Add ref-counted string slices #1175
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Instead of copying long substring slices, store a reference to and offset into the parent string. Gets more profitable as strings get longer. For 64k string slices, it's about 6.5x faster than copying. The downside of ref-counted string slices is that they can keep the parent string alive for longer than it otherwise would have been, leading to memory usage that is higher than without string slices. That's why this optimization is only applied to long-ish slices, currently slices > 1,024 bytes. Possible future enhancements are slicing only when the substring is > x% of the parent string, or copying lazily when the slice string is the only thing referencing the parent string.
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saghul
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Sep 30, 2025
bnoordhuis
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This change seems to have introduced a seemingly impossible bug in a real-world code base where values flip to JS_NULL for no discernible reason. My best guess is something manipulating string data directly, without being aware of slice strings, but it's impossible to track down where. The weird thing is that the value flipping to null isn't even a string, it appears to be an array. This reverts commit 08db51a. This reverts commit 62b4eed. Fixes: quickjs-ng#1178
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fixup: 08db51a ("Add ref-counted string slices", quickjs-ng#1175)
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Instead of copying long substring slices, store a reference to and offset into the parent string.
Gets more profitable as strings get longer. For 64k string slices, it's about 6.5x faster than copying.
The downside of ref-counted string slices is that they can keep the parent string alive for longer than it otherwise would have been, leading to memory usage that is higher than without string slices. That's why this optimization is only applied to long-ish slices, currently slices > 1,024 bytes.
Possible future enhancements are slicing only when the substring is > x% of the parent string, or copying lazily when the slice string is the only thing referencing the parent string.
edit: forgot to mention, the next step is adding cons/rope strings