1<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> 2 3<html> 4 5<head> 6<title>sparse-switch</title> 7<link rel=stylesheet href="opcode.css"> 8</head> 9 10<body> 11 12<h1>sparse-switch</h1> 13 14<h2>Purpose</h2> 15 16<p> 17Jump to a new instruction based on the value in the given register, using an 18ordered table of value-offset pairs, or fall through to the next instruction if 19there is no match. 20</p> 21<p> 22Note: The address of the table is guaranteed to be even (that is, 4-byte 23aligned). If the code size of the method is otherwise odd, then an extra code 24unit is inserted between the main code and the table whose value is the same as 25a nop. 26</p> 27 28<h2>Details</h2> 29 30<table class="instruc"> 31<thead> 32<tr> 33 <th>Op & Format</th> 34 <th>Mnemonic / Syntax</th> 35 <th>Arguments</th> 36</tr> 37</thead> 38<tbody> 39<tr> 40 <td>2c 31t</td> 41 <td>sparse-switch vAA, +BBBBBBBB <i>(with supplemental data as 42 specified below in "<code>sparse-switch</code> Format")</i></td> 43 <td><code>A:</code> register to test<br/> 44 <code>B:</code> signed "branch" offset to table data (32 bits)</td> 45</tr> 46</tbody> 47</table> 48 49<h2>Constraints</h2> 50 51<ul> 52 <li> 53 A must be a valid register index in the current stack frame. 54 </li> 55 <li> 56 Let PC be the address of the packed-switch instruction in the code array of 57 the current method. Then T = PC + B with the following properties: 58 <ul> 59 <li> 60 T must be 4-byte-aligned. 61 </li> 62 <li> 63 T must be in the same method. 64 </li> 65 <li> 66 T must point to a sparse-switch data table. 67 </li> 68 </ul> 69 </li> 70</ul> 71 72<h2>Behavior</h2> 73 74<ul> 75 <li> 76 The value of vA is used as a lookup key inside the sparse table data. 77 </li> 78 <li> 79 If there exists an I with 0 <= I < table.size such that table.keys[I] = vA, 80 then the jump target is determined as follows: 81 <ul> 82 <li> 83 PC' = PC + table.targets[I]. 84 </li> 85 <li> 86 Execution will resume at this address. 87 </li> 88 </ul> 89 </li> 90 <li> 91 Otherwise execution continues at the instruction following the sparse-switch 92 statement. 93 </li> 94</ul> 95 96<h2>Exceptions</h2> 97 98<p> 99None. 100</p> 101 102<h2>Notes</h2> 103 104<p> 105The low-to-high ordering of the keys allows the VM to employ binary search for 106the lookup, resulting in O(log table.size) comparisons. 107</p> 108 109</body> 110</html> 111