VMLA.f64

By: Wilco (Wilco.Dijkstra.delete@this.ntlworld.com), September 20, 2009 4:45 pm
Room: Moderated Discussions
Michael S (already5chosen@yahoo.com) on 9/20/09 wrote:
---------------------------
>Wilco (Wilco.Dijkstra@ntlworld.com) on 9/20/09 wrote:
>---------------------------
>>Michael S (already5chosen@yahoo.com) on 9/20/09 wrote:
>>---------------------------
>>>Wilco (Wilco.Dijkstra@ntlworld.com) on 9/18/09 wrote:
>>>---------------------------
>>>>
>>>>>2. Cortex-A9 claims "double the performance of previous ARM FPU", which probably
>>>>>means peak scalar double-precision throughput between 1/3 to 1/2 of Silverthorne.
>>>>
>>>>There is no official spec apart from "at least twice as fast as A8", so AFAICT,
>>>>single precision performance is 16 GFLOPS (quad FMAC) and double precision is 3GFLOPS
>>>>at 2GHz.
>>>
>>>How do you got to this number? 3 FMACD instructions every 4 clocks? "Twice as fast
>>>as A8" certainly wouldn't be that fast. Isn't A8 VPFlite engine around 0.2-0.25 DFlops/Hz?
>>
>>It's more like 0.07 DP FLOPS/Hz. VFPLite turned out quite a bit slower than I was
>>expecting - even moves take 4 cycles... I can get that sort of speed using software
>>emulation, no need to waste any hardware on it!
>
>0.07 DP FLOPS/Hz in software including call overhead, denormals and exceptions? Sounds quick.

As an example, my code for single precision IEEE multiply takes just 21 instructions for the typical case - special cases like zeroes, NaN, inf are quicker. I've never tried scheduling it on Cortex-A8, however getting it down to 14-15 cycles is feasible despite the multiply taking 8 cycles. VFPLite needs 10-12 cycles...

>>On A9, one VMLA.f64 can be issued every 2 cycles. In the other cycle you can issue
>>an add. The details of the latencies are in GCC4.4.1.
>>
>
>VMLA.f64 is new to me.

FMACD and VMLA.f64 are the same instruction. I designed a consistent naming scheme for the complete instruction set which meant renaming existing Thumb-1 and VFP instructions.

>BTW, why they say "twice as fast" if in fact it's closer to "20 times faster"?

Ahmdals' law. On ARM11 VFP has around 50 times the peak DP FLOPS compared to my FP emulation code. But most FP applications only sped up by a factor 5-6.

Wilco
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