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[fivemack]

Intel has announced the instruction set for its new vector-supercomputer-disguised-as-a-graphics-card 'Larrabee'.

http://software.intel.com/en-us/articles/prototype-primitives-guide/ has a C++ implementation using the data types and intrinsic names which the real thing will use.

It has a full set of the instructions you would expect, including count-set-bits and find-first-set-bit; it has vector gather and scatter (finally!), it has the normal-for-Intel irritating omissions (add-with-carry for 32-bit numbers only?), and it has one or two really quite surprising instructions:

BITINTERLEAVE21_PI - 2:1 Bit-Interleave Int32 Vectors

Performs an element-by-element bitwise interleave, using a 2:1 pattern, between int32 vector v2 and int32 vector v3. The low 21 bits from elements in v2 are interleaved with the low 11 bits from elements in v3 to form a vector of 32-bit values. Bits alternate 2:1, so that source elements A and B combine bitwise this way (high to low):
A20 B10 A19 A18 B9 A17 A16 B8 … A5 A4 B2 A3 A2 B1 A1 A0 B0

I will buy a chocolate pudding at the Carlton next Thursday for the person to give the least ludicrously contrived example in which this instruction might be useful. There is also a BITINTERLEAVE11_PI which takes alternate bits from the two source elements.

Comments

[mobbsy.livejournal.com]

Some sort of cunning way of hardening ECC schemes against bursty losses by interleving the ECC code with the data?

[stephdairy.livejournal.com]

BITINTERLEAVE11_PI is of vital use for implementing Intercal's mingle operator.

(S)

[beamjockey.livejournal.com]

I look forward to reading answers to your challenge from very smart people.

[angoel.livejournal.com]

I've a vague feeling it might be useful for fourier transforms.

[drsulak.livejournal.com]

Perhaps chopping data up for sending across multiple communication links, where the total bandwidth can vary.

In a graphics card? I don't know, as I have a cellular background. There may be interesting graphic algorithms that can use such specialized instructions.

[mobbsy.livejournal.com]

Ah! That'd be it (maybe)... 3D Morton numbers for proximity calculations. First 1:1 interleave X and Y (with BITINTERLEAVE11_PI), then take that result and then 2:1 interleave Z.

[tanac.livejournal.com]

No, this is used to improve 2D signal sets over weak transmission channels. A sort of error correction encoding or signal strengthening technique.

See http://eprints.iisc.ernet.in/5698/ and http://ieeexplore.ieee.org/Xplore/login.jsp?url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F4489030%2F4489031%2F04489200.pdf%3Farnumber%3D4489200&authDecision=-203 for examples of bitwise interleave uses. This one specifically can put the crc in the high bits for a coded stream in the lower bits to transmit ECC along with signal in a single DWORD

[tanac.livejournal.com]

(hee, in case you're wondering, that was Jeff, not me.)

[tanac.livejournal.com]

Oh, and he says not DWORD, but QuadWord. whatever that means.