[flang][runtime] Use dumber but faster check for list-directed repeti… #159867
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eugeneepshteyn
approved these changes
Sep 21, 2025
…tion When scanning list-directed input for nulls and repetition counts, the current library depends on having each record be prescanned for the presence of asterisk characters. It turns out that the overhead of calling memchr(...,'*',...) on each record doesn't pay off, especially on systems without SIMD-vectorized memchr implementations -- even on those, it's faster (about 10%) to just scan ahead for asterisks when decimal digits are encountered. Only when an asterisk is present, which is not common, should we then bother to convert the digits to their integer value.
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akuhlens
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Sep 22, 2025
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…tion
When scanning list-directed input for nulls and repetition counts, the current library depends on having each record be prescanned for the presence of asterisk characters. It turns out that the overhead of calling memchr(...,'*',...) on each record doesn't pay off, especially on systems without SIMD-vectorized memchr implementations -- even on those, it's faster (about 10%) to just scan ahead for asterisks when decimal digits are encountered. Only when an asterisk is present, which is not common, should we then bother to convert the digits to their integer value.