http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58915
Bug ID: 58915 Summary: [missed optimization] GCC fails to get the loop bound for some loops. Product: gcc Version: 4.9.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: congh at google dot com Getting the correct loop upper bound is important for some optimizations. GCC tries to get this bound by calling bound_difference() in tree-ssa-loop-niters.c, where GCC finds all control-dependent predicates of the loop and attempt to extract bound information from each predicate. However, GCC fails to get the bound for some loops. Below shows such an example: unsigned int i; if (i > 0) { ... if (i < 4) { do { ... --i; } while (i > 0); } } Clearly the upper bound is 3. But GCC could not get it for this loop. The reason is that GCC check i<4 (i could be zero) and i>0 separately and from neither condition can the upper bound be calculated. Those two conditions may not be combined into one as there may exist other statements between them. One possible solution is letting GCC collect all conditions first then merge them before calculating the upper bound. Any comments?