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SIMD-ized parsing of RFC date

This is a proof-of-concept of SIMD-assisted RFC date parser which is used, for instance, in HTML headers. Sample date in this format:

"Fri, 17 Apr 2015 16:14:11 GMT"

Scalar version

The baseline scalar implementation is derived from great implementation by Leandro A. F. Pereira (@lpereira), who introduced following important optimizations:

  • Translation from both weekday and month names into numbers 0..6/0..11 is done with a switch statement. This is a very clever solution. GCC translates such a switch into binary search algorithm.
  • All numeric conversions and range checking are done with custom, tailored functions. Input validation is done similarly.

This approach is obviously faster than the standard strptime function. See scalar.cpp for more details.

SSE version

SIMD implementation, available in file sse.cpp, does many things in parallel, i.e.:

  • Validation of input: check if constant fields contain valid characters (like spaces, commas, 'GMT' string at the end).
  • Translation from weekday and months names are vectorized; each require a few instructions and just one branch.
  • Most numeric conversion, digits validation, and numbers range checking is done in single step. Only the year requires more scalar code.

SSE with minimal perfect hashing

A much clever approach was developed by Kendall Willets; we discussed this by email, he kindly agreed to share his code.

One of the most difficult part in parsing is conversion from four-character abbreviations of month and weekday into numbers. Kendall observed that this transformation might be done by multiplication by a magic constant and then extraction of a few bits from the result word.

Moreover, hash values are not calculated for whole 32-bit values, but 16-bit subwords. Thanks to that multiplication can be done alongside other multiplications needed to convert from ASCII to numeric values.

Although the conversion itself can be done extremely fast -- approximately 4 cycles per input string! (SSE variant works at 27 cycles) -- the input validation kills the performance.

Known problems

  • My translation of Leandro's code might not be perfect.

How to compile

Type make test to compiler and run detailed unittests of scalar and SSE procedures.

Type make benchmark to build simple benchmark utility.

Performance comparison on Core i5

make benchmark && ./benchmark gives on my old laptop:

rdtsc_overhead set to 92
scalar                          :    77.805 cycle/op (best)   98.557 cycle/op (avg)
SSE                             :    33.979 cycle/op (best)   34.977 cycle/op (avg)

Performance comparison on Skylake

scalar                              :    34.330 cycle/op (best)   35.720 cycle/op (avg)
SSE                                 :    18.480 cycle/op (best)   18.765 cycle/op (avg)
SSE (Kendall)                       :    23.203 cycle/op (best)   23.293 cycle/op (avg)
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