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annotate docs/HistoricalNotes/2002-05-12-InstListChange.txt @ 136:6cd7e602c98b

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Fix compile error to update llvm 5.0.1
author mir3636
date Sat, 17 Feb 2018 11:50:31 +0900
parents 95c75e76d11b
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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0
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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1 Date: Sun, 12 May 2002 17:12:53 -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: LLVM change
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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 There is a fairly fundemental change that I would like to make to the LLVM
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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7 infrastructure, but I'd like to know if you see any drawbacks that I
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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8 don't...
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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 Basically right now at the basic block level, each basic block contains an
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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11 instruction list (returned by getInstList()) that is a ValueHolder of
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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12 instructions. To iterate over instructions, we must actually iterate over
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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13 the instlist, and access the instructions through the instlist.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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14
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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15 To add or remove an instruction from a basic block, we need to get an
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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16 iterator to an instruction, which, given just an Instruction*, requires a
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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17 linear search of the basic block the instruction is contained in... just
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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18 to insert an instruction before another instruction, or to delete an
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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19 instruction! This complicates algorithms that should be very simple (like
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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20 simple constant propagation), because they aren't actually sparse anymore,
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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21 they have to traverse basic blocks to remove constant propogated
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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22 instructions.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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23
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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24 Additionally, adding or removing instructions to a basic block
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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25 _invalidates all iterators_ pointing into that block, which is really
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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26 irritating.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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27
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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28 To fix these problems (and others), I would like to make the ordering of
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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29 the instructions be represented with a doubly linked list in the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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30 instructions themselves, instead of an external data structure. This is
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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31 how many other representations do it, and frankly I can't remember why I
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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32 originally implemented it the way I did.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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33
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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34 Long term, all of the code that depends on the nasty features in the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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35 instruction list (which can be found by grep'ing for getInstList()) will
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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36 be changed to do nice local transformations. In the short term, I'll
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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37 change the representation, but preserve the interface (including
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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38 getInstList()) so that all of the code doesn't have to change.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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39
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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40 Iteration over the instructions in a basic block remains the simple:
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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41 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) ...
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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42
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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43 But we will also support:
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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44 for (Instruction *I = BB->front(); I; I = I->getNext()) ...
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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45
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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46 After converting instructions over, I'll convert basic blocks and
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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47 functions to have a similar interface.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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48
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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49 The only negative aspect of this change that I see is that it increases
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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50 the amount of memory consumed by one pointer per instruction. Given the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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51 benefits, I think this is a very reasonable tradeoff.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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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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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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54
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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55 -Chris