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Speed up String encoding on CPUs without SIMD hardware support #812

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radiospiel wants to merge 1 commit into from
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Speed up String encoding on CPUs without SIMD hardware support #812

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17 changes: 17 additions & 0 deletions ext/json/ext/generator/generator.c
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Original file line number Diff line number Diff line change
Expand Up @@ -162,8 +162,25 @@ static const unsigned char escape_table_basic[256] = {

static unsigned char (*search_escape_basic_impl)(search_state *);

inline bool has_json_escapable_byte(uint64_t x) {
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@byroot byroot Jun 1, 2025

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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) {

uint64_t is_ascii = 0x8080808080808080ULL & ~x;
uint64_t xor2 = x ^ 0x0202020202020202ULL;
uint64_t lt32_or_eq34 = xor2 - 0x2121212121212121ULL;
uint64_t sub92 = x ^ 0x5C5C5C5C5C5C5C5CULL;
uint64_t eq92 = (sub92 - 0x0101010101010101ULL);
return ((lt32_or_eq34 | eq92) & is_ascii) != 0;
}

static inline unsigned char search_escape_basic(search_state *search)
{
while (search->ptr <= search->end - 8) {
uint64_t* pi = (uint64_t*)(search->ptr);
if(has_json_escapable_byte(*pi)) {
Comment on lines +177 to +178
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@byroot byroot Jun 1, 2025

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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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@byroot byroot Jun 1, 2025

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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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@byroot byroot Jun 1, 2025

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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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@radiospiel radiospiel Jun 20, 2025

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This PR comes from my curiosity how far I could take the C code without specialized hardware, and then wanted to share the results. But I share the hesitancy to merge.

Thanks for the review, closing this now.

break;
}
search->ptr += 8;
}

while (search->ptr < search->end) {
if (RB_UNLIKELY(escape_table_basic[(const unsigned char)*search->ptr])) {
search_flush(search);
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