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Re: [PATCH] Atom: Scheduler improvements for better imul placement

On 12.04.2012 16:38, Richard Guenther wrote:
On Thu, Apr 12, 2012 at 2:36 PM, Igor Zamyatin<> wrote:
On Thu, Apr 12, 2012 at 4:24 PM, Richard Guenther
<>  wrote:
On Thu, Apr 12, 2012 at 2:00 PM, Alexander Monakov<> wrote:

Can atom execute two IMUL in parallel?  Or what exactly is the pipeline

As I understand from Intel's optimization reference manual, the behavior is as follows: if the instruction immediately following IMUL has shorter latency, execution is stalled for 4 cycles (which is IMUL's latency); otherwise, a 4-or-more cycles latency instruction can be issued after IMUL without a stall. In other words, IMUL is pipelined with respect to other long-latency instructions, but not to short-latency instructions.

It seems to be modeled in the pipeline description though:

;;; imul insn has 5 cycles latency
(define_reservation "atom-imul-32"
                    "atom-imul-1, atom-imul-2, atom-imul-3, atom-imul-4,

;;; imul instruction excludes other non-FP instructions.
(exclusion_set "atom-eu-0, atom-eu-1"
               "atom-imul-1, atom-imul-2, atom-imul-3, atom-imul-4")

The main idea is quite simple:

If we are going to schedule IMUL instruction (it is on the top of
ready list) we try to find out producer of other (independent) IMUL
instruction that is in ready list too. The goal is try to schedule
such a producer to get another IMUL in ready list and get scheduling
of 2 successive IMUL instructions.

Why does that not happen without your patch? Does it never happen without your patch or does it merely not happen for one EEMBC benchmark (can you provide a testcase?)?

It does not happen because the scheduler by itself does not do such specific reordering. That said, it is easy to imagine the cases where this patch will make things worse rather than better.

Igor, why not try different subtler mechanisms like adjust_priority, which is get called when an insn is added to the ready list? E.g. increase the producer's priority.

The patch as is misses checks for NONDEBUG_INSN_P. Also, why bail out when you have more than one imul in the ready list? Don't you want to bump the priority of the other imul found?


And MD allows us only prefer scheduling of successive IMUL instructions, i.e.
If IMUL was just scheduled and ready list contains another IMUL
instruction then it will be chosen as the best candidate for

at least from my very limited guessing of what the above does.  So, did you
analyze why the scheduler does not properly schedule things for you?


From reading the patch, I could not understand the link between pipeline behavior and what the patch appears to do.

Hope that helps.


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