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GCC 4.6 Release Series — Changes, New Features, and Fixes - GNU Project

️Fri Jan 31 2025

GCC 4.6 Release Series
Changes, New Features, and Fixes

Caveats

The options -b <machine> and -V <version> have been removed because they were unreliable. Instead, users should directly run <machine>-gcc when cross-compiling, or <machine>-gcc-<version> to run a different version of gcc.
GCC now has stricter checks for invalid command-line options. In particular, when gcc was called to link object files rather than compile source code, it would previously accept and ignore all options starting with --, including linker options such as --as-needed and --export-dynamic, although such options would result in errors if any source code was compiled. Such options, if unknown to the compiler, are now rejected in all cases; if the intent was to pass them to the linker, options such as -Wl,--as-needed should be used.
Versions of the GNU C library up to and including 2.11.1 included an incorrect implementation of the cproj function. GCC optimizes its builtin cproj according to the behavior specified and allowed by the ISO C99 standard. If you want to avoid discrepancies between the C library and GCC's builtin transformations when using cproj in your code, use GLIBC 2.12 or later. If you are using an older GLIBC and actually rely on the incorrect behavior of cproj, then you can disable GCC's transformations using -fno-builtin-cproj.
The C-only intermodule optimization framework (IMA, enabled by -combine) has been removed in favor of the new generic link-time optimization framework (LTO) introduced in GCC 4.5.0.
GCC now ships with the LGPL-licensed libquadmath library, which provides quad-precision mathematical functions for targets with a __float128 datatype. __float128 is available for targets on 32-bit x86, x86-64 and Itanium architectures. The libquadmath library is automatically built on such targets when building the Fortran compiler.
New -Wunused-but-set-variable and -Wunused-but-set-parameter warnings were added for C, C++, Objective-C and Objective-C++. These warnings diagnose variables respective parameters which are only set in the code and never otherwise used. Usually such variables are useless and often even the value assigned to them is computed needlessly, sometimes expensively. The -Wunused-but-set-variable warning is enabled by default by -Wall flag and -Wunused-but-set-parameter by -Wall -Wextra flags.
On ARM, a bug has been fixed in GCC's implementation of the AAPCS rules for the layout of vectors that could lead to wrong code being generated. Vectors larger than 8 bytes in size are now by default aligned to an 8-byte boundary. This is an ABI change: code that makes explicit use of vector types may be incompatible with binary objects built with older versions of GCC. Auto-vectorized code is not affected by this change. (This change affects GCC versions 4.6.4 and later, with the exception of versions 4.7.0 and 4.7.1.)
On AVR, variables with the progmem attribute to locate data in flash memory must be qualified as const.
Support for a number of older systems and recently unmaintained or untested target ports of GCC has been declared obsolete in GCC 4.6. Unless there is activity to revive them, the next release of GCC will have their sources permanently removed.

All GCC ports for the following processor architectures have been declared obsolete:
- Argonaut ARC (arc-*)
- National Semiconductor CRX (crx-*)
- Motorola 68HC11 and 68HC12 (m68hc11-*-*, m6811-*-*, m68hc12-*-*, m6812-*-*)
- Sunplus S+core (score-*)
The following ports for individual systems on particular architectures have been obsoleted:
- Interix (i[34567]86-*-interix3*)
- NetWare x86 (i[3456x]86-*-netware*)
- Generic ARM PE (arm-*-pe* other than arm*-wince-pe*)
- MCore PE (mcore-*-pe*)
- SH SymbianOS (sh*-*-symbianelf*)
- GNU Hurd on Alpha and PowerPC (alpha*-*-gnu*, powerpc*-*-gnu*)
- M68K uClinux old ABI (m68k-*-uclinuxoldabi*)
- a.out NetBSD (arm*-*-netbsd*, i[34567]86-*-netbsd*, vax-*-netbsd*, but not *-*-netbsdelf*)
The i[34567]86-*-pe alias for Cygwin targets has also been obsoleted; users should configure for i[34567]86-*-cygwin* instead.

Certain configure options to control the set of libraries built with GCC on some targets have been obsoleted. On ARM targets, the options --disable-fpu, --disable-26bit, --disable-underscore, --disable-interwork, --disable-biendian and --disable-nofmult have been obsoleted. On MIPS targets, the options --disable-single-float, --disable-biendian and --disable-softfloat have been obsoleted.
Support has been removed for all the configurations obsoleted in GCC 4.5.
More information on porting to GCC 4.6 from previous versions of GCC can be found in the porting guide for this release.

