[Bug target/29256] [6/7/8 regression] loop performance regression

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=29256

Wilco <wilco at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |wilco at gcc dot gnu.org

--- Comment #66 from Wilco <wilco at gcc dot gnu.org> ---
(In reply to Aldy Hernandez from comment #65)
> (In reply to Jeffrey A. Law from comment #45)
> > This problem still exists and can be seen by making the arrays external and
> > using -fno-tree-loop-distribute-patterns.

> 
> Still a problem.  I get the same code Jeff got for comment 45.

A simple workaround for GCC8 might be to tweak the address costs when loop
unrolling is enabled, so offsets are preferred over indexing (either in IVOpt
or backend). For GCC9 a tree level loop optimization has been proposed which
will fix this issue.