Are FORTRAN 77 programs faster than Fortran 90 ones?

Question

Today I was reading code from some very popular numerical libraries written in FORTRAN 77 such as QUADPACK (last updated in 1987), and I was wondering if there is any reason not to rewrite those libraries in Fortran 90 apart from the big amount of work it would pose, given the great improvements Fortran 90 brought to the language, including free-form source, better control structures so GO TO could be forgotten, vectorization, interfaces and so on.

Is it because FORTRAN 77 compilers produce more optimized code, maybe it is better for parallel execution? Notice that I'm not even talking about Fortran 2003 for example, which is only 8 years old: I'm talking about Fortran 90, so I assume it has enough widespread and the compilers are ready. I don't have contact with the industry, anyway.

Edit: janneb is right: LAPACK is actually written in Fortran 90.

Answer 1

Well, like "pst" mentioned, the "big amount of work it would pose" is a pretty major reason.

Some further minor points:

• LAPACK IS Fortran 90, these days, in the sense that the latest version no longer compiles with a F77 compiler. That being said, it's far from a rewrite, only a few things that were changed.

• Most of the F90 features you mention make it easier and faster to write robust programs, it doesn't necessary make the resulting programs any faster.

• It wasn't that long ago that free F90 compilers were not available (plenty of people used g77!), so for a widely used library like LAPACK not using F90 features was likely a conscious decision.

• A F77 compiler does not, generally, produce faster code than a F90 compiler. If not for any other reason, then because it's likely obsolete and cannot optimize for the latest CPU's. A modern Fortran compiler might create faster code from F77 than from an equivalent F90 program which makes extensive use of things like pointers etc., but that is highly dependent on the program in question (e.g. using pointers and fancier data structures may allow usage of better algorithms, allowing the F90 program to produce results faster even though it might execute at a lower average utilization of the CPU arithmetic units).

• Vectorization, if by this you mean the F90+ array syntax, is mostly a programmer convenience issue rather than allowing faster code. A competent compiler will vectorize the equivalent DO loop just as well.

Answer 2

There is no reason to put in a ton of work to (maybe) improve something that works well. It should be noted that even though Fortran 90 introduced many new features, it did not change the language. Remember that Fortran 90 Standard is backwards compatible with FORTRAN 77. A modern compiler will be able to optimize a FORTRAN 77 code just as well. There are features introduced in Fortan 90 (e.g. pointers) that would hinder the efficiency and that should be avoided if one cares about execution time, e.g. in HPC.

So it would not really make a difference. For a modern compiler, well written FORTAN 77 is just as optimizable - it is like a cake without icing. Here, in case of Fortran 90 and later, icing looks better than it tastes - it is a convenience for the programmer, but does not necessarily improve the program efficiency.