CODEWITHSUNDEEP
ssevectorizationsimd
Mohammad Reza Selim
Mohammad Reza Selim

Reputation: 85

Efficient way to create a bit mask from multiple numbers possibly using SSE/SSE2/SSE3/SSE4 instructions

Suppose I have 16 ascii characters (hence 16 8 bit numbers) in a 128 bit variable/register. I want to create a bit mask in which those bits will be high whose bit positions (indexes) are represented by those 16 characters.

For example, if the string formed from those 16 characters is "CAD...", in the bit mask 67th bit, 65th bit, 68th bit and so on should be 1. The rest of the bits should be 0. What is the efficient way to do it specially using SIMD instructions?

I know that one of the technique is addition like this: 2^(67-1)+2^(65-1)+2^(68-1)+... But this will require a large number of operations. I want to do it in one/two operations/instructions if possible.

Please let me know a solution.

Upvotes: 4

Views: 1659

Answers (1)

Evgeny Kluev
Evgeny Kluev

Reputation: 24667

SSE4.2 contains one instruction, that performs almost what you want: PCMPISTRM with immediate operand 0. One of its operands should contain your ASCII characters, other - a constant vector with values like 32, 33, ... 47. You get the result in 16 least significant bits of XMM0. Since you need 128 bits, this instruction should be executed 8 times with different constant vectors (6 times if you need only printable ASCII characters). After each PCMPISTRM, use bitwise OR to accumulate the result in some XMM register.

There are 2 disadvantages of this method: (1) you need to read the Intel's architectures software developer's manual to understand PCMPISTRM's details because that's probably the most complicated SSE instruction ever, and (2) this instruction is pretty slow (throughput of 1/2 on Nehalem, 1/3 on Sandy Bridge, 1/4 on Bulldozer), so you'll hardly get any significant speed improvement over 'brute force' method.

Upvotes: 4

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