Re: [Lightning] Register liveness analysis and spilling

[Basile Starynkevitch]

On 09/03/2017 05:18 PM, Marc Nieper-Wißkirchen wrote:
> When generating lightning instructions from higher level code, scratch 
> registers are sometimes needed (for example, to code a move from 
> memory to memory).
>
> The generating code could analyze which registers do not contain live 
> data and use those as scratch registers (if there are none, it has to 
> spill some registers).
>
> Browsing through the lighting source code it seems that lightning 
> already contains some liveness analysis and some spilling algorithm. 
> It would be great if some interface to this could be offered to user 
> code, for example by offering two "virtual registers" that are 
> emulated by overlaying them to real registers (that are either dead or 
> spilled during the live time of the virtual registers).
>
> On some ports, e.g. the AMD64 port, these virtual registers could, in 
> fact, be real registers like RSI or RDI most of the time, which are 
> usually only used for arguments to C procedure calls, and are wasted 
> otherwise.

If you want real (and costly) optimizations, GNU lightning is perhaps 
not the best JIT library. In that case, consider instead libgccjit (see 
https://gcc.gnu.org/onlinedocs/jit/ for more) or LLVM (see 
http://llvm.org/ ...). Of course, the generated code would run faster, 
but the generation time (actually some compilation) would be slower. 
Alternatively, emit C code into a temporary file, compile it into some 
plugin, then use plugin loading facilities like dlopen (on POSIX).

Cheers

--
Basile STARYNKEVITCH   == http://starynkevitch.net/Basile
opinions are mine only - les opinions sont seulement miennes
Bourg La Reine, France