Re: auto vectorization in gcc

[Diego Novillo <dnovillo at redhat dot com>]

On Thu, 2003-07-17 at 08:45, Dorit Naishlos wrote:

> We are planning to implement auto-vectorization targeting vector
> extensions such as AltiVec.
>
Cool!

> (a) the tree-SSA branch is preparing the infrastructure for
>     auto-vectorization
> (b) the task of auto-vectorizaiton will probably be divided between
>     the RTL level and the tree level
> (c) since the tree-level is machine independent, actual vectorization
>     will take place in the RTL level
> (d) the tree-SSA branch will perform data-dependence analysis and
>     loop transformations to increase vectorizability of loops,
>     and somehow pass on the information to the RTL level.
> 
> How far is this from what has really been envisioned?
>
That's pretty much the idea we had in mind.

> Is it indeed the case that no target specific information will be
> available at the tree level, deferring transformations that rely on
> such information to the RTL level?
>
No.  I think we will need to have some target information at the tree
level.  But it's not clear how much or how detailed, yet.

>  (for example, detecting idioms
> like "substract and saturate" if they are supported, and unrolling
> or blocking the iterations according to the vector length, etc)?
> 
Something along those lines, yes.


> We would like to start making progress on auto-vectorization
> right away, and we're trying to figure out what's the best way to do
> that. On one hand, we don't want to duplicate work, and we want to
> take a path that could take advantage of the infrastructure that is
> being developed (tree-SSA branch, rtlopt branch), (and hopefully
> would be merged into the main trunk in the future).
>
I don't think there's much risk in duplicating effort at the moment. 
I'm not aware of anybody working on vectorization today.


> What is your opinion on the correct mode of work here?
> 
Since you want to start from the bottom up, I think the rtlopt branch
may be a better starting point.  At some point both tree-ssa and rtlopt
should merge, but I don't really know what the status of rtlopt is. 
Jan?  Zdenek?

Starting on the tree-ssa branch would also be possible, particularly if
you start with SLP.  There are some issues with RTL loops in tree-ssa
and you would need to implement some of the loop transformations at the
tree level, but we need to do that at some point, anyway.


> After the above questions are cleared out, the remaining major
> design decision is which vectorization scheme to follow -
> straight-line code vectorization (Superword Level Parallism (SLP)
> http://www.acm.org/pubs/citations/proceedings/pldi/349299/p145-larsen/),
> or the classic loop based vectorization, and how to partition it
> between the tree-level and the RTL level.
> 
This is what I had in mind as a starting point.  It's simple and doesn't
involve much of the heavy data dependency work from traditional
vectorization.


Diego.