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| <h5 class="subsubsection">9.55.4.3 Jump Relaxation</h5> |
| |
| <p><a name="index-relaxation-of-jump-instructions-2638"></a><a name="index-jump-instructions_002c-relaxation-2639"></a> |
| Jump instruction may require relaxation because the Xtensa jump instruction |
| (<code>J</code>) provide a PC-relative offset of only 128 Kbytes in either |
| direction. One option is to use jump long (<code>J.L</code>) instruction, which |
| depending on jump distance may be assembled as jump (<code>J</code>) or indirect |
| jump (<code>JX</code>). However it needs a free register. When there's no spare |
| register it is possible to plant intermediate jump sites (trampolines) |
| between the jump instruction and its target. These sites may be located in |
| areas unreachable by normal code execution flow, in that case they only |
| contain intermediate jumps, or they may be inserted in the middle of code |
| block, in which case there's an additional jump from the beginning of the |
| trampoline to the instruction past its end. So, for example: |
| |
| <pre class="smallexample"> j 1f |
| ... |
| retw |
| ... |
| mov a10, a2 |
| call8 func |
| ... |
| 1: |
| ... |
| </pre> |
| <p>might be relaxed to: |
| |
| <pre class="smallexample"> j .L0_TR_1 |
| ... |
| retw |
| .L0_TR_1: |
| j 1f |
| ... |
| mov a10, a2 |
| call8 func |
| ... |
| 1: |
| ... |
| </pre> |
| <p>or to: |
| |
| <pre class="smallexample"> j .L0_TR_1 |
| ... |
| retw |
| ... |
| mov a10, a2 |
| j .L0_TR_0 |
| .L0_TR_1: |
| j 1f |
| .L0_TR_0: |
| call8 func |
| ... |
| 1: |
| ... |
| </pre> |
| <p>The Xtensa assembler uses trampolines with jump around only when it cannot |
| find suitable unreachable trampoline. There may be multiple trampolines |
| between the jump instruction and its target. |
| |
| <p>This relaxation does not apply to jumps to undefined symbols, assuming they |
| will reach their targets once resolved. |
| |
| <p>Jump relaxation is enabled by default because it does not affect code size |
| or performance while the code itself is small. This relaxation may be |
| disabled completely with ‘<samp><span class="samp">--no-trampolines</span></samp>’ or ‘<samp><span class="samp">--no-transform</span></samp>’ |
| command-line options (see <a href="Xtensa-Options.html#Xtensa-Options">Command-line Options</a>). |
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