in reply to Re^4: [OT] The interesting problem of comparing bit-strings.
in thread [OT] The interesting problem of comparing bit-strings.

You cannot search for bits using byte-wise instructions

But the thinkg is that, actually, you can! Only that you will have to run eight searches instead of one.

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Re^6: [OT] The interesting problem of comparing bit-strings.
by BrowserUk (Pope) on Mar 25, 2015 at 11:40 UTC
    But the think thing is that, actually, you can! Only that you will have to run eight searches instead of one.

    Hm. Pedantically, that still searching for bytes, but no matter, it still doesn't solve the problem. What if what you are looking for spans a byte boundary?

    Eg. The needle below appears twice in the haystack, but no combination of 8 (or any number) of repne scasb instructions will ever find either:

    needle: 00100100 haystack: + * * * * 1 2 3 4 5 6 +7 8 9 0 1 2 0123456789012345678901234567890123456789012345678901234567890123456789 +0123456789012345678901234567890123456789012345678901234567 1111000101001010110110101100101011101001010100010111100010011001010100 +0111111111001000000011100100100111011100100100111000110111

    Because, repne scasb scans the string looking for a fixed byte value, but the needle doesn't appear as a byte anywhere in the string.

    Both occurrences span byte boundaries, thus to find them, you need to do two compares, using two different masks on two adjacent bytes, against two different byte values, neither of which are or include the needle.

    Nor can you pad the needle (on both ends) to 16-bits and use repne scabw to find it, because again, a masking operation is required to extract the relevant 7 bits from the surrounding 16.

    The same is true (and then some) for my use of 64-bit comparisons, but I didn't suggest I was going to microcode loop instructions.

    So, I'll say it again, with a minor addition, that (I hope) will satisfy you; and will go right over his head:

    You cannot search for bits using byte-wise instructions alone

    What he pontificated about was utterly pointless; and wrong.

    And that before we get into:

    1. I'm coding in C not assembler.

      And my compiler doesn't support inline assembler.

    2. the whole guff about: "If you actually discover a matching 64-bit quantity within the buffer, then the odds are 64 in 2^64 that the entire value-being-sought is present".

      We already know how completely screwed his sens eof probabilities are: The odds of an actual collision for a 7-digit random number are so astronomically small that I quite frankly would not bother to check for it.

      I mentioned that my haystacks are frequently >1/4GB in size"; that's 33 million quads; the chances that two the same appear somewhere are (by the Birthday Paradox) not so slim in totally random data; but in structured, correlated, meaningful data, quite likely to be much much higher.

      And that's before you consider that the patterns I'll be searching for are not quad-aligned, thus every bit in that 1/4GB (bar the first and last 63 bit) is actually a part of 63 (potentially) entirely different quads. Ie. that 33 million just became 2.1 billion.

    Beyond that, using bytes is a bad idea for this in any number of ways:

    • Byte aligned data is the most expensive size to load/store for a 64 bit processor.

      Effectively 64-bits is moved across the data bus, but only 8-bits are used. Even reading from cache, every byte except the low byte of each quad has to be manipulated into the low 8 bits of a register before it can be used.

    • <l>Byte operators effectively waste 7/8ths of each register used to hold the byte.

      Most of them use AL or CL leaving the rest of the register useless for the duration.

    If it is of interest, this is my currently incomplete code (a few edge cases to handle) locating a 1Mi bit needle near the end of a 1GiB buffer of random bits:

    C:\test\C>bvSearch 1745337664 1048576 Found it at 67108800 Took 0.255063793859 secs

    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority". I'm with torvalds on this
    In the absence of evidence, opinion is indistinguishable from prejudice. Agile (and TDD) debunked
      The idea is that for long needles, you can discard their borders (the unaligned bits), search for the middle string using some efficient algorithm and only when it matches, check the borders.

      update: oh, and BTW, I was not endorsing sundialsvc4 response, just stating that this time, besides all his typical non-sense there may actually be also some insight.

        The idea is that for long needles, you can discard their borders (the unaligned bits), search for the middle string using some efficient algorithm and only when it matches, check the borders

        I get that. But even then, there is not "the middle bit", but rather 'eight middle bits' (and upto 7 different unaligned bits, at either end):

        haystack: + * * * * 1 2 3 4 5 6 +7 8 9 0 1 2 0123456789012345678901234567890123456789012345678901234567890123456789 +0123456789012345678901234567890123456789012345678901234567 1111000101001010110110101100101011101001010100010111100010011001010100 +0111111111001000000011100100100111011100100100111000110111 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm bbbbbbbmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmme bbbbbbmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmee bbbbbmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmeee bbbbmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmeeee bbbmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmeeeee bbmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmeeeeee bmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmeeeeee +e mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm +m bbbbbbbmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm +me bbbbbbmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm +mee bbbbbmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm +meee bbbbmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm +meeee bbbmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm +meeeee bbmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm +meeeeee bmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm +meeeeeee mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm +mmmmmmmmm ...

        Note that the "aligned middle bits" are different every bit offset, cycling every 8 bits.

        Of course the same problem occurs with 64-bit units, but rotating the 64-bit units using 64, single instruction, 128-bit shiftlefts, is far more efficient than doing 64 x 8 x 16-bit shifts to achieve the same thing.


        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority". I'm with torvalds on this
        In the absence of evidence, opinion is indistinguishable from prejudice. Agile (and TDD) debunked
        A reply falls below the community's threshold of quality. You may see it by logging in.
      locating a 1Mi bit needle near the end of a 1GiB buffer of random bits

      Have you considered that using random test data may not be a good idea. You are never going to generate the performance-degrading worst cases.

      For instance, it is very unlikely that you will find partial needle matches longer than a few bits.

        Have you considered that using random test data may not be a good idea.

        I'm not really at the testing stage.

        Still trying to cover off the edge cases without upsetting the compiler's ability to maintain the important values in registers, to avoid reloading values from memory.

        The difference in performance between /Od (none) and /Ox (full) is currently over an order of magnitude -- about as big a difference as I ever recall seeing -- and I think I might be able to get two orders if I can move some of the conditional stuff out of the critical path; but it is all too easy to upset things badly by adding an extra test or two. (And I need an extra condition or two :()

        For instance, it is very unlikely that you will find partial needle matches longer than a few bits.

        Currently I'm generating a haystack of random quads and then extracting a needle of a specified size and (0..62) bit-offset from within that, somewhere close to the end.

        This means I not only know that the needle exists, but where it should be found. More importantly, if it isn't found, then I can quickly detect patterns in the combinations of parameters that cause it to fail.

        For example, it currently handles leading bit offsets, but not non-multiples of 64-bits. And that's because I tried (unsuccessfully) to move the terminating condition out of the main path of the code.


        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority". I'm with torvalds on this
        In the absence of evidence, opinion is indistinguishable from prejudice. Agile (and TDD) debunked