General Optimizer Improvements

A new general optimization level, -Ofast, has been introduced. It combines the existing optimization level -O3 with options that can affect standards compliance but result in better optimized code. For example, -Ofast enables -ffast-math.
Link-time optimization improvements:
- The Scalable Whole Program Optimizer (WHOPR) project has stabilized to the point of being usable. It has become the default mode when using the LTO optimization model. Link time optimization can now split itself into multiple parallel compilations. Parallelism is controlled with -flto=n (where n specifies the number of compilations to execute in parallel). GCC can also cooperate with a GNU make job server by specifying the -flto=jobserver option and adding + to the beginning of the Makefile rule executing the linker.
  Classical LTO mode can be enforced by -flto-partition=none. This may result in small code quality improvements.
- A large number of bugs were fixed. GCC itself, Mozilla Firefox and other large applications can be built with LTO enabled.
- The linker plugin support improvements
  - Linker plugin is now enabled by default when the linker is detected to have plugin support. This is the case for GNU ld 2.21.51 or newer (on ELF and Cygwin targets) and the Gold linker on ELF targets. Plugin support of the Apple linker on Darwin is not compatible with GCC. The linker plugin can also be controlled by the -fuse-linker-plugin command-line option.
  - Resolution information from the linker plugin is used to drive whole program assumptions. Use of the linker plugin results in more aggressive optimization on binaries and on shared libraries that use the hidden visibility attribute. Consequently the use of -fwhole-program is not necessary in addition to LTO.
- Hidden symbols used from non-LTO objects now have to be explicitly annotated with externally_visible when the linker plugin is not used.
- C++ inline functions and virtual tables are now privatized more aggressively, leading to better inter-procedural optimization and faster dynamic linking.
- Memory usage and intermediate language streaming performance have been improved.
- Static constructors and destructors from individual units are inlined into a single function. This can significantly improve startup times of large C++ applications where static constructors are very common. For example, static constructors are used when including the iostream header.
- Support for the Ada language has been added.
Interprocedural optimization improvements
- The interprocedural framework was re-tuned for link time optimization. Several scalability issues were resolved.
- Improved auto-detection of const and pure functions. Newly, noreturn functions are auto-detected.
  The -Wsuggest-attribute=[const|pure|noreturn] flag is available that informs users when adding attributes to headers might improve code generation.
- A number of inlining heuristic improvements. In particular:
  - Partial inlining is now supported and enabled by default at -O2 and greater. The feature can be controlled via -fpartial-inlining.
    Partial inlining splits functions with short hot path to return. This allows more aggressive inlining of the hot path leading to better performance and often to code size reductions (because cold parts of functions are not duplicated).
  - Scalability for large compilation units was improved significantly.
  - Inlining of callbacks is now more aggressive.
  - Virtual methods are considered for inlining when the caller is inlined and devirtualization is then possible.
  - Inlining when optimizing for size (either in cold regions of a program or when compiling with -Os) was improved to better handle C++ programs with larger abstraction penalty, leading to smaller and faster code.
- The IPA reference optimization pass detecting global variables used or modified by functions was strengthened and sped up.
- Functions whose address was taken are now optimized out when all references to them are dead.
- A new inter-procedural static profile estimation pass detects functions that are executed once or unlikely to be executed. Unlikely executed functions are optimized for size. Functions executed once are optimized for size except for the inner loops.
- On most targets with named section support, functions used only at startup (static constructors and main), functions used only at exit and functions detected to be cold are placed into separate text segment subsections. This extends the -freorder-functions feature and is controlled by the same switch. The goal is to improve the startup time of large C++ programs.
  Proper function placement requires linker support. GNU ld 2.21.51 on ELF targets was updated to place those functions together within the text section leading to better code locality and faster startup times of large C++ programs. The feature is also supported in the Apple linker. Support in the gold linker is planned.
A new switch -fstack-usage has been added. It makes the compiler output stack usage information for the program, on a per-function basis, in an auxiliary file.
A new switch -fcombine-stack-adjustments has been added. It can be used to enable or disable the compiler's stack-slot combining pass which before was enabled automatically at -O1 and above, but could not be controlled on its own.
A new switch -fstrict-volatile-bitfields has been added. Using it indicates that accesses to volatile bitfields should use a single access of the width of the field's type. This option can be useful for precisely defining and accessing memory-mapped peripheral registers from C or C++.

Compile time and memory usage improvements

Datastructures used by the dataflow framework in GCC were reorganized for better memory usage and more cache locality. Compile time is improved especially on units with large functions (possibly resulting from a lot of inlining) not fitting into the processor cache. The compile time of the GCC C compiler binary with link-time optimization went down by over 10% (benchmarked on x86-64 target).

