7 ms·

Pretty similar article from very recently: https://nullprogram.com/blog/2021/12/04/ https://nullprogram.com/blog/2021/12/04/ Discussion: https://news.ycombinat

Pretty similar article from very recently: https://nullprogram.com/blog/2021/12/04/ https://nullprogram.com/blog/2021/12/04/ Discussion: https://news.ycombinator.com/item?id=29439403 https://news.ycombinator.com/item?id=29439403 The article mentions in an addendum (and BeeOnRope also pointed it out in the HN thread) a nice CLMUL trick for dealing with quotes originally discovered by Geoff Langdale. That should work here for a nice speedup. But without the CLMUL trick, I'd guess that the unaligned loads that generally occur after a vector containing both quotes and newlines in this version (the "else" case on lines 34-40) would hamper the performance somewhat, since it would eat up twice as much L1 cache bandwidth. I'd suggest dealing with the masks using bitwise operations in a loop, and letting i stay divisible by 16. Or just use CLMUL :)

davidm1729 5y ago
Hi, I'm one of the authors of the post Thanks for pointing us to CLMUL, I'm not familiar with these kind of multiplications, but, converting the quote bitmask to a quoted bitmask would certainly make it faster. With this new bitmask, we could negate it and AND it with the newline mask, generating a mask of newlines that are not inside quotes. Getting the last newline then would be a simple CLZ of that mask. And there wouldn't be a need to resort to byte to byte processing. In our tests, going byte to byte for more iterations to keep the alignment when hitting the "else case" performed worse than making the unaligned loads, but as you say "just use CLMUL" (as all loads will be aligned) :D
- jart 5y ago
  PMOVMSKB/BSF/POPCNT takes serious wizardry, but instructions like PCLMULLQLQDQ make you feel like Gandalf. It's defined: pair clmul(uint64_t a, uint64_t b) { uint64_t t, x = 0, y = 0; if (a && b) { if (bsr(a) < bsr(b)) t = a, a = b, b = t; /* optional */ for (t = 0; b; a <<= 1, b >>= 1) { if (b & 1) x ^= a, y ^= t; t = t << 1 | a >> 63; } } return (pair){x, y}; } There's a famous paper on how it can perform polynomial division at 40gbps. It's really cool that it has practical applications in things like CSV too. https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/fast-crc-computation-generic-polynomials-pclmulqdq-paper.pdf https://www.intel.com/content/dam/www/public/us/en/documents...
- zwegner 5y ago
  CLMUL in general is a bit weird to wrap your head around, but a CLMUL with -1 isn't too tricky: it's like a running 1-bit sum, or in other words, each bit in the result is the parity of all the bits up to that point in the multiplier. > In our tests, going byte to byte for more iterations to keep the alignment when hitting the "else case" performed worse than making the unaligned loads, but as you say "just use CLMUL" (as all loads will be aligned) :D I was talking about using bitwise operations with the quote/escape/newline masks already computed (like in the blog post I linked), rather than a byte-by-byte loop. But yeah, CLMUL is better anyways :)
- gpderetta 5y ago
  CLMUL is quite interesting. I learned about it when going in depth on how multiplications help with hashing. A multiplication is in practice: - a sum over - a series (i.e. one for each bit set in the multiplier) - of shifts (where the shift amount is the index of that bit in the multiplier) The shifting and the combining are great for hashing as they "distribute" each bit around. CLMUL simply replaces the addition in step one with xor (which can also be thought as the single bit carryless addition).
