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<h5 class="subsubsection">9.36.4.2 Function Call Relaxation</h5>
<p><a name="index-relaxation-of-call-instructions-1866"></a><a name="index-call-instructions_002c-relaxation-1867"></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 <span class="samp">--longcalls</span> command-line option or the
<code>longcalls</code> directive (see <a href="Longcalls-Directive.html#Longcalls-Directive">longcalls</a>).
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