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@REPLYADDR Terje Mathisen
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@REPLYTO 2:5075/128 Terje Mathisen
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robf...@gmail.com wrote:
> On Friday, September 22, 2023 at 10:26:38???AM UTC-4, MitchAlsup wrote:
>> One builds FP calculation resources as big as longest container
needed at full throughput.
>> In a 64-bit machine, this is one with a 11-bit exponent and a
52-bit fraction.
>> On such a machine, the latency is set by the calculations on
this sized number.
>> AND
>> Smaller width numbers do not save any cycles.
>> <
>> So, the only advantage one has with 48-bit, ... numbers is memory footprint.
>> There is NO (nada, zero, zilch) advantage in calculation latency.
>> <
> Does that include complicated calculations too? What about trig
> functions, square root, or other iterative functions? As I have
> implemented reciprocal square root in micro-code it takes longer for
> greater precision. Makes me think there is some benefit to supporting
> varying precisions.
This is easy to verify: Lookup the latency for both 32 and 64-bit
versions of the function you are interested in!
If they differ by less then 25%, then anything intermediate really
doesn`t make sense for a HW op.
In software you can more easily play tricks like the infamous InvSqrt()
function of Quake III fame, where just 10 bits was sufficient to make
lighting calculations look OK. Today you can do the same, using the
approximate reciprocal square root vector op, and simply skip all the
normal NR stages to follow.
Terje
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