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