CTFE JIT

[bearophile]

If CTFE gets used a lot, for example to parse a piece of D code at compile-time (as I am trying to do now), or it's used a lot, it can slow down the whole compilation. To solve this some kind of Just In Time compilation can be used to compile CTFE code and speed it up a lot.

There are several ways to do this. For example LLVM is designed to allow the creation of JIT compilers too, so LDC can use LLVM to JIT the CTFE code. But simpler solutions can be found, that can be used with DMD too.

DMD can keep an extra size_t state for each function run at compile-time. This word keeps it total running time in microseconds, summed for all the calls, of this function (this word is never present in the final binary). When this value for a function becomes bigger (for example) than 0.3 seconds, dmd can use itself to compile the function, so successive calls to it are run by the compiled version of the function.

The disadvantage of such simple solution is that a function that takes a lot to run the first time it is called will not be compiled. But I think this can be acceptable in many situations.

I think DMD can generally compile a CTFE function in less than 0.5 seconds. A possible optimization: I think compiling few functions doesn't take much more time than compiling just one, so when one of such "nested compilations" gets triggered, then CTFE functions that have more than 0.2 run time too can be compiled in the same block of code.

Bye,
bearophile