New Languages and Language specific improvements

Ada

Stack checking has been improved on selected architectures (Alpha, IA-32/x86-64, RS/6000 and SPARC): it now will detect stack overflows in all cases on these architectures.
Initial support for Ada 2012 has been added.

C family

A new warning, enabled by -Wdouble-promotion, has been added that warns about cases where a value of type float is implicitly promoted to double. This is especially helpful for CPUs that handle the former in hardware, but emulate the latter in software.
A new function attribute leaf was introduced. This attribute allows better inter-procedural optimization across calls to functions that return to the current unit only via returning or exception handling. This is the case for most library functions that have no callbacks.
Support for a new data type __int128 for targets having wide enough machine-mode support.
The new function attribute callee_pop_aggregate allows to specify if the caller or callee is responsible for popping the aggregate return pointer value from the stack.

Support for selectively enabling and disabling warnings via #pragma GCC diagnostic has been added. For instance:

#pragma GCC diagnostic error "-Wuninitialized"
  foo(a);			/* error is given for this one */
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wuninitialized"
  foo(b);			/* no diagnostic for this one */
#pragma GCC diagnostic pop
  foo(c);			/* error is given for this one */
#pragma GCC diagnostic pop
  foo(d);			/* depends on command-line options */

The -fmax-errors=N option is now supported. Using this option causes the compiler to exit after N errors have been issued.

C

There is now experimental support for some features from the upcoming C1X revision of the ISO C standard. This support may be selected with -std=c1x, or -std=gnu1x for C1X with GNU extensions. Note that this support is experimental and may change incompatibly in future releases for consistency with changes to the C1X standard draft. The following features are newly supported as described in the N1539 draft of C1X (with changes agreed at the March 2011 WG14 meeting); some other features were already supported with no compiler changes being needed, or have some support but not in full accord with N1539 (as amended).
- Static assertions (_Static_assert keyword)
- Typedef redefinition
- New macros in <float.h>
- Anonymous structures and unions
The new -fplan9-extensions option directs the compiler to support some extensions for anonymous struct fields which are implemented by the Plan 9 compiler. A pointer to a struct may be automatically converted to a pointer to an anonymous field when calling a function, in order to make the types match. An anonymous struct field whose type is a typedef name may be referred to using the typedef name.

C++

Improved experimental support for the upcoming C++0x ISO C++ standard, including support for constexpr (thanks to Gabriel Dos Reis and Jason Merrill), nullptr (thanks to Magnus Fromreide), noexcept, unrestricted unions, range-based for loops (thanks to Rodrigo Rivas Costa), opaque enum declarations (thanks also to Rodrigo), implicitly deleted functions and implicit move constructors.
When an extern declaration within a function does not match a declaration in the enclosing context, G++ now properly declares the name within the namespace of the function rather than the namespace which was open just before the function definition (c++/43145).
GCC now warns by default when casting integers to larger pointer types. These warnings can be disabled with the option -Wno-int-to-pointer-cast, which is now also available in C++.
G++ no longer optimizes using the assumption that a value of enumeration type will fall within the range specified by the standard, since that assumption is easily violated with a conversion from integer type (c++/43680). The old behavior can be restored with -fstrict-enums.
The new -fnothrow-opt flag changes the semantics of a throw() exception specification to match the proposed semantics of the noexcept specification: just call terminate if an exception tries to propagate out of a function with such an exception specification. This dramatically reduces or eliminates the code size overhead from adding the exception specification.
The new -Wnoexcept flag will suggest adding a noexcept qualifier to a function that the compiler can tell doesn't throw if it would change the value of a noexcept expression.
The -Wshadow option now warns if a local variable or type declaration shadows another type in C++. Note that the compiler will not warn if a local variable shadows a struct/class/enum, but will warn if it shadows an explicit typedef.
When an identifier is not found in the current scope, G++ now offers suggestions about which identifier might have been intended.
G++ now issues clearer diagnostics for missing semicolons after class, struct, and union definitions.
G++ now issues clearer diagnostics for missing semicolons after class member declarations.
G++ now issues clearer diagnostics when a colon is used in a place where a double-colon was intended.
G++ no longer accepts mutable on reference members (c++/33558). Use -fpermissive to allow the old, non-conforming behaviour.
A few mangling fixes have been made, to attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter. By default the compiler still uses the old mangling, but emits aliases with the new mangling on targets that support strong aliases. Users can switch over entirely to the new mangling with -fabi-version=5 or -fabi-version=0. -Wabi will now warn about code that uses the old mangling.
In 4.6.0 and 4.6.1 G++ no longer allows objects of const-qualified type to be default initialized unless the type has a user-declared default constructor. In 4.6.2 G++ implements the proposed resolution of DR 253, so default initialization is allowed if it initializes all subobjects. Code that fails to compile can be fixed by providing an initializer e.g.
```
    struct A { A(); };
    struct B : A { int i; };
    const B b = B();
```
Use -fpermissive to allow the old, non-conforming behaviour.

