Measure RGBDS performance

[ISSOtm]

Some people have been complaining about RGBDS performance being subpar. With the growing complexity of the codebase, particularly the increasing amount of features—especially for RGBASM—, I am worried about how taxing a given change may be on the overall performance.

This is split in two sub-problems:

Profiling

To know how slow the programs currently are, we should identify their processing bottlenecks. I did that with perf once, but the data was fairly lackluster beyond "60% of your time is spent inside yyparse". Maybe gprof would be better, or something else?

• Decide what profiler(s) to use.
• Pick codebases to profile on.
• Interpret data, identify improvements.
• DO IT

Measuring the performance impact of changes

• Decide on a strategy to follow regarding breaking changes
• Pick codebase(s) to measure on
• Set up measurement script
• Integrate with CI (?)
• Measure whether mmap is actually worth it, in several scenarios (macro-heavy vs. largely linear, for ex.)

[Rangi42]

For reference, gprof output after running current master rgbasm on pokecrystal's main.asm (which produces a 5 MB main.o file): https://pastebin.com/F3n5vfA6

[Rangi42]

gprof is probably not a good choice: https://stackoverflow.com/a/1779343

Valgrind could be useful, though not on Windows or Cygwin.

[daid]

gprof can point to hotspots, however, it has all kinds of problems with the optimizer and static functions. Quite likely the fstk_Init count is actually a static function that is somewhere after it in the binary.
Disabling the compiler optimizations isn't really an option, as that makes the measurement invalid.
And it looks like gprof is spending a lot of time in it's own accounting function (_mcount_private), which makes any time based result invalid.

[Rangi42]

valgrind --tool=callgrind --dump-instr=yes --simulate-cache=yes --collect-jumps=yes ./rgbasm -o main.o main.asm produces accurate-looking results.

Maybe peekInternal could be optimized for the common peek(0) and peek(1) cases, and for when nothing is getting expanded further yet.

[daid]

I think peekInternal is also reading from disk, so I would read from tmpfs to make sure it's not some kind of disk performance thing you are seeing here.

[Rangi42]

Also regarding performance, look into whether mmap is actually an improvement, as noted in #557.

[ISSOtm]

According to perf annotate on pokecrystal's main.asm file, 31% of the CPU time is spent copying the yylval type, which is 264 bytes large (0x108). Moving to variable-size strings (#650) should help reduce this; I think the next larger member of the %union is the struct Expression, but that's already much shorter.

[Rangi42]

The struct Expression copying could also be improved by arranging its members from largest to smallest for better packing; same for any other widely-copied structs. (Unfortunately this can mean losing some meaningful order to them, but unlike Rust, the compiler won't optimize it for you.)