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annotate docs/HistoricalNotes/2001-09-18-OptimizeExceptions.txt @ 121:803732b1fca8

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LLVM 5.0
author kono
date Fri, 27 Oct 2017 17:07:41 +0900
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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1 Date: Tue, 18 Sep 2001 00:38:37 -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: Idea for a simple, useful link time optimization
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5
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6
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7 In C++ programs, exceptions suck, and here's why:
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8
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9 1. In virtually all function calls, you must assume that the function
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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10 throws an exception, unless it is defined as 'nothrow'. This means
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11 that every function call has to have code to invoke dtors on objects
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12 locally if one is thrown by the function. Most functions don't throw
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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13 exceptions, so this code is dead [with all the bad effects of dead
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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14 code, including icache pollution].
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15 2. Declaring a function nothrow causes catch blocks to be added to every
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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16 call that isnot provably nothrow. This makes them very slow.
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17 3. Extra extraneous exception edges reduce the opportunity for code
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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18 motion.
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19 4. EH is typically implemented with large lookup tables. Ours is going to
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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20 be much smaller (than the "standard" way of doing it) to start with,
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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21 but eliminating it entirely would be nice. :)
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22 5. It is physically impossible to correctly put (accurate, correct)
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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23 exception specifications on generic, templated code. But it is trivial
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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24 to analyze instantiations of said code.
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25 6. Most large C++ programs throw few exceptions. Most well designed
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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26 programs only throw exceptions in specific planned portions of the
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27 code.
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28
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29 Given our _planned_ model of handling exceptions, all of this would be
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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30 pretty trivial to eliminate through some pretty simplistic interprocedural
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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31 analysis. The DCE factor alone could probably be pretty significant. The
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32 extra code motion opportunities could also be exploited though...
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33
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34 Additionally, this optimization can be implemented in a straight forward
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35 conservative manner, allowing libraries to be optimized or individual
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36 files even (if there are leaf functions visible in the translation unit
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37 that are called).
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38
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39 I think it's a reasonable optimization that hasn't really been addressed
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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40 (because assembly is way too low level for this), and could have decent
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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41 payoffs... without being a overly complex optimization.
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42
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43 After I wrote all of that, I found this page that is talking about
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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44 basically the same thing I just wrote, except that it is translation unit
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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45 at a time, tree based approach:
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46 http://www.ocston.org/~jls/ehopt.html
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47
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48 but is very useful from "expected gain" and references perspective. Note
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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49 that their compiler is apparently unable to inline functions that use
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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50 exceptions, so there numbers are pretty worthless... also our results
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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51 would (hopefully) be better because it's interprocedural...
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52
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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53 What do you think?
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54
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55 -Chris
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56