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Kevin Newton
c0e41097b0
Apple's libc implementation of memmem is super slow (it is a forked version of freebsd's that never got vectorized). Instead, we should fall back to the rolling hash on Apple. In the attached benchmark, I'm seeing 1.07% slower to 30.34% slower, depending on the haystack. For reference, here are the various implementations I checked: * musl: https://git.musl-libc.org/cgit/musl/tree/src/string/memmem.c * freebsd: https://github.com/freebsd/freebsd-src/blob/main/lib/libc/string/memmem.c * apple: https://github.com/apple-oss-distributions/Libc/blob/main/string/FreeBSD/memmem.c You can see Apple just linearly searches through the string and calls memcmp each time, whereas the other two do a window'd rolling hash similar to the fallback Ruby already has.
76 lines
2.2 KiB
YAML
76 lines
2.2 KiB
YAML
prelude: |
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# Haystacks of various sizes
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small_hay = "a" * 256
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medium_hay = "a" * 4096
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large_hay = "a" * 65536
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# Short needles (2-8 bytes) that exercise rb_memsearch_ss
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needle_2 = "xy"
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needle_4 = "xyzw"
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needle_8 = "xyzwabcd"
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# Needle whose first byte is absent from the haystack (memchr fast-path)
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# vs needle whose first byte is common (rolling hash comparison)
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first_byte_absent = "x" + "a" * 3
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first_byte_common = "a" + "x" * 3
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# Haystack with match at the end
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hay_match_end = "a" * 4095 + "xy"
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# Haystack with match at the start
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hay_match_start = "xy" + "a" * 4094
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# Mixed content haystack (more realistic)
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mixed_hay = (("abcdefghij" * 100) + "z") * 10
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benchmark:
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# === First byte absent from haystack (biggest win for rolling hash) ===
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index_first_byte_absent_small: |
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small_hay.index(first_byte_absent)
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index_first_byte_absent_medium: |
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medium_hay.index(first_byte_absent)
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index_first_byte_absent_large: |
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large_hay.index(first_byte_absent)
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# === First byte common in haystack (stresses comparison loop) ===
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index_first_byte_common_small: |
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small_hay.index(first_byte_common)
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index_first_byte_common_medium: |
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medium_hay.index(first_byte_common)
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index_first_byte_common_large: |
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large_hay.index(first_byte_common)
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# === Needle length variations (all absent) ===
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index_needle_2_absent: |
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medium_hay.index(needle_2)
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index_needle_4_absent: |
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medium_hay.index(needle_4)
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index_needle_8_absent: |
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medium_hay.index(needle_8)
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# === Match at end of haystack ===
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index_match_at_end: |
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hay_match_end.index(needle_2)
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# === Match at start of haystack ===
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index_match_at_start: |
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hay_match_start.index(needle_2)
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# === include? (same code path) ===
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include_first_byte_absent: |
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medium_hay.include?(first_byte_absent)
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include_first_byte_common: |
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medium_hay.include?(first_byte_common)
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# === byteindex ===
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byteindex_first_byte_absent: |
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medium_hay.byteindex(first_byte_absent)
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byteindex_first_byte_common: |
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medium_hay.byteindex(first_byte_common)
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# === Mixed/realistic haystack ===
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index_mixed_absent: |
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mixed_hay.index(needle_4)
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index_mixed_present: |
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mixed_hay.index("ijab")
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