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Re: [4.5] Find more autoinc addressing for induction variables


Steven Bosscher wrote:

But in any case I think the patch should not go onto the trunk in its
current form for other technical reasons.

And these would be...? So far you've only given alternatives which aren't equivalent and wouldn't work.


FWIW I do understand the important of this optimization and I know
auto-inc/dec support in GCC should be improved. But there has to be a
cheaper and easier, more generic way

Feel free to suggest one if one occurs to you that actually works. As long as you don't have a realistic suggestion, "there has to be" isn't patch review, it's wishful thinking and obstructionism. I'm trying to solve a long-standing problem with gcc here.


(one that can also be extended to handle more than just FORM_POST_INC).

As far as I understood the code in auto-inc-dec.c, it can also generate preincrement if we find such a candidate. If we have a memory reference with offset 4, followed by an inc by 4, that can be turned into preincrement. If my ivopts patch works as intended, it should generate such cases for targets that have preincrement, but I haven't had a chance to test this.


We could look for increment/memory reference pairs in the other order, but given how most loops are structured, that seems unlikely to trigger very often - bivs tend to be incremented at the end.

x = &y[0];
for (i = 0; i < n; i++) {
 for (j = 0; j < m; j++) {
   foo (*x);
   if (bar ())
     break;
   x++;
 }
}

If the outer loop isn't there, or the initialization is moved into it, we
can use autoincrement, but here we need to avoid doing so.

Would things simplify if you can make sure that B and C are in the same loop? This is easy to test (they must have the same loop_father).

That's a necessary condition, and I've experimented with using it to bail out early if it isn't true. However, it didn't seem to have any effect on run time.
It's not a sufficient condition, since either one may be inside an if statement. Also, if you look at the example above, you'll find that both B and C _are_ in the same loop; whether we can use autoinc depends on the code _outside_ the loop.


Thanks for the test case.
Are you preparing an updated tree-ssa-loop-ivopts patch or can I use
the one you already posted?

I'll send another one out eventually; I'll probably try for another improvement in the heuristic first. For testing on the Blackfin, you can use either one of the versions I already posted.



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