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strcase: some optimisations. #8875

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strcase: some optimisations. #8875

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gevaerts
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@gevaerts gevaerts commented May 18, 2022

Lookup tables for toupper() and tolower() make Curl_strcasecompare()
about 1.5 times faster. Reorganising Curl_strcasecompare() to fully
exit early then also allows simplifying the check at the end, for
another 15%. In total, the changes make Curl_strcasecompare() around
1.6 to 1.7 times faster.

Note that these optimisation assume ASCII. The original Curl_raw_toupper()
and raw_tolower() look like they already made that assumption, but confirmation would be good.

For short strings (I tested 4 to 8 characters) this is competitive with glibc, with this code being slightly faster on some CPUs and some lengths, and glibc being slightly faster on others. For longer strings glibc starts getting significantly faster (but the locale issue of course remains)

Lookup tables for toupper() and tolower() make Curl_strcasecompare()
about 1.5 times faster. Reorganising Curl_strcasecompare() to fully
exit early then also allows simplifying the check at the end, for
another 15%. In total, the changes make Curl_strcasecompare() around
1.6 to 1.7 times faster.

Note that these optimisation assume ASCII. The original Curl_raw_toupper()
and raw_tolower() look like they already made that assumption.
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@bagder bagder commented May 18, 2022

Confirmed: we only (need to) support ASCII with these.

bagder
bagder approved these changes May 18, 2022
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@bagder bagder commented May 19, 2022

Thanks!

@bagder bagder closed this in b12c2b9 May 19, 2022
@gevaerts gevaerts deleted the strcasecmp branch May 19, 2022
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