<li><a href="#targetimpls">Target-specific Implementation Notes</a>
<ul>
<li><a href="#tailcallopt">Tail call optimization</a></li>
+ <li><a href="#sibcallopt">Sibling call optimization</a></li>
<li><a href="#x86">The X86 backend</a></li>
<li><a href="#ppc">The PowerPC backend</a>
<ul>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
+ <a name="sibcallopt">Sibling call optimization</a>
+</div>
+
+<div class="doc_text">
+
+<p>Sibling call optimization is a restricted form of tail call optimization.
+ Unlike tail call optimization described in the previous section, it can be
+ performed automatically on any tail calls when <tt>-tailcallopt</tt> option
+ is not specified.</p>
+
+<p>Sibling call optimization is currently performed on x86/x86-64 when the
+ following constraints are met:</p>
+
+<ul>
+ <li>Caller and callee have the same calling convention. It can be either
+ <tt>c</tt> or <tt>fastcc</tt>.
+
+ <li>The call is a tail call - in tail position (ret immediately follows call
+ and ret uses value of call or is void).</li>
+
+ <li>Caller and callee have matching return type or the callee result is not
+ used.
+
+ <li>If any of the callee arguments are being passed in stack, they must be
+ available in caller's own incoming argument stack and the frame offsets
+ must be the same.
+</ul>
+
+<p>Example:</p>
+<div class="doc_code">
+<pre>
+declare i32 @bar(i32, i32)
+
+define i32 @foo(i32 %a, i32 %b, i32 %c) {
+entry:
+ %0 = tail call i32 @bar(i32 %a, i32 %b)
+ ret i32 %0
+}
+</pre>
+</div>
+
+</div>
+<!-- ======================================================================= -->
+<div class="doc_subsection">
<a name="x86">The X86 backend</a>
</div>
a <a href="#i_ret"><tt>ret</tt></a> instruction. If the "tail" marker is
present, the function call is eligible for tail call optimization,
but <a href="CodeGenerator.html#tailcallopt">might not in fact be
- optimized into a jump</a>. As of this writing, the extra requirements for
- a call to actually be optimized are:
+ optimized into a jump</a>. The code generator may optimize calls marked
+ "tail" with either 1) automatic <a href="CodeGenerator.html#sibcallopt">
+ sibling call optimization</a> when the caller and callee have
+ matching signatures, or 2) forced tail call optimization when the
+ following extra requirements are met:
<ul>
<li>Caller and callee both have the calling
convention <tt>fastcc</tt>.</li>