Precompute embedded string literals hash code #10596
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mame
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Apr 22, 2024
string.c
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| RUBY_ASSERT(free_bytes >= sizeof(st_index_t)); | ||
| #endif | ||
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| *(st_index_t *)(RSTRING_END(str) + TERM_LEN(str)) = rb_str_hash(str); |
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I am afraid if this may cause a bus error on some platform. Is it safe?
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this may cause a bus error on some platform
Because of alignment? If it's a concern, I think we could ensure it's always aligned, the overhead would be very slightly larger.
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Yes > because of alignment
It would be safer to use aligned address if possible.
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Thanks, I'll look into it.
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Suggested change
| *(st_index_t *)(RSTRING_END(str) + TERM_LEN(str)) = rb_str_hash(str); | |
| typedef struct {char bytes[sizeof(st_index_t)];} unaligned_index; | |
| union {st_index_t i; unaligned_index b;} u = {.i = rb_str_hash(str)}; | |
| *(unaligned_index *)(RSTRING_END(str) + TERM_LEN(str)) = u.b; |
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Thank @nobu, interesting trick. I suppose we need it on read as well?
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Looks like we broke something after the refactor, I'll investigate. |
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| if (FL_TEST_RAW(str, STR_PRECOMPUTED_HASH)) { | ||
| real_size += sizeof(st_index_t); | ||
| } |
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This was the bug. The compactor thought the string could fit in a smaller slot, corrupting the precomputed hash.
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Alright, I think the bugs have been squashed now. |
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With embedded strings we often have some space left in the slot, which we can use to store the string Hash code. It's probably only worth it for string literals, as they are the ones likely to be used as hash keys. We chose to store the Hash code right after the string terminator as to make it easy/fast to compute, and not require one more union in RString. ``` compare-ruby: ruby 3.4.0dev (2024-04-22T06:32:21Z main f77618c) [arm64-darwin23] built-ruby: ruby 3.4.0dev (2024-04-22T10:13:03Z interned-string-ha.. 8a1a32331b) [arm64-darwin23] last_commit=Precompute embedded string literals hash code | |compare-ruby|built-ruby| |:-----------|-----------:|---------:| |symbol | 39.275M| 39.753M| | | -| 1.01x| |dyn_symbol | 37.348M| 37.704M| | | -| 1.01x| |small_lit | 29.514M| 33.948M| | | -| 1.15x| |frozen_lit | 27.180M| 33.056M| | | -| 1.22x| |iseq_lit | 27.391M| 32.242M| | | -| 1.18x| ``` Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>
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This was referenced Nov 4, 2024
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[Feature #20415]
With embedded strings we often have some space left in the slot, which we can use to store the string Hash code.
It's probably only worth it for string literals, as they are the ones likely to be used as hash keys.
We chose to store the Hash code right after the string terminator as to make it easy/fast to compute, and not require one more union in RString.