http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53778
Bug #: 53778 Summary: bad code (delivering NaN instead of proper result) with -foptimize-sibling-calls Classification: Unclassified Product: gcc Version: 4.6.3 Status: UNCONFIRMED Severity: major Priority: P3 Component: fortran AssignedTo: unassig...@gcc.gnu.org ReportedBy: thomas.or...@awi.de Host: x86_64-pc-linux-gnu Target: x86_64-pc-linux-gnu Build: x86_64-pc-linux-gnu I have a function in my Fortran code base that looks like this: function dat_init_wind(handle, x, perturb) result(wind) type(datafield_t), intent(in) :: handle real(kind=8), dimension(:), intent(in) :: x logical, intent(in), optional :: perturb real(kind=8), dimension(handle%world%dims) :: wind logical :: pert pert = .true. if(present(perturb)) pert = perturb; select case(handle%initial_state) case(dat_init_geosiso) wind = geosiso_wind(handle, handle%bottomwind, x) case(dat_init_baroiso) wind = baroiso_wind(handle, handle%bottomwind, x) case(dat_init_baroclinpoly) wind = baroclinpoly_wind(handle, handle%baroclinpoly, x) case default wind = handle%base_speeds * bottomwind_profile(handle%bottomwind, world_map_y(handle%world, x)) end select_ if(pert .and. handle%perturb_wind /= 0) call pert_perturb(handle%pert, wind(handle%perturb_wind), x, handle%world%scale%space /& & handle%world%scale%time) end function This worked fine until a recent change, where I changed innerworkings of pert_perturb(). Suddenly the result (wind) was a set of NaN instead of 0 (in the considered configuration). Note that pert is false, as is (handle%perturb_wind /= 0), so the changes to pert_perturb() should have no influence on the result. Also, I noticed that adding a printout to the function fixes things, even if it is not actually called: select case(handle%initial_state) case(-100) write(0,0) 'This is a stub that never is executed but prevent compiler BUG 25 from triggering. Apparently.' case(dat_init_geosiso) Also, dropping the call to bottomwind_profile() from the 'default' case, which is what is actually executed, fixes the issue, but makes the code rather no-op for me. While the recent changes in my code also touch that function, it itself still computes correclty (as does pert_perturb()). That somehow fits with me narrowing down the issue to the minimal optimization flags needed (down from simple -g -O2): -O -g -foptimize-sibling-calls (The -g can be dropped, I strongly assume.) So it is something with optimizing function calls / stack mess-around. Note that I was unable to further reduce what is behind -O, I can activate all flags that make up -O here individually and the issue does not come into play. But also, setting -O and disabling all flags also does not trigger (it's a combination I guess). So it is <some unknown basic setting> plus sibling call optimization. Now, does this description ring a bell? It would be so swell if this situation was clear enough to diagnose the error in gfortran's optimization. If not, I will have to try to extract something self-contained out of my codebase again ... which might need considerable time. If this issue is already known and something along that fixed (in 4.7, perhaps?), that would be a nice surprise. I apologize if this turns out to be a bug in my code after all, but since I work with -fbounds-check -ffpe-trap=invalid,zero,overflow usually, I don't see much possibility to produce such breakage in Fortran. If this were a C program, I'd look harder for me messing up someplace;-)