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| <h5 class="subsubsection">9.42.4.2 Function Call Relaxation</h5> |
| |
| <p><a name="index-relaxation-of-call-instructions-2027"></a><a name="index-call-instructions_002c-relaxation-2028"></a> |
| Function calls may require relaxation because the Xtensa immediate call |
| instructions (<code>CALL0</code>, <code>CALL4</code>, <code>CALL8</code> and |
| <code>CALL12</code>) provide a PC-relative offset of only 512 Kbytes in either |
| direction. For larger programs, it may be necessary to use indirect |
| calls (<code>CALLX0</code>, <code>CALLX4</code>, <code>CALLX8</code> and <code>CALLX12</code>) |
| where the target address is specified in a register. The Xtensa |
| assembler can automatically relax immediate call instructions into |
| indirect call instructions. This relaxation is done by loading the |
| address of the called function into the callee's return address register |
| and then using a <code>CALLX</code> instruction. So, for example: |
| |
| <pre class="smallexample"> call8 func |
| </pre> |
| <p>might be relaxed to: |
| |
| <pre class="smallexample"> .literal .L1, func |
| l32r a8, .L1 |
| callx8 a8 |
| </pre> |
| <p>Because the addresses of targets of function calls are not generally |
| known until link-time, the assembler must assume the worst and relax all |
| the calls to functions in other source files, not just those that really |
| will be out of range. The linker can recognize calls that were |
| unnecessarily relaxed, and it will remove the overhead introduced by the |
| assembler for those cases where direct calls are sufficient. |
| |
| <p>Call relaxation is disabled by default because it can have a negative |
| effect on both code size and performance, although the linker can |
| usually eliminate the unnecessary overhead. If a program is too large |
| and some of the calls are out of range, function call relaxation can be |
| enabled using the ‘<samp><span class="samp">--longcalls</span></samp>’ command-line option or the |
| <code>longcalls</code> directive (see <a href="Longcalls-Directive.html#Longcalls-Directive">longcalls</a>). |
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