pclmulqdq 5y ago
The carryless multiplication instructions are amazing and people should use them more often. They are just so poorly explained that they feel like magic.
mattewong 5y ago
Even with the CLMUL trick, CSV parsing does not play nice. It can be made to work for JSON parsing because you can make more assumptions. With CSV, it only works smoothly if you are willing to accept a subset of what most spreadsheet programs accept i.e. to assume your CSV is "well-formed". Considering for example that the following three cells: AA"A,"AA""A","A"A"A when opened in Excel will all give you the same value, using CLMUL to normalize will require many repeated additional SIMD operations-- probably at least 8 if not more. At some vector size it will be worth it, but not clear at 256. The irony is, if you are stuck with CSV input, then the fact that you couldn't get a better format/encoding also suggests that you can't assume your CSV is "well-formed"
- zwegner 5y ago
  Good points all around. Not sure what the OP's requirements are, but judging by their current code, CLMUL should do nicely (or they have a bug). And also, thanks for that example. Clearly I don't know CSV well enough--are quotes in fields that don't start with a quote not special?
  - mattewong 5y ago
    If their data comes from a controlled bubble and they need not assume "real-world" data, then CLMUL might do nicely but best case it would likely only be a marginal improvement (and even then I would be willing to bet, not at anything less than 512 bit vector sizes). Best case, it still needs additional vector calls to support quoting. Obviously, if no quoting will be supported, it's even simpler, but then you also cannot support commas or newlines inside of cell values and are getting so far from "CSV" that you might as well just say you have pipe-delimited data which happens to use comma instead of pipe in which case you don't need CLMUL. If quoting needs to be supported, CLMUL will still require a number of repeated passes, shifts etc to deal with the various cases including an escaped first quote char, last quote char, non-first-or-last quote char and embedded commas/newlines.
    - zwegner 5y ago
      Thread's getting a bit old, hope you see this :) Referring to the OP, I'm not sure what exact dialect of csv they're using, but its quoting rules are nothing like Excel's: quotes are escaped with a separate character (possibly backslash), and quoted regions can start anywhere, not just the start of a field. This makes CLMUL much easier to apply, quite similarly to how it's used in simdjson. Finding escaped characters can be done branchlessly with a handful of scalar operations on the masks (this code is in simdjson too). For proper Excel-style parsing, you're quite possibly right (and looking at ZSV, I trust that you've thought about this problem a lot more than me). I'm not certain that it can't all be done efficiently with very few branches, though, using mostly SIMD and some scalar operations on masks. CLMUL might not be useful here, since quotes are treated completely differently depending on whether the field started with a quote. Instead you'd have a first phase basically looking for a comma or newline followed by a quote, then looking for the next unescaped quote. Escaped quotes within quoted fields can be found with a similar sequence to simdjson's backslash support (and removed with something like vcompressb on newer AVX-512 chips).

