Improve performance of fmt::format on integer formatting

[vitaut]

Current results:

['Method'                      , 'Time, s' , 'Time ratio' ],
['fmt::FormatInt'              ,   0.130366,           1.0],
['fmt::Writer'                 ,    0.15293, 1.17308193854],
['cppx::decimal_from'          ,   0.158245, 1.21385177117],
['karma::generate'             ,   0.163913, 1.25732936502],
['karma::generate+std::string' ,   0.317622, 2.43638678797],
['fmt::Writer+std::string'     ,   0.327253, 2.51026341224],
['ltoa'                        ,   0.489808, 3.75717595078],
['itostr'                      ,   0.615213,  4.7191215501],
['sprintf'                     ,   0.883171, 6.77455011276],
['fmt::format'                 ,   0.956393, 7.33621496402], <---
['boost::lexical_cast'         ,   0.994558, 7.62896767562],
['sprintf+std::string'         ,    1.02793, 7.88495466609],
['std::to_string'              ,    1.34569, 10.3224000123],
['std::stringstream'           ,    1.38597, 10.6313762791],
['boost::format'               ,    4.07773, 31.2790911741]

For comparison, here are results from 1b80148:

['Method'                      , 'Time, s' , 'Time ratio' ],
['fmt::FormatInt'              ,   0.141162,           1.0],
['karma::generate'             ,   0.162126,  1.1485102223],
['cppx::decimal_from'          ,   0.167754, 1.18837930888],
['fmt::Writer'                 ,   0.192687, 1.36500616313],
['karma::generate+std::string' ,   0.321725, 2.27911902637],
['fmt::Writer+std::string'     ,   0.387114, 2.74233858971],
['fmt::format'                 ,   0.462457,  3.2760728808], <---
['ltoa'                        ,   0.487219, 3.45148836089],
['itostr'                      ,   0.665112, 4.71169294853],
['sprintf'                     ,   0.894235, 6.33481390176],
['boost::lexical_cast'         ,    1.04254, 7.38541533841],
['sprintf+std::string'         ,    1.04865, 7.42869894164],
['std::stringstream'           ,    1.27094, 9.00341451665],
['std::to_string'              ,    1.34653, 9.53889857044],
['boost::format'               ,    4.40832, 31.2288009521]

[patlecat]

Cool now you beat even Karma and C, impressive 👯

[vitaut]

Thanks! It's been for a while already, but some regression crept in.

[vitaut]

Change in performance is caused by making fmt::format return std::string in a126492. Constructing string objects takes about the same time as formatting.

[vitaut]

The problem with the int_generator benchmark is that some tests construct temporary strings (std::to_string, std::stringstream, fmt::format) and some don't (sprintf+std::string, karma::generate+std::string) which makes comparison meaningless. Need to fix this.

[vitaut]

Fixed the benchmark in fmtlib/format-benchmark@bd23f14. Updated results:

['Method'                      , 'Time, s' , 'Time ratio' ],
['fmt::FormatInt'              ,   0.147907,           1.0],
['karma::generate'             ,   0.167082, 1.12964227521],
['fmt::Writer'                 ,   0.168074,  1.1363491924],
['cppx::decimal_from'          ,   0.170386, 1.15198063648],
['ltoa'                        ,   0.487298, 3.29462432474],
['itostr'                      ,   0.664632, 4.49358042554],
['fmt::Writer+std::string'     ,   0.694602, 4.69620775217],
['karma::generate+std::string' ,   0.735435, 4.97227987857],
['sprintf'                     ,   0.884361,  5.9791693429],
['boost::lexical_cast'         ,   0.967354, 6.54028544964],
['fmt::format'                 ,    1.09314, 7.39072525303],
['std::stringstream'           ,    1.21641, 8.22415436727],
['std::to_string'              ,    1.31157,  8.8675316246],
['sprintf+std::string'         ,     1.3987, 9.45661800997],
['boost::format'               ,    4.19163, 28.3396323365]

fmt::format's performance can be improved a bit bringing it back to sprintf level. However, std::string construction is a limiting factor as can be seen from the results above - it adds about half a second to the run time for each method. So for better performance one should avoid std::string-based methods and use fmt::Writer or fmt::FormatInt.

[vitaut]

Corrected results after fixing an std::string copying issue in fmtlib/format-benchmark@fc4cd13:

['Method'                      , 'Time, s' , 'Time ratio' ],
['fmt::FormatInt'              ,   0.131329,           1.0],
['fmt::Writer'                 ,   0.151219, 1.15145169764],
['karma::generate'             ,   0.161277, 1.22803798095],
['cppx::decimal_from'          ,    0.16367, 1.24625939435],
['ltoa'                        ,   0.488084, 3.71649826009],
['itostr'                      ,   0.575497,  4.3821014399],
['karma::generate+std::string' ,   0.649735, 4.94738405074],
['fmt::Writer+std::string'     ,   0.665199, 5.06513412879],
['sprintf'                     ,   0.883342, 6.72617624439],
['boost::lexical_cast'         ,   0.891263,  6.7864904172],
['fmt::format'                 ,   0.988134, 7.52411120164],
['std::stringstream'           ,    1.22015, 9.29078878237],
['std::to_string'              ,    1.28301, 9.76943401686],
['sprintf+std::string'         ,    1.35638, 10.3281072726],
['boost::format'               ,    4.12666, 31.4223058121]

[vitaut]

One possibility to improve fmt::format performance is to format directly to std::string which is now possible because buffer management is decoupled from writers.

[vitaut]

The current numbers (on std) look pretty good:

['Method'                      , 'Time, s' , 'Time ratio' ],
['fmt::FormatInt'              ,   0.133837,           1.0],
['cppx::decimal_from'          ,   0.150949, 1.12785702011],
['fmt::basic_writer'           ,   0.151999, 1.13570238424],
['karma::generate'             ,   0.167353, 1.25042402325],
['itostr'                      ,   0.242164, 1.80939501035],
['fmt::basic_writer+std::string',   0.280116, 2.09296382914],
['karma::generate+std::string' ,   0.304879, 2.27798740259],
['boost::lexical_cast'         ,   0.454893, 3.39885831272],
['ltoa'                        ,   0.490139, 3.66220850736],
['fmt::format'                 ,    0.55959, 4.18113077848],
['sprintf'                     ,   0.943337, 7.04840216084],
['sprintf+std::string'         ,    1.07682, 8.04575715236],
['std::to_string'              ,    1.26133, 9.42437442561],
['std::stringstream'           ,     2.2325, 16.6807385103],
['boost::format'               ,    4.28298,  32.001464468],

Formatting directly to std::string may actually result in more re-allocations so probably not worth it.