Compile something with -ftree-vectorizer-verbose, and you'll probably get: "note: not vectorized: too many BBs in loop."
The meaning of 'BB' in this context is most certainly not universally known, so instead of a mysterious two-letter acronym the warning should talk about basic blocks. (I couldn't even find out the meaning of 'BB' by using Google, then I ended up asking on IRC.) -- Summary: The meaning of 'BB' in "too many BBs in loop" Product: gcc Version: 4.3.2 Status: UNCONFIRMED Severity: trivial Priority: P3 Component: tree-optimization AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: lanurmi at iki dot fi http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39129