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Comment by yvdriess | Hacker News Reader

Finger crossed then that Azule can get a proper moving GC to work with LLVM then, until then a CL on LLVM (even Julia) are stuck with kinda bad GC.

I wonder why start from scratch for an LLVM backend, can't SBCL be used to generate LLVM code?

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SBCL doesn't generate LLVM code. My primary goal was Common Lisp with C++ interoperation. It seemed easier at the time to start from the ECL Common Lisp code base and write a new C++ core and a Common Lisp interpreter that always interoperated with C++. As I wrote the compiler and expanded the system I maintained C++ interoperation all the time. There were a hundred problems that I had to discover and solve along the way to maintain a Common Lisp that interoperated with C++. You can get some level of interoperation between ECL and C++ up in a weekend but it won't work with C++ exceptions and RAII and there are dozens of other problems. In retrospect I don't think I would have gotten here starting from ECL because I never really understood the ECL code.

It wasn't from scratch, it was from ECL. And did you mean Azul?

Is the compacting garbage collector mentioned in the article not a proper moving GC? I am not familiar with it.

Yes, the Memory Pool System by Ravenbrook (https://www.ravenbrook.com/project/mps/) is a proper, moving garbage collector. It uses precise GC on the heap and conservative GC on the stack, as does the garbage collector in Steel Bank Common Lisp on x86 chips. I need it because I need my code to run on 100,000 CPU supercomputers with a controlled memory footprint to develop organic nano machines (seriously).

To be honest I was just kind of thinking "oh great another language implementation," before I read what you're actually doing and why you needed to create clasp. I appreciate the difficulty of writing precisely GC'd C/C++. It's pretty awesome that you were able to use clang to (I assume this is mainly what the analyzer does) track roots in C & C++ code.

Best of luck.

Thanks - yes the analyzer tracks roots through about 300 C++ classes. It also finds global variables and builds C++ code to interface with the MPS library. I exposed the Clang libraries to search the AST and describe the AST in Common Lisp and then wrote the static analyzer in Lisp. I shudder at the thought of doing this all in C++ and I write a lot of complicated stuff like Common Lisp implementations in C++ :-). Common Lisp is the language of trees and pattern recognition. Common Lisp is the perfect tool for this job.

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