We've found a serious audio distortion with the standard GSM 6.10 library, when optimizations are turned on in gcc 4.2. No such problem exists in gcc 4.1. Sorry I can't be more specific as to where the problem lies.
Asterisk bugtracker: http://bugs.digium.com/view.php?id=11243 -- Summary: [4.2 regression] Optimizations cause audio distortion in GSM decompression Product: gcc Version: 4.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: gcc-gnu-org at the-tilghman dot com GCC build triplet: i486-linux-gnu GCC host triplet: i486-linux-gnu GCC target triplet: i486-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34216