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by wholesomepotato 927 days ago
it's one XOR, SHL, OR, SHR, AND, on some archs the shifts might come free with the other instruction. I'd expect it to be faster.

a = v ^ 0x30303030lu // normalize to digits 0xXX0a0b0c

b = (a << 12) | a // combine into XXXXXbacb0c

idx = (b >> 12) & 0xfff // get bac

res = lookup[idx]
1 comments
RaisingSpear 926 days ago
That's kinda neat. Actually, if you're doing that, you may as well reduce it to a single shift+or:

a = ((a >> 12) | a) & 0xfff

You could also skip the xor with 0x303030 by adjusting the lookup table accordingly.

Unfortunately, you'd still need to factor in the length argument somehow. That is, if given "23" with length=1, it should parse to 2, not 23. You could address this with a variable shift, but at that point, I can't see it being any better than a multiply+shift, even assuming the lookup table is fully cached.

The other major issue is validation, which the lookup table doesn't help much with.

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wholesomepotato 926 days ago
Might be wrong but this shortcut corrupts the lower bits with garbage from the higher byte.

The lookup table can detect some, but not all errors, so yeah, it relies on valid input.

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RaisingSpear 926 days ago
I can't see how it's any different. If you have 0x0y0z, a shift+or gives 0x0yxz, so the result is same as what you have, just with fewer operations.
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wholesomepotato 925 days ago
It's unclear what is in the highest byte, so I assume not 0x000x0y0z, but 0xab0x0y0z where ab is unknown (in the past comment I used XX for this). If highest byte is known, then sure, even better.
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RaisingSpear 925 days ago
The '& 0xfff' eliminates the highest byte, so it doesn't matter what it is.

Your code doesn't handle the 'length' parameter, so the problem isn't the highest byte, it's bytes beyond 'length'.

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wholesomepotato 924 days ago
I think you're right. Even better. Did you have time to bench it, etc.?

I see what you mean by length. I just skimmed over the text originally as I don't have time for rather lame problems like this. I'd just add 3 bits of length to be part of the index, job done. 12KB lookup table instead of 4KB, assuming 0 is not a valid value (negate to avoid needing 0b11).

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