The Prominent Changes For The GCC 4.7 Compiler

With GCC 4.7 being released soon, new compiler benchmarks at Phoronix will be published in the coming weeks (beginning next week Monday), but for those wondering what's different on the feature side, here's a look.

Most of the key GCC 4.7 features have already been talked about in a number of different Phoronix articles, but here's a concise summary of what to expect from this open-source compiler collection.

- Several LTO (Link-Time Optimization) improvements. In particular, there's improved scalability and reduced memory usage, reduced object file size, stream performance improvements, and several bug-fixes. Using LTO while building Firefox now only takes up 3GB of RAM where as previously 8GB was needed for optimizing the browser. The linking time itself is also shorter now, with claims of Firefox being sped up by about a factor of 10.

- Other optimization work: Interprocedural optimization improvements and a string-length optimization pass.

- IRIX 6.5, MIPS OpenBSD, Solaris 7, and Tru64 UNIX v5.1 have been obseleted with GCC 4.7. Unless any of those ports reach a maintained state by GCC 4.8, they will be eliminated.

- Experimental support for transactional memory with the C compiler and the supporting libitm run-time library. This transactional memory support is available at this point for x86_32, x86_64, and Alpha platforms.

- Better support for the C++11 and C11 languages (there's also some C99 changes), such as support for atomic operations in their new memory model. With C11 there's also support for Unicode strings, non-returning functions, alignment support, and support for the C library implementation of the CMPLX family of macros. The C++11 support improvements include implementing extended friend syntax, explicit override control, non-static data member initializers, user-defined literals, alias-declarations, delegating constructors, and atomic classes.

- Major Google Go language improvements.

- The libstdc++ run-time library also has experimental support for C++11.

- Various Fortran language improvements.

- Support for ARM Cortex-A7 processors using the -mcpu=cortex-a7 switch.

- Support for Texas Instruments C6X processors.

- Support for Tilera TILE-Gx and TILEPro processors.

- Support for Intel Ivy Bridge processors. The support improvements come via building upon the existing Intel Sandy Bridge CPU support and added support for Bull Mountain (RDRND; the new random number generator), FSGSBASE, and F16C extensions. The Intel Ivy Bridge CPU support in the GCC compiler can be tapped via -march=core-avx-i. The first Ivy Bridge hardware will begin shipping in a few months.

- Going out even further, preliminary support for Intel Haswell processors has been added. Haswell won't be here for another year, but GCC 4.7 introduces support for AVX2, FMA, BMI, BMI2, and LZCNT instructions as to be first-introduced with Haswell processors. This support comes via the -march=core-avx2 flag. Find out more by reading Compilers Mature For Intel Sandy/Ivy Bridge, Prep For Haswell.

- Initial support for AMD family 15h "Piledriver" processors via -march=bdver2. The first Piledriver-based AMD Fusion APUs are expected to arrive soon with AMD Trinity.

The full list of official changes for GCC 4.7 can be found out via this GNU.org page. Meanwhile, there's already new feature work towards GCC 4.8. New Phoronix benchmarks of GCC 4.7 are coming soon.