When it comes to platform changes there's now support for MIPS64 LE/BE running Linux, support for MIPS DSP ASE on MIPS32, support for s390x decimal floating point instructions on DFP, POWER8 Power ISA 2.07 instructions, support for Intel AVX2 64-bit instructions, Intel Transactional Synchronization Extensions (TSX) with RTM and HLE, initial support for hardware transactional memory on IBM POWER8, and improved OS X 10.8 64-bit support.
Tooling changes for Valgrind 3.9 include memcheck improvements in handling vectorized code, better control over the leak checker, reduced possible leak reports, better control for stack-trace acquisition, and better reporting of leak suppression usage. The Helgrind tool has been updated to report less false errors.
There's also many fixed bugs and other improvements in the new Valgrind 3.9 release from late last week. More details on the open-source Valgrind 3.9 tool can be found via the release notes.