Hello! Andy Wingo <wi...@igalia.com> skribis:
> A few points of information :) Much appreciated! > The guile.arities section starts with a sorted array of fixed-size > headers, then is followed by a sequence of ULEB128 references to local > variable names, including non-arguments. The size is a bit perplexing, > I agree. I can think of a number of ways to encode that section > differently but we'd need to understand a bit more about it and why the > baseline compiler is significantly different. ‘.guile.arities’ size should be proportional to the number of procedures, right? Additionally, if there are only/mostly thunks, the string table for argument names should be small if not empty. For N thunks, I would expect roughly N 28-byte headers + NxM UL128, say 100 bytes per thunk; there’s 1000 of them, so we should be ~100,000 bytes. This is roughly what we get observe with the baseline compiler. >> “.rtl-text” is 38% smaller and “.guile.arities” is almost a tenth of >> what it was. > > The difference in the text are the new baseline intrinsics, > e.g. $vector-ref. It goes in the opposite direction from instruction > explosion, which sought to (1) make the JIT compiler easier by > decomposing compound operations into their atomic parts, (2) make the > optimizer learn more information from flow rather than type-checking > side effects, and (3) allow the optimizer to eliminate / hoist / move > the component pieces of macro-operations. > > However in the baseline compiler (2) and (3) aren't possible because > there is no optimizer on that level, and therefore the result is > actually a lose -- 10 micro-ops cost more than 1 macro-op because of > stack traffic overhead, which isn't currently mitigated by the JIT (1). > > So instruction explosion is residual code explosion, which should pay > off in theory, but not for the baseline compiler. So I added new > intrinsics for e.g. $vector-ref et al. Thus the smaller code size. Yes, that makes a lot of sense. In particular, this file must use the struct intrinsics a lot. > There are more possibilities for making code size smaller, e.g. having > two equivalent encodings for bytecode, where one is smaller: > > https://webkit.org/blog/9329/a-new-bytecode-format-for-javascriptcore/ Like THUMB, but for bytecode. :-) I guess we could first analyze the generated code more closely and see if there are opportunities there. Thanks for the explanations! Ludo’.