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annotate docs/HistoricalNotes/2001-05-19-ExceptionResponse.txt @ 100:7d135dc70f03

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LLVM 3.9
author Miyagi Mitsuki <e135756@ie.u-ryukyu.ac.jp>
date Tue, 26 Jan 2016 22:53:40 +0900
parents 95c75e76d11b
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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1 Date: Sat, 19 May 2001 19:09:13 -0500 (CDT)
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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2 From: Chris Lattner <sabre@nondot.org>
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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3 To: Vikram S. Adve <vadve@cs.uiuc.edu>
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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4 Subject: RE: Meeting writeup
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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5
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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6 > I read it through and it looks great!
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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7
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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8 Thanks!
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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9
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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10 > The finally clause in Java may need more thought. The code for this clause
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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11 > is like a subroutine because it needs to be entered from many points (end of
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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12 > try block and beginning of each catch block), and then needs to *return to
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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13 > the place from where the code was entered*. That's why JVM has the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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14 > jsr/jsr_w instruction.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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15
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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16 Hrm... I guess that is an implementation decision. It can either be
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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17 modelled as a subroutine (as java bytecodes do), which is really
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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18 gross... or it can be modelled as code duplication (emitted once inline,
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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19 then once in the exception path). Because this could, at worst,
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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20 slightly less than double the amount of code in a function (it is
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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21 bounded) I don't think this is a big deal. One of the really nice things
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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22 about the LLVM representation is that it still allows for runtime code
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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23 generation for exception paths (exceptions paths are not compiled until
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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24 needed). Obviously a static compiler couldn't do this though. :)
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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25
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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26 In this case, only one copy of the code would be compiled... until the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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27 other one is needed on demand. Also this strategy fits with the "zero
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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28 cost" exception model... the standard case is not burdened with extra
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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29 branches or "call"s.
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30
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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31 > I suppose you could save the return address in a particular register
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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32 > (specific to this finally block), jump to the finally block, and then at the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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33 > end of the finally block, jump back indirectly through this register. It
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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34 > will complicate building the CFG but I suppose that can be handled. It is
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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35 > also unsafe in terms of checking where control returns (which is I suppose
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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36 > why the JVM doesn't use this).
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37
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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38 I think that a code duplication method would be cleaner, and would avoid
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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39 the caveats that you mention. Also, it does not slow down the normal case
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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40 with an indirect branch...
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41
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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42 Like everything, we can probably defer a final decision until later. :)
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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43
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44 -Chris
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45