Puzzling performance difference between ifort and gfortran

Question

Recently, I read a post on Stack Overflow about finding integers that are perfect squares. As I wanted to play with this, I wrote the following small program:

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Answer 1

What compiler versions are you using? Interestingly, it looks like a case where there is a performance regression from 11.1 to 12.0 -- e.g. for me, 11.1 (ifort -fast square.f90) takes 3.96s, and 12.0 (same options) took 13.3s. gfortran (4.6.1) (-O3) is still faster (3.35s). I have seen this kind of a regression before, although not quite as dramatic. BTW, replacing the if statement with

is_square = any(m == [0, 1, 4, 9])
if(.not. is_square) return

makes it run twice as fast with ifort 12.0, but slower in gfortran and ifort 11.1.

It looks like part of the problem is that 12.0 is overly aggressive in trying to vectorize things: adding

!DEC$ NOVECTOR

right before the DO loop (without changing anything else in the code) cuts the run time down to 4.0 sec.

Also, as a side benefit: if you have a multi-core CPU, try adding -parallel to the ifort command line :)