Runtime Library (libstdc++)

Improved experimental support for the upcoming ISO C++ standard, C++0x, including using constexpr and nullptr.
Performance improvements to the Debug Mode, thanks to François Dumont.
Atomic operations used for reference-counting are annotated so that they can be understood by race detectors such as Helgrind, see Data Race Hunting.
Most libstdc++ standard headers have been changed to no longer include the cstddef header as an implementation detail. Code that relied on that header being included as side-effect of including other standard headers will need to include cstddef explicitly.

Fortran

On systems supporting the libquadmath library, GNU Fortran now also supports a quad-precision, kind=16 floating-point data type (REAL(16), COMPLEX(16)). As the data type is not fully supported in hardware, calculations might be one to two orders of magnitude slower than with the 4, 8 or 10 bytes floating-point data types. This change does not affect systems which support REAL(16) in hardware nor those which do not support libquadmath.
Much improved compile time for large array constructors.
In order to reduce execution time and memory consumption, use of temporary arrays in assignment expressions is avoided for many cases. The compiler now reverses loops in order to avoid generating a temporary array where possible.
Improved diagnostics, especially with -fwhole-file.
The -fwhole-file flag is now enabled by default. This improves code generation and diagnostics. It can be disabled using the deprecated -fno-whole-file flag.
Support the generation of Makefile dependencies via the -M... flags of GCC; you may need to specify the -cpp option in addition. The dependencies take modules, Fortran's include, and CPP's #include into account. Note: Using -M for the module path is no longer supported, use -J instead.
The flag -Wconversion has been modified to only issue warnings where a conversion leads to information loss. This drastically reduces the number of warnings; -Wconversion is thus now enabled with -Wall. The flag -Wconversion-extra has been added and also warns about other conversions; -Wconversion-extra typically issues a huge number of warnings, most of which can be ignored.
A new command-line option -Wunused-dummy-argument warns about unused dummy arguments and is included in -Wall. Before, -Wunused-variable also warned about unused dummy arguments.
Fortran 2003 support has been extended:
- Improved support for polymorphism between libraries and programs and for complicated inheritance patterns (cf. object-oriented programming).
- Experimental support of the ASSOCIATE construct.
- In pointer assignments it is now possible to specify the lower bounds of the pointer and, for a rank-1 or a simply contiguous data-target, to remap the bounds.
- Automatic (re)allocation: In intrinsic assignments to allocatable variables the left-hand side will be automatically allocated (if unallocated) or reallocated (if the shape or type parameter is different). To avoid the small performance penalty, you can use a(:) = ... instead of a = ... for arrays and character strings – or disable the feature using -std=f95 or -fno-realloc-lhs.
- Deferred type parameter: For scalar allocatable and pointer variables the character length can be deferred.
- Namelist variables with allocatable and pointer attribute and nonconstant length type parameter are supported.
Fortran 2008 support has been extended:
- Experimental coarray support (for one image only, i.e. num_images() == 1); use the -fcoarray=single flag to enable it.
- The STOP and the new ERROR STOP statements now support all constant expressions.
- Support for the CONTIGUOUS attribute.
- Support for ALLOCATE with MOLD.
- Support for the STORAGE_SIZE intrinsic inquiry function.
- Support of the NORM2 and PARITY intrinsic functions.
- The following bit intrinsics were added: POPCNT and POPPAR for counting the number of 1 bits and returning the parity; BGE, BGT, BLE, and BLT for bitwise comparisons; DSHIFTL and DSHIFTR for combined left and right shifts, MASKL and MASKR for simple left and right justified masks, MERGE_BITS for a bitwise merge using a mask, SHIFTA, SHIFTL and SHIFTR for shift operations, and the transformational bit intrinsics IALL, IANY and IPARITY.
- Support of the EXECUTE_COMMAND_LINE intrinsic subroutine.
- Support for the IMPURE attribute for procedures, which allows for ELEMENTAL procedures without the restrictions of PURE.
- Null pointers (including NULL()) and not allocated variables can be used as actual argument to optional non-pointer, non-allocatable dummy arguments, denoting an absent argument.
- Non-pointer variables with TARGET attribute can be used as actual argument to POINTER dummies with INTENT(IN)
- Pointers including procedure pointers and those in a derived type (pointer components) can now be initialized by a target instead of only by NULL.
- The EXIT statement (with construct-name) can now be used to leave not only the DO but also the ASSOCIATE, BLOCK, IF, SELECT CASE and SELECT TYPE constructs.
- Internal procedures can now be used as actual argument.
- The named constants INTEGER_KINDS, LOGICAL_KINDS, REAL_KINDS and CHARACTER_KINDS of the intrinsic module ISO_FORTRAN_ENV have been added; these arrays contain the supported kind values for the respective types.
- The module procedures C_SIZEOF of the intrinsic module ISO_C_BINDINGS and COMPILER_VERSION and COMPILER_OPTIONS of ISO_FORTRAN_ENV have been implemented.
- Minor changes: obsolescence diagnostics for ENTRY was added for -std=f2008; a line may start with a semicolon; for internal and module procedures END can be used instead of END SUBROUTINE and END FUNCTION; SELECTED_REAL_KIND now also takes a RADIX argument; intrinsic types are supported for TYPE(intrinsic-type-spec); multiple type-bound procedures can be declared in a single PROCEDURE statement; implied-shape arrays are supported for named constants (PARAMETER). The transformational, three argument versions of BESSEL_JN and BESSEL_YN were added – the elemental, two-argument version had been added in GCC 4.4; note that the transformational functions use a recurrence algorithm.

