[Bug fortran/64530] [4.9 Regression] Incorrect calculation when assigning to array with -O3

["anlauf at gmx dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64530

Harald Anlauf <anlauf at gmx dot de> changed:

           What    |Removed                     |Added
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                 CC|                            |anlauf at gmx dot de

--- Comment #1 from Harald Anlauf <anlauf at gmx dot de> ---
(In reply to jkaus from comment #0)
> With O3 we get the wrong result (0.0):
> gfortran -O3 a.F90
> ./a.out
> > buggy   0.0000000000000000        0.0000000000000000        0.0000000000000000     
> > works   3.5000000000000000        3.5000000000000000        3.5000000000000000  

Interesting bug.  I can confirm that it does not occur with 4.8,
but with 4.9 and 5.

Note that if one interchanges in subroutine "buggy" the lines

    old(i) = ave(i)
    tmp(i) = tmp(i) / scale_factor

to

    tmp(i) = tmp(i) / scale_factor
    old(i) = ave(i)

I also get the correct result.

With gfortran 4.9 and 5 I also get the correct result with -O3
when e.g. using UBSAN (-fsanitize=undefined); it probably also
reduces the optimization.

Since the dump tree looks the same for 4.8 and 4.9, could it be
a middle-end bug?