Re: [f-cpu] Popcount usage ?

[whygee@club-internet.fr]

hi !

>Hi,
>what is popcount used for ? From my analysis of
>compiled codes it is used in IA64 mainly for implementing
>ffs/ffz.
>So what is it good for ? I can be computed for 64bits
>using existing operations by cca 22 insns in about
>30 cycles. Is it the op so critical for some high
>performance algos ?

it is used for 2 main reasons :
 - during powerup, it is a piece of the BIST system.
    it is used for signature compaction in front of
    a LFSR, it compacts 128 bits down to 6 bits.
 - For some communication, storage or signal processing
    algos, it is used to compute the Hamming distance
    (it has a XOR of two operands in front of it).
And it looks sexy for all the spooks out there.

But it is optionnal, that is : you can forget it,
if you want, in case you use a FPGA. However, if
you implement it in silicon, then power-up BIST
will need it.

>devik
YG

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