Support for the Go programming language has been added to GCC. It is not enabled by default when you build GCC; use the --enable-languages configure option to build it. The driver program for compiling Go code is gccgo.

Go is currently known to work on GNU/Linux and RTEMS. Solaris support is in progress. It may or may not work on other platforms.

Objective-C and Objective-C++

The -fobjc-exceptions flag is now required to enable Objective-C exception and synchronization syntax (introduced by the keywords @try, @catch, @finally and @synchronized).
A number of Objective-C 2.0 features and extensions are now supported by GCC. These features are enabled by default; you can disable them by using the new -fobjc-std=objc1 command-line option.
The Objective-C 2.0 dot-syntax is now supported. It is an alternative syntax for using getters and setters; object.count is automatically converted into [object count] or [object setCount: ...] depending on context; for example if (object.count > 0) is automatically compiled into the equivalent of if ([object count] > 0) while object.count = 0; is automatically compiled into the equivalent ot [object setCount: 0];. The dot-syntax can be used with instance and class objects and with any setters or getters, no matter if they are part of a declared property or not.
Objective-C 2.0 declared properties are now supported. They are declared using the new @property keyword, and are most commonly used in conjunction with the new Objective-C 2.0 dot-syntax. The nonatomic, readonly, readwrite, assign, retain, copy, setter and getter attributes are all supported. Marking declared properties with __attribute__ ((deprecated)) is supported too.
The Objective-C 2.0 @synthesize and @dynamic keywords are supported. @synthesize causes the compiler to automatically synthesize a declared property, while @dynamic is used to disable all warnings for a declared property for which no implementation is provided at compile time. Synthesizing declared properties requires runtime support in most useful cases; to be able to use it with the GNU runtime, appropriate helper functions have been added to the GNU Objective-C runtime ABI, and are implemented by the GNU Objective-C runtime library shipped with GCC.
The Objective-C 2.0 fast enumeration syntax is supported in Objective-C. This is currently not yet available in Objective-C++. Fast enumeration requires support in the runtime, and such support has been added to the GNU Objective-C runtime library (shipped with GCC).
The Objective-C 2.0 @optional keyword is supported. It allows you to mark methods or properties in a protocol as optional as opposed to required.
The Objective-C 2.0 @package keyword is supported. It has currently the same effect as the @public keyword.
Objective-C 2.0 method attributes are supported. Currently the supported attributes are deprecated, sentinel, noreturn and format.
Objective-C 2.0 method argument attributes are supported. The most widely used attribute is unused, to mark an argument as unused in the implementation.
Objective-C 2.0 class and protocol attributes are supported. Currently the only supported attribute is deprecated.
Objective-C 2.0 class extensions are supported. A class extension has the same syntax as a category declaration with no category name, and the methods and properties declared in it are added directly to the main class. It is mostly used as an alternative to a category to add methods to a class without advertising them in the public headers, with the advantage that for class extensions the compiler checks that all the privately declared methods are actually implemented.
As a result of these enhancements, GCC can now be used to build Objective-C and Objective-C++ software that uses Foundation and other important system frameworks with the NeXT runtime on Darwin 9 and Darwin 10 (OSX 10.5 and 10.6).
Many bugs in the compiler have been fixed in this release; in particular, LTO can now be used when compiling Objective-C and Objective-C++ and the parser is much more robust in dealing with invalid code.

