http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59121
--- Comment #17 from Mircea Namolaru <mircea.namolaru at inria dot fr> --- Yes, data dependencies computation is expensive in the polyehdral model and it could take considerable time - but it is worrying that in too many cases fails to provide (after a few hours left running, when I stop it) an answer for very simple programs. I will ckeck with the isl people if this is the expected behaviour of the isl_union_map_compute_flow (this is the function where the data dependency computation is stuck) and how (and if) they could help us. Many Fortran programs with loops having no-constants bounds and n-dimensional arrays (n>=3) may be affected by this problem and may work only for small dimensions of the arrays. The problem touches especially Fortran, that uses linearized accesses to multi-dimensional arrays - this creates patterns leading to this problem (in this example we have an array acc of dimension 55,48,49 and the array access acc(j,k,l) is transformed to acc(j + 55*k + 2640*l). I've checked the constraints passed to isl_union_map_compute - see that wrapping is perfromed. But wrapping requires modulo operation, expressed by constraints with existential quantifier that may be harder to solve. By disabling the wrapping, some simple examples that before were stuck in data dependency computation finish immediately. In what measure is wrapping necessary ? - as a side-effect it may increase compilation time (that may be already considerable).
