Thread: What is the faster way to add byte before each byte in char *

  1. #1
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    What is the faster way to add byte before each byte in char *

    If I have a char* for example :
    Code:
    {0x01,0x02,0x03,0x04,0x05}
    and I want to add 0x00 before each byte .

    the result will be :
    Code:
    {0x00,0x01,0x00,0x02,0x00,0x03,0x00,0x04,0x00,0x05}
    What is the faster way to do that ?

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    Probably picking up more than one character at a time, which would be processor dependent. In assembly on X86 with xmm registers, you could load 16 bytes at a time, copy to another xmm register, then use pshufb on both xmm registers to insert the zeroes, then store 16 bytes at a time (twice).
    Last edited by rcgldr; 01-03-2019 at 01:13 PM.

  3. #3
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    > What is the faster way to do that ?
    Faster than what?
    You haven't shown us what you're doing at the moment.

    Unless you have some woefully underpowered micro-controller, or needing to do 100's of MB/sec, this looks like a premature optimisation.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    Quote Originally Posted by rcgldr View Post
    Probably picking up more than one character at a time, which would be processor dependent. In assembly on X86 with xmm registers, you could load 16 bytes at a time, copy to another xmm register, then use pshufb on both xmm registers to insert the zeroes, then store 16 bytes at a time (twice).
    For those of us not familiar with assembly, couldn't the same be achieved by memcpy'ing into a uint16_t pointer? Would probably need to account for endianness though.





    EDIT: offtopic, BTW, do u guys know if the forums were under ddos attack? It was hard to access the site the past 24 hours. I was getting this error:What is the faster way to add byte before each byte in char *-screen-shot-2019-01-03-3-37-36-am-jpg
    Last edited by Dren; 01-03-2019 at 07:24 PM.

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    Quote Originally Posted by Dren View Post
    For those of us not familiar with assembly, couldn't the same be achieved by memcpy'ing into a uint16_t pointer? Would probably need to account for endianness though.
    Not as fast. The instruction pshufb will shuffle the bytes and insert 0's in 1 cycle, and the operations on two separate xmm registers occur in parallel.

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