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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 13:05, Richard Guenther
<richard.guenther@gmail.com> wrote:
> 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.
>
The dependence analysis in the vectorizer is done only in the innermost
loop that is vectorized, whereas prefetch does the analysis of data deps
for every loop.
Sebastian