Speed up String encoding on CPUs without SIMD hardware support #812
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This change tests for the presence of characters to be escaped on CPUs without special SIMD instructions. We are testing 8 chars in one round, following a code excerpt from https://lemire.me/blog/2025/04/13/detect-control-characters-quotes-and-backslashes-efficiently-using-swar/ For strings that don't require escapes I have seen speedups of around 15% with this change.
byroot
reviewed
Jun 1, 2025
| @@ -162,8 +162,25 @@ static const unsigned char escape_table_basic[256] = { | |||
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| static unsigned char (*search_escape_basic_impl)(search_state *); | |||
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| inline bool has_json_escapable_byte(uint64_t x) { | |||
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Suggested change
| inline bool has_json_escapable_byte(uint64_t x) { | |
| static inline bool has_json_escapable_byte(uint64_t x) { |
byroot
reviewed
Jun 1, 2025
Comment on lines
+177
to
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| uint64_t* pi = (uint64_t*)(search->ptr); | ||
| if(has_json_escapable_byte(*pi)) { |
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Suggested change
| uint64_t* pi = (uint64_t*)(search->ptr); | |
| if(has_json_escapable_byte(*pi)) { | |
| uint64_t *pi = (uint64_t *)search->ptr; | |
| if (has_json_escapable_byte(*pi)) { |
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Not a huge deal, but rather than a function that return a boolean and fallback to the slow path, we could use the same structure as SIMD codepaths (return a mask that gives us the position of characters to escape).
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That being said, I'm not sure if this PR is really worth it, as we kinda assume the overwhelming majority of users will have SIMD available. So it's questionable whether complexifying the fallback implementation bring any benefit.
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This change tests for the presence of characters to be escaped on CPUs without special SIMD instructions. We are testing 8 chars in one round, following a code excerpt from https://lemire.me/blog/2025/04/13/detect-control-characters-quotes-and-backslashes-efficiently-using-swar/
For strings that don't require escapes I have seen speedups of around 15% with this change.