Runtime Library (libobjc)

The GNU Objective-C runtime library now defines the macro __GNU_LIBOBJC__ (with a value that is increased at every release where there is any change to the API) in objc/objc.h, making it easy to determine if the GNU Objective-C runtime library is being used, and if so, which version. Previous versions of the GNU Objective-C runtime library (and other Objective-C runtime libraries such as the Apple one) do not define this macro.
A new Objective-C 2.0 API, almost identical to the one implemented by the Apple Objective-C runtime, has been implemented in the GNU Objective-C runtime library. The new API hides the internals of most runtime structures but provides a more extensive set of functions to operate on them. It is much easier, for example, to create or modify classes at runtime. The new API also makes it easier to port software from Apple to GNU as almost no changes should be required. The old API is still supported for backwards compatibility; including the old objc/objc-api.h header file automatically selects the old API, while including the new objc/runtime.h header file automatically selects the new API. Support for the old API is being phased out and upgrading the software to use the new API is strongly recommended. To check for the availability of the new API, the __GNU_LIBOBJC__ macro can be used as older versions of the GNU Objective-C runtime library, which do not support the new API, do not define such a macro.
Runtime support for @synchronized has been added.
Runtime support for Objective-C 2.0 synthesized property accessors has been added.
Runtime support for Objective-C 2.0 fast enumeration has been added.

New Targets and Target Specific Improvements

ARM

GCC now supports the Cortex-M4 processor implementing the v7-em version of the architecture using the option -mcpu=cortex-m4.
Scheduling descriptions for the Cortex-M4, the Neon and the floating point units of the Cortex-A9 and a pipeline description for the Cortex-A5 have been added.
Synchronization primitives such as __sync_fetch_and_add and friends are now inlined for supported architectures rather than calling into a kernel helper function.
SSA loop prefetching is enabled by default for the Cortex-A9 at -O3.
Several improvements were committed to improve code generation for the ARM architecture including a rewritten implementation for load and store multiples.
Several enhancements were committed to improve SIMD code generation for NEON by adding support for widening instructions, misaligned loads and stores, vector conditionals and support for 64-bit arithmetic.
Support was added for the Faraday cores fa526, fa606te, fa626te, fmp626te, fmp626 and fa726te and can be used with the respective names as parameters to the -mcpu= option.
Basic support was added for Cortex-A15 and is available through -mcpu=cortex-a15.
GCC for AAPCS configurations now more closely adheres to the AAPCS specification by enabling -fstrict-volatile-bitfields by default.

IA-32/x86-64

The new -fsplit-stack option permits programs to use a discontiguous stack. This is useful for threaded programs, in that it is no longer necessary to specify the maximum stack size when creating a thread. This feature is currently only implemented for 32-bit and 64-bit x86 GNU/Linux targets.
Support for emitting profiler counter calls before function prologues. This is enabled via a new command-line option -mfentry.
Optimization for the Intel Core 2 processors is now available through the -march=core2 and -mtune=core2 options.
Support for Intel Core i3/i5/i7 processors is now available through the -march=corei7 and -mtune=corei7 options.
Support for Intel Core i3/i5/i7 processors with AVX is now available through the -march=corei7-avx and -mtune=corei7-avx options.
Support for AMD Bobcat (family 14) processors is now available through the -march=btver1 and -mtune=btver1 options.
Support for AMD Bulldozer (family 15) processors is now available through the -march=bdver1 and -mtune=bdver1 options.
The default setting (when not optimizing for size) for 32-bit GNU/Linux and Darwin x86 targets has been changed to -fomit-frame-pointer. The default can be reverted to -fno-omit-frame-pointer by configuring GCC with the --enable-frame-pointer configure option.
Darwin, FreeBSD, Solaris 2, MinGW and Cygwin now all support __float128 on 32-bit and 64-bit x86 targets.
AVX floating-point arithmetic can now be enabled by default at configure time with the new --with-fpmath=avx option.
The SSA loop prefetching pass is enabled when using -O3 when optimizing for CPUs where prefetching is beneficial (AMD CPUs newer than K6).
Support for TBM (Trailing Bit Manipulation) built-in functions and code generation is available via -mtbm.
Support for AMD's BMI (Bit Manipulation) built-in functions and code generation is available via -mbmi.

