| Date: Sat, 19 May 2001 19:09:13 -0500 (CDT) |
| From: Chris Lattner <sabre@nondot.org> |
| To: Vikram S. Adve <vadve@cs.uiuc.edu> |
| Subject: RE: Meeting writeup |
| |
| > I read it through and it looks great! |
| |
| Thanks! |
| |
| > The finally clause in Java may need more thought. The code for this clause |
| > is like a subroutine because it needs to be entered from many points (end of |
| > try block and beginning of each catch block), and then needs to *return to |
| > the place from where the code was entered*. That's why JVM has the |
| > jsr/jsr_w instruction. |
| |
| Hrm... I guess that is an implementation decision. It can either be |
| modelled as a subroutine (as java bytecodes do), which is really |
| gross... or it can be modelled as code duplication (emitted once inline, |
| then once in the exception path). Because this could, at worst, |
| slightly less than double the amount of code in a function (it is |
| bounded) I don't think this is a big deal. One of the really nice things |
| about the LLVM representation is that it still allows for runtime code |
| generation for exception paths (exceptions paths are not compiled until |
| needed). Obviously a static compiler couldn't do this though. :) |
| |
| In this case, only one copy of the code would be compiled... until the |
| other one is needed on demand. Also this strategy fits with the "zero |
| cost" exception model... the standard case is not burdened with extra |
| branches or "call"s. |
| |
| > I suppose you could save the return address in a particular register |
| > (specific to this finally block), jump to the finally block, and then at the |
| > end of the finally block, jump back indirectly through this register. It |
| > will complicate building the CFG but I suppose that can be handled. It is |
| > also unsafe in terms of checking where control returns (which is I suppose |
| > why the JVM doesn't use this). |
| |
| I think that a code duplication method would be cleaner, and would avoid |
| the caveats that you mention. Also, it does not slow down the normal case |
| with an indirect branch... |
| |
| Like everything, we can probably defer a final decision until later. :) |
| |
| -Chris |
| |