davidm1729 5y ago

Hi, I'm one of the authors of the post Thanks for pointing us to CLMUL, I'm not familiar with these kind of multiplications, but, converting the quote bitmask to a quoted bitmask would certainly make it faster. With this new bitmask, we could negate it and AND it with the newline mask, generating a mask of newlines that are not inside quotes. Getting the last newline then would be a simple CLZ of that mask. And there wouldn't be a need to resort to byte to byte processing. In our tests, going byte to byte for more iterations to keep the alignment when hitting the "else case" performed worse than making the unaligned loads, but as you say "just use CLMUL" (as all loads will be aligned) :D

jart 5y ago
PMOVMSKB/BSF/POPCNT takes serious wizardry, but instructions like PCLMULLQLQDQ make you feel like Gandalf. It's defined: pair clmul(uint64_t a, uint64_t b) { uint64_t t, x = 0, y = 0; if (a && b) { if (bsr(a) < bsr(b)) t = a, a = b, b = t; /* optional */ for (t = 0; b; a <<= 1, b >>= 1) { if (b & 1) x ^= a, y ^= t; t = t << 1 | a >> 63; } } return (pair){x, y}; } There's a famous paper on how it can perform polynomial division at 40gbps. It's really cool that it has practical applications in things like CSV too. https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/fast-crc-computation-generic-polynomials-pclmulqdq-paper.pdf https://www.intel.com/content/dam/www/public/us/en/documents...
zwegner 5y ago
CLMUL in general is a bit weird to wrap your head around, but a CLMUL with -1 isn't too tricky: it's like a running 1-bit sum, or in other words, each bit in the result is the parity of all the bits up to that point in the multiplier. > In our tests, going byte to byte for more iterations to keep the alignment when hitting the "else case" performed worse than making the unaligned loads, but as you say "just use CLMUL" (as all loads will be aligned) :D I was talking about using bitwise operations with the quote/escape/newline masks already computed (like in the blog post I linked), rather than a byte-by-byte loop. But yeah, CLMUL is better anyways :)
gpderetta 5y ago
CLMUL is quite interesting. I learned about it when going in depth on how multiplications help with hashing. A multiplication is in practice: - a sum over - a series (i.e. one for each bit set in the multiplier) - of shifts (where the shift amount is the index of that bit in the multiplier) The shifting and the combining are great for hashing as they "distribute" each bit around. CLMUL simply replaces the addition in step one with xor (which can also be thought as the single bit carryless addition).

pclmulqdq 5y ago

The carryless multiplication instructions are amazing and people should use them more often. They are just so poorly explained that they feel like magic.

mattewong 5y ago

Even with the CLMUL trick, CSV parsing does not play nice. It can be made to work for JSON parsing because you can make more assumptions. With CSV, it only works smoothly if you are willing to accept a subset of what most spreadsheet programs accept i.e. to assume your CSV is "well-formed". Considering for example that the following three cells: AA"A,"AA""A","A"A"A when opened in Excel will all give you the same value, using CLMUL to normalize will require many repeated additional SIMD operations-- probably at least 8 if not more. At some vector size it will be worth it, but not clear at 256. The irony is, if you are stuck with CSV input, then the fact that you couldn't get a better format/encoding also suggests that you can't assume your CSV is "well-formed"

zwegner 5y ago
Good points all around. Not sure what the OP's requirements are, but judging by their current code, CLMUL should do nicely (or they have a bug). And also, thanks for that example. Clearly I don't know CSV well enough--are quotes in fields that don't start with a quote not special?
- mattewong 5y ago
  If their data comes from a controlled bubble and they need not assume "real-world" data, then CLMUL might do nicely but best case it would likely only be a marginal improvement (and even then I would be willing to bet, not at anything less than 512 bit vector sizes). Best case, it still needs additional vector calls to support quoting. Obviously, if no quoting will be supported, it's even simpler, but then you also cannot support commas or newlines inside of cell values and are getting so far from "CSV" that you might as well just say you have pipe-delimited data which happens to use comma instead of pipe in which case you don't need CLMUL. If quoting needs to be supported, CLMUL will still require a number of repeated passes, shifts etc to deal with the various cases including an escaped first quote char, last quote char, non-first-or-last quote char and embedded commas/newlines.
  - zwegner 5y ago
    Thread's getting a bit old, hope you see this :) Referring to the OP, I'm not sure what exact dialect of csv they're using, but its quoting rules are nothing like Excel's: quotes are escaped with a separate character (possibly backslash), and quoted regions can start anywhere, not just the start of a field. This makes CLMUL much easier to apply, quite similarly to how it's used in simdjson. Finding escaped characters can be done branchlessly with a handful of scalar operations on the masks (this code is in simdjson too). For proper Excel-style parsing, you're quite possibly right (and looking at ZSV, I trust that you've thought about this problem a lot more than me). I'm not certain that it can't all be done efficiently with very few branches, though, using mostly SIMD and some scalar operations on masks. CLMUL might not be useful here, since quotes are treated completely differently depending on whether the field started with a quote. Instead you'd have a first phase basically looking for a comma or newline followed by a quote, then looking for the next unescaped quote. Escaped quotes within quoted fields can be found with a similar sequence to simdjson's backslash support (and removed with something like vcompressb on newer AVX-512 chips).