MicroBlaze

Support has been added for the Xilinx MicroBlaze softcore processor (microblaze-elf) embedded target. This configurable processor is supported on several Xilinx Spartan and Virtex FPGAs.

MIPS

GCC now supports the Loongson 3A processor. Its canonical -march= and -mtune= name is loongson3a.

MN10300 / AM33

The inline assembly register constraint "A" has been renamed "c". This constraint is used to select a floating-point register that can be used as the destination of a multiply-accumulate instruction.
New inline assembly register constraints "A" and "D" have been added. These constraint letters resolve to all general registers when compiling for AM33, and resolve to address registers only or data registers only when compiling for MN10300.
The MDR register is represented in the compiler. One can access the register via the "z" constraint in inline assembly. It can be marked as clobbered or used as a local register variable via the "mdr" name. The compiler uses the RETF instruction if the function does not modify the MDR register, so it is important that inline assembly properly annotate any usage of the register.

PowerPC/PowerPC64

GCC now supports the Applied Micro Titan processor with -mcpu=titan.
The -mrecip option has been added, which indicates whether the reciprocal and reciprocal square root instructions should be used.
The -mveclibabi=mass option can be used to enable the compiler to autovectorize mathematical functions using the Mathematical Acceleration Subsystem library.
The -msingle-pic-base option has been added, which instructs the compiler to avoid loading the PIC base register in function prologues. The PIC base register must be initialized by the runtime system.
The -mblock-move-inline-limit option has been added, which enables the user to control the maximum size of inlined memcpy calls and similar.
PowerPC64 GNU/Linux support for applications requiring a large TOC section has been improved. A new command-line option, -mcmodel=MODEL, controls this feature; valid values for MODEL are small, medium, or large.
The Altivec builtin functions vec_ld and vec_st have been modified to generate the Altivec memory instructions LVX and STVX, even if the -mvsx option is used. In the initial GCC 4.5 release, these builtin functions were changed to generate VSX memory reference instructions instead of Altivec memory instructions, but there are differences between the two instructions. If the VSX instruction set is available, you can now use the new builtin functions vec_vsx_ld and vec_vsx_st which always generates the VSX memory instructions.
The GCC compiler on AIX now defaults to a process layout with a larger data space allowing larger programs to be compiled.
The GCC long double type on AIX 6.1 and above has reverted to 64-bit double precision, matching the AIX XL compiler default, because of missing C99 symbols required by the GCC runtime.
The default processor scheduling model and tuning for PowerPC64 GNU/Linux and for AIX 6.1 and above now is POWER7.
Starting with GCC 4.6.1, vectors of type vector long long or vector long are passed and returned in 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 is also fixed in the GCC 4.5.4 release.

S/390, zSeries and System z9/z10, IBM zEnterprise z196

Support for the zEnterprise z196 processor has been added. When using the -march=z196 option, the compiler will generate code making use of the following instruction facilities:
- Conditional load/store
- Distinct-operands
- Floating-point-extension
- Interlocked-access
- Population-count
The -mtune=z196 option avoids the compare and branch instructions as well as the load address instruction with an index register as much as possible and performs instruction scheduling appropriate for the new out-of-order pipeline architecture.
When using the -m31 -mzarch options the generated code still conforms to the 32-bit ABI but uses the general purpose registers as 64-bit registers internally. This requires a Linux kernel saving the whole 64-bit registers when doing a context switch. Kernels providing that feature indicate that by the 'highgprs' string in /proc/cpuinfo.
The SSA loop prefetching pass is enabled when using -O3.

SPARC

GCC now supports the LEON series of SPARC V8 processors. The code generated by the compiler can either be tuned to it by means of the --with-tune=leon configure option and -mtune=leon compilation option, or the compiler can be built for the sparc-leon-{elf,linux} and sparc-leon3-{elf,linux} targets directly.
GCC has stopped sign/zero-extending parameter registers in the callee for functions taking parameters with sub-word size in 32-bit mode, since this is redundant with the specification of the ABI. GCC has never done so in 64-bit mode since this is also redundant.
The command-line option -mfix-at697f has been added to enable the documented workaround for the single erratum of the Atmel AT697F processor.

Operating Systems

Android

GCC now supports the Bionic C library and provides a convenient way of building native libraries and applications for the Android platform. Refer to the documentation of the -mandroid and -mbionic options for details on building native code. At the moment, Android support is enabled only for ARM.

