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| <title>sparse-switch</title> |
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| <h1>sparse-switch</h1> |
| |
| <h2>Purpose</h2> |
| |
| <p> |
| Jump to a new instruction based on the value in the given register, using an |
| ordered table of value-offset pairs, or fall through to the next instruction if |
| there is no match. |
| </p> |
| <p> |
| Note: The address of the table is guaranteed to be even (that is, 4-byte |
| aligned). If the code size of the method is otherwise odd, then an extra code |
| unit is inserted between the main code and the table whose value is the same as |
| a nop. |
| </p> |
| |
| <h2>Details</h2> |
| |
| <table class="instruc"> |
| <thead> |
| <tr> |
| <th>Op & Format</th> |
| <th>Mnemonic / Syntax</th> |
| <th>Arguments</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td>2c 31t</td> |
| <td>sparse-switch vAA, +BBBBBBBB <i>(with supplemental data as |
| specified below in "<code>sparse-switch</code> Format")</i></td> |
| <td><code>A:</code> register to test<br/> |
| <code>B:</code> signed "branch" offset to table data (32 bits)</td> |
| </tr> |
| </tbody> |
| </table> |
| |
| <h2>Constraints</h2> |
| |
| <ul> |
| <li> |
| A must be a valid register index in the current stack frame. |
| </li> |
| <li> |
| Let PC be the address of the packed-switch instruction in the code array of |
| the current method. Then T = PC + B with the following properties: |
| <ul> |
| <li> |
| T must be 4-byte-aligned. |
| </li> |
| <li> |
| T must be in the same method. |
| </li> |
| <li> |
| T must point to a sparse-switch data table. |
| </li> |
| </ul> |
| </li> |
| </ul> |
| |
| <h2>Behavior</h2> |
| |
| <ul> |
| <li> |
| The value of vA is used as a lookup key inside the sparse table data. |
| </li> |
| <li> |
| If there exists an I with 0 <= I < table.size such that table.keys[I] = vA, |
| then the jump target is determined as follows: |
| <ul> |
| <li> |
| PC' = PC + table.targets[I]. |
| </li> |
| <li> |
| Execution will resume at this address. |
| </li> |
| </ul> |
| </li> |
| <li> |
| Otherwise execution continues at the instruction following the sparse-switch |
| statement. |
| </li> |
| </ul> |
| |
| <h2>Exceptions</h2> |
| |
| <p> |
| None. |
| </p> |
| |
| <h2>Notes</h2> |
| |
| <p> |
| The low-to-high ordering of the keys allows the VM to employ binary search for |
| the lookup, resulting in O(log table.size) comparisons. |
| </p> |
| |
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