[Bug tree-optimization/42108] [4.8/4.9/5 Regression] 50% performance regression

["burnus at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42108

--- Comment #64 from Tobias Burnus <burnus at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #63)
> Unfortunately for the testcase this doesn't allow moving the division at all
> and we are lucky that we have range information at all because of the fortran
> frontend casting 'n' to unsigned before dividing by it.

If it helps and the semantic is preserved, there is no reason not to completely
change what tree code the Fortran FE generates for loops.

[I think one reason for the odd way tree code for loops is generated is: The
current code makes it simple to permit loops which are always run once, as some
Fortran 66 compilers did. For instance, "DO i = 2, 1" would then be executed
once. (Such loops are not permitted in F66 - and some compilers executed them
once others zero times; since F77, such loops are permitted and executed zero
times. Unsurprisingly, some old code from the 60s relies on the execute once
feature.)

g77 and some commercial compilers have a compile flag like "-f66", gfortran
hasn't and I don't think it ever will.]