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Re: [Patch PR 44576]: Reduce the computation cost in compute_miss_rate for prefetching loop arrays
On Wed, Jun 30, 2010 at 8:01 PM, Zdenek Dvorak <rakdver@kam.mff.cuni.cz> wrote:
> Hi,
>
>> On Wed, Jun 30, 2010 at 7:34 PM, Fang, Changpeng <Changpeng.Fang@amd.com> wrote:
>> >> FOR_EACH_LOOP (li, loop, LI_ONLY_INNERMOST)
>> >
>> >>does that make a difference?
>> >
>> > This doesn't help, because "compute_all_dependences" was called the same number of time
>> > as before.
>> >
>> > (BTW, should we limit prefetching only to the innermost one?)
>> >
>> > In this test case, there are 6 large loops, where each loop has 729 memory reference.
>> > It takes 4~5 seconds to "compute_all_dependence" for one such loop.
>>
>> It shouldn't take that long. ?Can you gather a more detailed profile?
>
> actually, compute_all_dependences is quadratic in the number of memory
> references, and in more complicated cases, it can perform rather complex
> computations, so 5 seconds on 729 references does seem like a realistic time.
> Of course, we need to either speed up the analysis or add an cut-off to avoid
> it on loops with too many memory references (or both),
Well, but at -O3 the vectorizer computes dependences as well, and it
doesn't take that much of time, So there must be something obvious
going wrong.
Richard.
> Zdenek
>