Six times faster than C

Re:Six times faster than C by xoranth on Friday July 07, 2023 @07:11AM (#36630004)

That's likely the fastest way to do that without vectorization. But you'd need to upcast 's' to an uint64 (or at least an uint32). That means that vectorization would operate on 32/64 bit lanes.

With vectorization, I think the way to go is to have two nested loops, an outer advances by 32 * 255 elements at a time, and an inner one that loads 32 bytes, compares each character to 's', and accumulates on 8 bit lanes.

Then in the outer loop you do an horizontal sum of the 8 bit accumulators.

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Re:Six times faster than C by fuber2018 on Friday July 07, 2023 @07:19AM (#36630058)

My SWAR version almost does what your vectorization algorithm description does - just that the SWAR-code looks rather gnarly because the compiler isn't auto-generating the vector code for you, it's hand-coded in C by me and I'm limited to 64 bits at a time.

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Re:Six times faster than C by gpderetta on Friday July 07, 2023 @07:19AM (#36630056)

Indeed, the blocked vectorization with 8 bits accumulators shown elsethread is going to be faster and there reducing the sum to 1 bit per iteration is worth it.

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