Optimize string concatenation

[vitaut]

Look if we can beat string append with or without reserve on this benchmark: https://godbolt.org/z/EZE7Dn

% ./naive-benchmark
2020-05-15 17:44:56
Running ./naive-benchmark
Run on (8 X 2800 MHz CPU s)
CPU Caches:
  L1 Data 32K (x4)
  L1 Instruction 32K (x4)
  L2 Unified 262K (x4)
  L3 Unified 8388K (x1)
Load Average: 1.50, 1.52, 1.63
------------------------------------------------------------
Benchmark                  Time             CPU   Iterations
------------------------------------------------------------
naive                    137 ns          137 ns      5184417
append                   109 ns          109 ns      6200012
appendWithReserve        117 ns          116 ns      5923469
fmtlib                   185 ns          185 ns      3792887
nullop                 0.220 ns        0.220 ns   1000000000

[vitaut]

format_to+memory_buffer results are similar to appendWithReserve:

% ./naive-benchmark
2020-05-15 17:53:33
Running ./naive-benchmark
Run on (8 X 2800 MHz CPU s)
CPU Caches:
  L1 Data 32K (x4)
  L1 Instruction 32K (x4)
  L2 Unified 262K (x4)
  L3 Unified 8388K (x1)
Load Average: 2.01, 4.79, 3.79
------------------------------------------------------------
Benchmark                  Time             CPU   Iterations
------------------------------------------------------------
naive                    136 ns          136 ns      5073493
append                   110 ns          110 ns      6052327
appendWithReserve        118 ns          118 ns      5823676
fmtlib                   118 ns          118 ns      5977542
nullop                 0.221 ns        0.221 ns   1000000000

[vitaut]

With a small parsing optimization:

------------------------------------------------------------
Benchmark                  Time             CPU   Iterations
------------------------------------------------------------
naive                    133 ns          133 ns      5160377
append                   118 ns          118 ns      6391760
appendWithReserve        121 ns          121 ns      5549698
format                   165 ns          165 ns      4162108
format_to                104 ns          103 ns      6547073
nullop                 0.221 ns        0.221 ns   1000000000

[vitaut]

Second iteration:

------------------------------------------------------------
Benchmark                  Time             CPU   Iterations
------------------------------------------------------------
naive                    135 ns          135 ns      5096209
append                   107 ns          107 ns      6414956
appendWithReserve        111 ns          111 ns      6131745
format                   153 ns          153 ns      4523980
format_to               94.6 ns         94.6 ns      7208840
nullop                 0.218 ns        0.218 ns   1000000000

[vitaut]

fmt::format with format string compilation

auto output = fmt::format(FMT_COMPILE("Result: {}: ({},{},{},{})"), str1, str2, str3, str4, str5);

beats concat even with preallocation, so I guess not much to do here:

------------------------------------------------------------
Benchmark                  Time             CPU   Iterations
------------------------------------------------------------
naive                    137 ns          137 ns      4801855
append                   129 ns          128 ns      6219734
appendWithReserve        117 ns          117 ns      5995461
format_compile           101 ns          101 ns      6953205
format_runtime           161 ns          161 ns      4368884
format_to                103 ns          103 ns      6706715
nullop                 0.230 ns        0.230 ns   1000000000

Might reintroduce small format string optimization to improve format_runtime perf.

[vitaut]

With small optimization:

------------------------------------------------------------
Benchmark                  Time             CPU   Iterations
------------------------------------------------------------
naive                    139 ns          139 ns      4840941
append                   113 ns          113 ns      6207325
appendWithReserve        116 ns          116 ns      6029597
format_compile           108 ns          107 ns      6392344
format_runtime           129 ns          129 ns      5341350
format_to               67.7 ns         67.6 ns     10185967
nullop                 0.233 ns        0.232 ns   1000000000