table.concat is idiomatic and should be the fastest way to concatenate all table array elements together, but apparently you can beat it by using string.format, string.rep and table.unpack:
... this just won't do, so we should fix table.concat performance.
The deficit comes from two places:
rawgeti overhead followed by other stack accesses, all to extract a string from what is almost always an in-bounds array lookup
addlstring overhead in case separator is empty (extra function calls)
This change fixes this by using a fast path for in-bounds array lookup for a string. Note that table.concat also supports numbers (these need to be converted to strings which is a little cumbersome and has innate overhead), and out-of-bounds accesses*. In these cases we fall back to the old implementation.
To trigger out-of-bounds accesses, you need to skip the past-array-end element (which is nil per array invariant), but this is achievable because table.concat supports offset+length arguments. This should almost never come up in practice but the per-element branches et al are fairly cheap compared to the eventual string copy/alloc anyway.
This change makes table.concat ~2x faster when the separator is empty; the table.concat benchmark shows +40% gains but it uses a variety of string separators of different lengths so it doesn't get the full benefit from this change.
table.concat is idiomatic and should be the fastest way to concatenate all table array elements together, but apparently you can beat it by using
string.format,string.repandtable.unpack:... this just won't do, so we should fix table.concat performance.
The deficit comes from two places:
This change fixes this by using a fast path for in-bounds array lookup for a string. Note that
table.concatalso supports numbers (these need to be converted to strings which is a little cumbersome and has innate overhead), and out-of-bounds accesses*. In these cases we fall back to the old implementation.To trigger out-of-bounds accesses, you need to skip the past-array-end element (which is nil per array invariant), but this is achievable because table.concat supports offset+length arguments. This should almost never come up in practice but the per-element branches et al are fairly cheap compared to the eventual string copy/alloc anyway.
This change makes table.concat ~2x faster when the separator is empty; the table.concat benchmark shows +40% gains but it uses a variety of string separators of different lengths so it doesn't get the full benefit from this change.