now always abort the program. Whether a core dump is generated depends on the user environment settings; see the ulimit -c setting for POSIX shells, limit coredumpsize for C shells, and the WER user-mode dumps settings on Windows.
• The -fbacktrace option is now enabled by default. When encountering a fatal error, gfortran will attempt to print a backtrace to standard error before aborting. It can be disabled with -fno-backtrace. Note: On POSIX targets with the addr2line utility from GNU binutils, GNU Fortran can print a backtrace with function name, file name, line number information in addition to the addresses; otherwise only the addresses are printed.
• Fortran 2003:
  • Generic interface names which have the same name as derived types are now supported, which allows to write constructor functions. Note that Fortran does not support static constructor functions; only default initialization or an explicit structure-constructor initialization are available.
  • Polymorphic (class) arrays are now supported.
• Fortran 2008:
  • Support for the DO CONCURRENT construct has been added, which allows the user to specify that individual loop iterations have no interdependencies.
  • Coarrays: Full single-image support except for polymorphic coarrays. Additionally, preliminary support for multiple images via an MPI-based coarray communication library has been added. Note: The library version is not yet usable as remote coarray access is not yet possible.
• TS 29113:
  • New flag -std=f2008ts permits programs that are expected to conform to the Fortran 2008 standard and the draft Technical Specification (TS) 29113 on Further Interoperability of Fortran with C.
  • The OPTIONAL attribute is now allowed for dummy arguments of BIND(C) procedures.
  • The RANK intrinsic has been added.
  • The implementation of the ASYNCHRONOUS attribute in GCC is compatible with the candidate draft of TS 29113 (since GCC 4.6).

Go

• GCC 4.7 implements the Go 1 language standard. The library support in 4.7.0 is not quite complete, due to release timing. Release 4.7.1 is expected to include complete support.
• Go has been tested on GNU/Linux and Solaris platforms. It may work on other platforms as well.

New Targets and Target Specific Improvements

ARM

• GCC now supports the Cortex-A7 processor implementing the v7-a version of the architecture using the option -mcpu=cortex-a7.
• The default vector size in auto-vectorization for NEON is now 128 bits. If vectorization fails thusly, the vectorizer tries again with 64-bit vectors.
• A new option -mvectorize-with-neon-double was added to allow users to change the vector size to 64 bits.

AVR

• GCC now supports the XMEGA architecture. This requires GNU binutils 2.22 or later.
• Support for the named address spaces __flash, __flash1, …, __flash5 and __memx has been added. These address spaces locate read-only data in flash memory and allow reading from flash memory by means of ordinary C code, i.e. without the need of (inline) assembler code:

  const __flash int values[] = { 42, 31 };
  
  int add_values (const __flash int *p, int i)
  {
      return values[i] + *p;
  }

• Support for AVR-specific built-in functions has been added.
• Support has been added for the signed and unsigned 24-bit scalar integer types __int24 and __uint24.
• New command-line options -maccumulate-args, -mbranch-cost=cost and -mstrict-X were added to allow better fine-tuning of code optimization.
• Many optimizations to:
  • 64-bit integer arithmetic
  • Widening multiplication
  • Integer division by a constant
  • Avoid constant reloading in multi-byte instructions.
  • Micro-optimizations for special instruction sequences.
  • Generic built-in functions like __builtin_ffs*, __builtin_clz*, etc.
  • If-else decision trees generated by switch instructions
  • Merging of data located in flash memory
  • New libgcc variants for devices with 8-bit wide stack pointer
  • …
• Better documentation:
  • Handling of EIND and indirect jumps on devices with more than 128 KiB of program memory.
  • Handling of the RAMPD, RAMPX, RAMPY and RAMPZ special function registers.
  • Function attributes OS_main and OS_task.
  • AVR-specific built-in macros.

C6X

• Support has been added for the Texas Instruments C6X family of processors.

CR16

• Support has been added for National Semiconductor's CR16 architecture.

Epiphany

• Support has been added for Adapteva's Epiphany architecture.

IA-32/x86-64

• Support for Intel AVX2 intrinsics, built-in functions and code generation is available via -mavx2.
• Support for Intel BMI2 intrinsics, built-in functions and code generation is available via -mbmi2.
• Implementation and automatic generation of __builtin_clz* using the lzcnt instruction is available via -mlzcnt.
• Support for Intel FMA3 intrinsics and code generation is available via -mfma.
• A new -mfsgsbase command-line option is available that makes GCC generate new segment register read/write instructions through dedicated built-ins.
• Support for the new Intel rdrnd instruction is available via -mrdrnd.
• Two additional AVX vector conversion instructions are available via -mf16c.
• Support for new Intel processor codename IvyBridge with RDRND, FSGSBASE and F16C is available through -march=core-avx-i.
• Support for the new Intel processor codename Haswell with AVX2, FMA, BMI, BMI2, LZCNT is available through -march=core-avx2.
• Support for new AMD family 15h processors (Piledriver core) is now available through -march=bdver2 and -mtune=bdver2 options.
• Support for the x32 psABI is now available through the -mx32 option.
• Windows mingw targets are using the -mms-bitfields option by default.
• Windows x86 targets are using the __thiscall calling convention for C++ class-member functions.
• Support for the configure option --with-threads=posix for Windows mingw targets.

