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Re: [Lightning] Register liveness analysis and spilling

From: Basile Starynkevitch
Subject: Re: [Lightning] Register liveness analysis and spilling
Date: Sun, 3 Sep 2017 18:52:32 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0

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 for more) or LLVM (see ...). 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).


opinions are mine only - les opinions sont seulement miennes
Bourg La Reine, France

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