[llvm-commits] [LLVMdev] [PATCH] BasicBlock Autovectorization Pass

[Hal Finkel]

On Tue, 2012-01-24 at 16:53 +0100, Tobias Grosser wrote:
> On 01/24/2012 05:13 AM, Hal Finkel wrote:
> > On Tue, 2012-01-17 at 13:25 -0600, Sebastian Pop wrote:
> >> Hi,
> >>
> >> On Fri, Dec 30, 2011 at 3:09 AM, Tobias Grosser<tobias at grosser.es>  wrote:
> >>> As it seems my intuition is wrong, I am very eager to see and understand
> >>> an example where a search limit of 4000 is really needed.
> >>>
> >>
> >> To make the ball roll again, I attached a testcase that can be tuned
> >> to understand the impact on compile time for different sizes of a
> >> basic block.  One can also set the number of iterations in the loop to
> >> 1 to test the vectorizer with no loops around.
> >>
> >> Hal, could you please report the compile times with/without the
> >> vectorizer for different basic block sizes?
> >
> > I've looked at your test case, and I am pleased to report a negligible
> > compile-time increase!
> That is nice. But does this example actually trigger the search limit of 
> 4000? I think that is the case I am especially interested in.

I know (and the answer is yes, it could, but not in an interesting way),
but I reduced the default search limit to 400. I did this because, when
used in combination with my load/store-reordering patch, such a high
limit is no longer optimal. As I suspected, it appears that the high
limit was compensating for the lack of the ability to schedule
non-aliasing loads after stores. I would like to deal with the
load/store reording problem on its own merits (and have already
submitted a patch that does this), and so I'll leave the lower default
on the vectorizer search limit.

In addition, Sebastian's test case highlights why, with the current
implementation, having such a high search limit would be bad for compile
times. A limit in the hundreds, not thousands, is necessary to provide
reasonable compile times for unrolled loops with long dependency chains
such as the ones in his example.

Thanks again,
Hal

> 
> Cheers
> Tobi

-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory