initial import of GCC 12.3.0.

major changes in GCC 11 included:

- The default mode for C++ is now -std=gnu++17 instead of -std=gnu++14.
- When building GCC itself, the host compiler must now support C++11,
rather than C++98.
- Some short options of the gcov tool have been renamed: -i to -j and
-j to -H.
- ThreadSanitizer improvements.
- Introduce Hardware-assisted AddressSanitizer support.
- For targets that produce DWARF debugging information GCC now defaults
to DWARF version 5. This can produce up to 25% more compact debug
information compared to earlier versions.
- Many optimisations.
- The existing malloc attribute has been extended so that it can be
used to identify allocator/deallocator API pairs. A pair of new
-Wmismatched-dealloc and -Wmismatched-new-delete warnings are added.
- Other new warnings:
-Wsizeof-array-div, enabled by -Wall, warns about divisions of two
sizeof operators when the first one is applied to an array and the
divisor does not equal the size of the array element.
-Wstringop-overread, enabled by default, warns about calls to string
functions reading past the end of the arrays passed to them as
arguments.
-Wtsan, enabled by default, warns about unsupported features in
ThreadSanitizer (currently std::atomic_thread_fence).
- Enchanced warnings:
-Wfree-nonheap-object detects many more instances of calls to
deallocation functions with pointers not returned from a dynamic
memory allocation function.
-Wmaybe-uninitialized diagnoses passing pointers or references to
uninitialized memory to functions taking const-qualified arguments.
-Wuninitialized detects reads from uninitialized dynamically
allocated memory.
-Warray-parameter warns about functions with inconsistent array forms.
-Wvla-parameter warns about functions with inconsistent VLA forms.
- Several new features from the upcoming C2X revision of the ISO C
standard are supported with -std=c2x and -std=gnu2x.
- Several C++20 features have been implemented.
- The C++ front end has experimental support for some of the upcoming
C++23 draft.
- Several new C++ warnings.
- Enhanced Arm, AArch64, x86, and RISC-V CPU support.
- The implementation of how program state is tracked within
-fanalyzer has been completely rewritten with many enhancements.

see https://gcc.gnu.org/gcc-11/changes.html for a full list.

major changes in GCC 12 include:

- An ABI incompatibility between C and C++ when passing or returning
by value certain aggregates containing zero width bit-fields has
been discovered on various targets. x86-64, ARM and AArch64
will always ignore them (so there is a C ABI incompatibility
between GCC 11 and earlier with GCC 12 or later), PowerPC64 ELFv2
always take them into account (so there is a C++ ABI
incompatibility, GCC 4.4 and earlier compatible with GCC 12 or
later, incompatible with GCC 4.5 through GCC 11). RISC-V has
changed the handling of these already starting with GCC 10. As
the ABI requires, MIPS takes them into account handling function
return values so there is a C++ ABI incompatibility with GCC 4.5
through 11.
- STABS: Support for emitting the STABS debugging format is
deprecated and will be removed in the next release. All ports now
default to emit DWARF (version 2 or later) debugging info or are
obsoleted.
- Vectorization is enabled at -O2 which is now equivalent to the
original -O2 -ftree-vectorize -fvect-cost-model=very-cheap.
- GCC now supports the ShadowCallStack sanitizer.
- Support for __builtin_shufflevector compatible with the clang
language extension was added.
- Support for attribute unavailable was added.
- Support for __builtin_dynamic_object_size compatible with the
clang language extension was added.
- New warnings:
-Wbidi-chars warns about potentially misleading UTF-8
bidirectional control characters.
-Warray-compare warns about comparisons between two operands of
array type.
- Some new features from the upcoming C2X revision of the ISO C
standard are supported with -std=c2x and -std=gnu2x.
- Several C++23 features have been implemented.
- Many C++ enhancements across warnings and -f options.

see https://gcc.gnu.org/gcc-12/changes.html for a full list.