[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Ideal performance of ELisp (was: Why tree-sitter instead of Semantic

From: Lynn Winebarger
Subject: Re: Ideal performance of ELisp (was: Why tree-sitter instead of Semantic? (was Re: CC Mode with font-lock-maximum-decoration 2))
Date: Fri, 12 Aug 2022 19:26:14 -0400

On Fri, Aug 12, 2022, 5:50 PM Stefan Monnier <monnier@iro.umontreal.ca> wrote:
>> > I don't have this information.  Maybe someone else does.  But in
>> > general, it is a very small wonder that a parser written in optimized
>> > C is much faster than anything written in Emacs Lisp, given that Lisp
>> > is an interpreted language that has no special support for writing
>> > parsers.
>> That can be cured over time, now that the bulk of the core of emacs
>> uses lexical scoping.
> I very much doubt that.


I noted the switch to lexical scope, because dynamic scope prevents any call from ever being a tail call in any meaningful sense.  It's not automatic, but when the core of Emacs lisp code used dynamic scoping, changes to the VM to support proper tail recursion would have been meaningless.  That is no longer the case.
Once the VM supports proper tail recursion, it's straightforward to generate automata that never perform a function call, at least not as part of the recognizer.  Lisp with proper tail recursion gives you the equivalent of computed goto's, which are not available in standards-conformant C AFAIK.  If the compiler can prove certain local variables are always fixnums and remove the tag/untag overhead, then the generated IR passed to libgccjit (or clang) should be able to match anything expressible in C for those automata.
The code in the actions may not be as optimizable as the recognition automata, I'll grant you.

> ELisp code cannot match the speed of optimized C code.

I suspect it could, to some extent, in theory.  Getting there would
require a fair bit more work, probably using a different compilation
strategy than the AOT compiler we have now.

A good reference is _javascript_ which shouldn't be noticeably easier to
compile efficiently and where many millions of dollars have been poured
over several years to speed it up.  The result is indeed able to match
the performance of C nowadays for several non-trivial examples of code,
but it's far from obvious that we have the resources to reproduce this
feat for ELisp.

The claim wasn't that every piece of ELisp code could be as optimized as sharply written C or C++ code, but that there is a subset that is (that can be targeted by tools like parser and lexer generators), at least at the level of an abstract RISC machine that the IRs for compiler backends like libgccjit or llvm let you target.  

I don't think the compiler has to match the level of optimization of the V8 compiler to get significant improvements in the performance of ELisp.  We're really only talking about optimizations known to Common Lisp and Scheme compiler writers in the 1980s.  However, the most significant improvements in performance will probably only come after changes that remove impediments to parallelizing work and improved memory management.


reply via email to

[Prev in Thread] Current Thread [Next in Thread]