Speed up the branchless UTF-8 decoder by removing !len #7
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In your post, you say: "Adding that !len is actually somewhat costly, though I couldn’t figure out why." My suspicion was that it is because the "!" operator would essentially behave like a branch, returning 1 if the input is 0 and 0 otherwise. So, my idea was to copy the table of lengths you have and create another one for "error lengths" to get that same effect (0 when it's OK and 1 when there is an error, to ensure that it moves forward at least one byte, as mentioned). The throughput went up from 504 MB/s to 557 MB/s on my machine.
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For what it's worth, I actually see the speed drop from |
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In your post, you say: "Adding that !len is actually somewhat costly,
though I couldn’t figure out why."
My suspicion was that it is because the "!" operator would essentially
behave like a branch, returning 1 if the input is 0 and 0 otherwise.
So, my idea was to copy the table of lengths you have and create another
one for "error lengths" to get that same effect (0 when it's OK and 1
when there is an error, to ensure that it moves forward at least one
byte, as mentioned).
The throughput went up from 504 MB/s to 557 MB/s on my machine.