Re: Optimization of spread

[Mikael Morin <morin-mikael@orange.fr>]

Hello,

welcome, and thanks for your interest.

Le 03/11/2022 à 11:48, Théo Cavignac via Fortran a écrit :
> Hello,
> I am currently writing some numerical code in Fortran 2003 and I want
> to use the spread intrinsic because having used NumPy heavily for the
> past few years, it feels natural to use such an array primitive.
> I naturally wondered what would be the effect on performance and found
> this on Stack Overflow: https://stackoverflow.com/a/55732905/6324751
> 
> TLDR: spread is as fast, if not faster than a do loop, when using
> ifort. However, it is significantly slower (up to 100% in my
> microbenchmarks) with gfortran 12.2.0.
> 
> Investigating the matter a bit more, I noticed that ifort recognize
> the pattern and essentially produce the same code for both the do loop
> and the spread call, while gfortran “naively” call spread, even with
> -O3.
> 
> Here is a demonstration on godbolt.org: https://godbolt.org/z/dcYEPj8bP
> 
> So, my question is: is this something that could be better optimized?
> I wonder if simply allowing the compiler to inline spread wouldn't
> already enable further optimizations that would lead to the same kind
> of performance as found in ifort.
Well, obviously you can get the same performance gfortran gets with do 
loops if you make gfortran generate do loops in place for spread.

> I also think other array intrinsic may benefit from this effort if
> similar strategies can be applied.
> While I have never been contributing to GCC, but I would be willing to
> do this implementation if it is in the reach of my C++ skills, and if
> someone can point me in the right direction.
> 
The first step to do is get a work environment and build the latest gcc 
git master from source.
The source is actually more C than C++ (the fortran front-end at least). 
  It requires little C++ skills, but time and willingness to decipher 
its complexity.

There are two places where inlining can be done:
  * In front-end passes where the parsed fortran code is rewritten 
before generating the intermediary code for the optimizers.  Thomas 
König can help you there.
  * Directly in the code generation for the optimizers.  It is (much) 
more complex but can avoid the need for temporaries.  I can help you there.

Some links about our development process and conventions:
https://gcc.gnu.org/contribute.html
https://gcc.gnu.org/git.html

How to build GCC:
https://gcc.gnu.org/wiki/InstallingGCC


Mikael