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Re: Loop fusion.

On 04/22/2015 09:10 PM, Steven Bosscher wrote:

On Wed, Apr 22, 2015 at 6:59 PM, Toon Moene wrote:

Why is loop fusion important, especially in Fortran 90 and later programs ?

Because without it, every array assignment is a single loop nest, isolated
from related, same-shape assignments.

Why is this a bad thing? When you're talking about single-node
machines, separate loops is probably faster if your arrays are large
enough: better cache locality and easier to vectorize.

Loop fusion is only a win if you iterate through the same array
variables. Writing such a pass is not so hard for the simple, most
common cases. The front end could do some of the rewriting from
F90-style array assignments to fused loops if it notices consecutive
array assignments/operations on the same variables.

It could well be that my artificial example was not what my colleague measured ...

Indeed, I thought about the front end doing this, but that would limit it to those that the front end could recognize; on the other hand, that might be the right limitation.

Thanks !

Toon Moene - e-mail: - phone: +31 346 214290
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
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