MIPS

• GCC now supports thread-local storage (TLS) for MIPS16. This requires GNU binutils 2.22 or later.
• GCC can now generate code specifically for the Cavium Octeon+ and Octeon2 processors. The associated command-line options are -march=octeon+ and -march=octeon2 respectively. Both options require GNU binutils 2.22 or later.
• GCC can now work around certain 24k errata, under the control of the command-line option -mfix-24k. These workarounds require GNU binutils 2.20 or later.
• 32-bit MIPS GNU/Linux targets such as mips-linux-gnu can now build n32 and n64 multilibs. The result is effectively a 64-bit GNU/Linux toolchain that generates 32-bit code by default. Use the configure-time option --enable-targets=all to select these extra multilibs.
• Passing -fno-delayed-branch now also stops the assembler from automatically filling delay slots.

PowerPC/PowerPC64

• Vectors of type vector long long or vector long are passed and returned using the same method as other vectors with the VSX instruction set. Previously the GCC compiler did not adhere to the ABI for 128-bit vectors with 64-bit integer base types (PR 48857). This will also be fixed in the GCC 4.6.1 and 4.5.4 releases.
• A new option -mno-pointers-to-nested-functions was added to allow AIX 32-bit/64-bit and GNU/Linux 64-bit PowerPC users to specify that the compiler should not load up the chain register (r11) before calling a function through a pointer. If you use this option, you cannot call nested functions through a pointer, or call other languages that might use the static chain.
• A new option msave-toc-indirect was added to allow AIX 32-bit/64-bit and GNU/Linux 64-bit PowerPC users control whether we save the TOC in the prologue for indirect calls or generate the save inline. This can speed up some programs that call through a function pointer a lot, but it can slow down other functions that only call through a function pointer in exceptional cases.
• The PowerPC port will now enable machine-specific builtin functions when the user switches the target machine using the #pragma GCC target or __attribute__ ((__target__ ("target"))) code sequences. In additon, the target macros are updated. However, due to the way the -save-temps switch is implemented, you won't see the effect of these additional macros being defined in preprocessor output.

SH

• A new option -msoft-atomic has been added. When it is specified, GCC will generate GNU/Linux-compatible gUSA atomic sequences for the new __atomic routines.
• Since it is neither supported by GAS nor officially documented, code generation for little endian SH2A has been disabled. Specifying -ml with -m2a* will now result in a compiler error.
• The defunct -mbranch-cost option has been fixed.
• Some improvements to the generated code of:
  • Utilization of the tst #imm,R0 instruction.
  • Dynamic shift instructions on SH2A.
  • Integer absolute value calculations.

SPARC

• The option -mflat has been reinstated. When it is specified, the compiler will generate code for a single register window model. This is essentially a new implementation and the corresponding debugger support has been added to GDB 7.4.
• Support for the options -mtune=native and -mcpu=native has been added on selected native platforms (GNU/Linux and Solaris).
• Support for the SPARC T3 (Niagara 3) processor has been added.
• VIS:
  • An intrinsics header visintrin.h has been added.
  • Builtin intrinsics for the VIS 1.0 edge handling and pixel compare instructions have been added.
  • The little-endian version of alignaddr is now supported.
  • When possible, VIS builtins are marked const, which should increase the compiler's ability to optimize VIS operations.
  • The compiler now properly tracks the %gsr register and how it behaves as an input for various VIS instructions.
  • Akin to fzero, the compiler can now generate fone instructions in order to set all of the bits of a floating-point register to 1.
  • The documentation for the VIS intrinsics in the GCC manual has been brought up to date and many inaccuracies were fixed.
  • Intrinsics for the VIS 2.0 bmask, bshuffle, and non-condition-code setting edge instructions have been added. Their availability is controlled by the new -mvis2 and -mno-vis2 options. They are enabled by default on UltraSPARC-III and later CPUs.
• Support for UltraSPARC Fused Multiply-Add floating-point extensions has been added. These instructions are enabled by default on SPARC T3 (Niagara 3) and later CPUs.

TILE-Gx/TILEPro

• Support has been added for the Tilera TILE-Gx and TILEPro families of processors.

Other significant improvements

• A new option (-grecord-gcc-switches) was added that appends compiler command-line options that might affect code generation to the DW_AT_producer attribute string in the DWARF debugging information.
• GCC now supports various new GNU extensions to the DWARF debugging information format, like entry value and call site information, typed DWARF stack or a more compact macro representation. Support for these extensions has been added to GDB 7.4. They can be disabled through the -gstrict-dwarf command-line option.