Loop fusion.

[Toon Moene <toon@moene.org>]

A few days ago there was a rant on the Fortran Standardization 
Committee's e-mail list about Fortran's "whole array arithmetic" being 
unoptimizable.

An example picked at random from our weather forecasting code:

     ZQICE(1:NPROMA,1:NFLEVG) = PGFL(1:NPROMA,1:NFLEVG,YI%MP)
     ZQLI(1:NPROMA,1:NFLEVG) = PGFL(1:NPROMA,1:NFLEVG,YL%MP)
     ZQRAIN(1:NPROMA,1:NFLEVG) = PGFL(1:NPROMA,1:NFLEVG,YR%MP)
     ZQSNOW(1:NPROMA,1:NFLEVG) = PGFL(1:NPROMA,1:NFLEVG,YS%MP)

The reaction from one of the members of the committee (about "their" 
compiler):

'And multiple consecutive array statements with the same shape are 
“fused” exactly so that the compiler can generate good cache use. This 
sort of optimization is pretty low hanging fruit.'

As far as I can see loop fusion as a stand-alone optimization is not 
supported as yet, although some mention is made in the context of graphite.

Is this something that should be pursued ?

Kind regards,

-- 
Toon Moene - e-mail: toon@moene.org - phone: +31 346 214290
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
At home: http://moene.org/~toon/; weather: http://moene.org/~hirlam/
Progress of GNU Fortran: http://gcc.gnu.org/wiki/GFortran#news