Darwin/Mac OS X

General
- Initial support for CFString types has been added.
  This allows GCC to build projects including the system Core Foundation frameworks. The GCC Objective-C family supports CFString "toll-free bridged" as per the Mac OS X system tools. CFString is also recognized in the context of format attributes and arguments (see the documentation for format attributes for limitations). At present, 8-bit character types are supported.
- Object file size reduction.
  The Darwin zeroed memory allocators have been re-written to make more use of .zerofill sections. For non-debug code, this can reduce object file size significantly.
- Objective-C family 64-bit support (NeXT ABI 2).
  Initial support has been added to support 64-bit Objective-C code using the Darwin/OS X native (NeXT) runtime. ABI version 2 will be selected automatically when 64-bit code is built.
- Objective-C family 32-bit ABI 1.
  For 32-bit code ABI 1 is also now also allowed. At present it must be selected manually using -fobjc-abi-version=1 where applicable - i.e. on Darwin 9/10 (OS X 10.5/10.6).
x86 Architecture
- The -mdynamic-no-pic option has been enabled.
  Code supporting -mdynamic-no-pic optimization has been added and is applicable to -m32 builds. The compiler bootstrap uses the option where appropriate.
- The default value for -mtune= has been changed.
  Since Darwin systems are primarily Xeon, Core-2 or similar the default tuning has been changed to -mtune=core2.
- Enable 128-bit long double (__float128) support on Darwin.
PPC Architecture
- Darwin64 ABI.
  Several significant bugs have been fixed, such that GCC now produces code compatible with the Darwin64 PowerPC ABI.
- libffi and boehm-gc.
  The Darwin ports of the libffi and boehm-gc libraries have been upgraded to include a Darwin64 implementation. This means that powerpc*-*-darwin9 platforms may now, for example, build Java applications with -m64 enabled.
- Plug-in support has been enabled.
- The -fsection-anchors option is now available although, presently, not heavily tested.

Solaris 2

New Features

Support symbol versioning with the Sun linker.
Allow libstdc++ to leverage full ISO C99 support on Solaris 10+.
Support thread-local storage (TLS) with the Sun assembler on Solaris 2/x86.
Support TLS on Solaris 8/9 if prerequisites are met.
Support COMDAT group with the GNU assembler and recent Sun linker.
Support the Sun assembler visibility syntax.
Default Solaris 2/x86 to -march=pentium4 (Solaris 10+) resp. -march=pentiumpro (Solaris 8/9).
Don't use SSE on Solaris 8/9 x86 by default.
Enable 128-bit long double (__float128) support on Solaris 2/x86.

ABI Change

Change the ABI for returning 8-byte vectors like __m64 in MMX registers on Solaris 10+/x86 to match the Sun Studio 12.1+ compilers. This is an incompatible change. If you use such types, you must either recompile all your code with the new compiler or use the new -mvect8-ret-in-mem option to remain compatible with previous versions of GCC and Sun Studio.

Windows x86/x86_64

Initial support for decimal floating point.
Support for the __thiscall calling-convention.
Support for hot-patchable function prologues via the ms_hook_prologue attribute for x86_64 in addition to 32-bit x86.
Improvements of stack-probing and stack-allocation mechanisms.
Support of push/pop-macro pragma as preprocessor command.
With #pragma push_macro("macro-name") the current definition of macro-name is saved and can be restored with #pragma pop_macro("macro-name") to its saved definition.
Enable 128-bit long double (__float128) support on MinGW and Cygwin.

Other significant improvements

Installation changes

An install-strip make target is provided that installs stripped executables, and may install libraries with unneeded or debugging sections stripped.
On Power7 systems, there is a potential problem if you build the GCC compiler with a host compiler using options that enable the VSX instruction set generation. If the host compiler has been patched so that the vec_ld and vec_st builtin functions generate Altivec memory instructions instead of VSX memory instructions, then you should be able to build the compiler with VSX instruction generation.

Changes for GCC Developers

Note: these changes concern developers that develop GCC itself or software that integrates with GCC, such as plugins, and not the general GCC users.

The gengtype utility, which previously was internal to the GCC build process, has been enchanced to provide GC root information for plugins as necessary.
The old GC allocation interface of ggc_alloc and friends was replaced with a type-safe alternative.

This is the list of problem reports (PRs) from GCC's bug tracking system that are known to be fixed in the 4.6.1 release. This list might not be complete (that is, it is possible that some PRs that have been fixed are not listed here).

GCC 4.6 Release Series — Changes, New Features, and Fixes - GNU Project