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Add documentation for tail call optimization to CodeGenerator.html. Add a link

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referring to it to LangRef.html.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51097 91177308-0d34-0410-b5e6-96231b3b80d8
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Arnold Schwaighofer 2008-05-14 09:17:12 +00:00
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45
docs/CodeGenerator.html
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@ -84,6 +84,7 @@
</li>
<li><a href="#targetimpls">Target-specific Implementation Notes</a>
<ul>
<li><a href="#tailcallopt">Tail call optimization</a></li>
<li><a href="#x86">The X86 backend</a></li>
<li><a href="#ppc">The PowerPC backend</a>
<ul>
@ -1620,7 +1621,51 @@ are specific to the code generator for a particular target.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="tailcallopt">Tail call optimization</a>
</div>
<div class="doc_text">
<p>Tail call optimization, callee reusing the stack of the caller, is currently supported on x86/x86-64 and PowerPC. It is performed if:
<ul>
<li>Caller and callee have the calling convention <tt>fastcc</tt>.</li>
<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>Option <tt>-tailcallopt</tt> is enabled.</li>
<li>Platform specific constraints are met.</li>
</ul>
</p>
<p>x86/x86-64 constraints:
<ul>
<li>No variable argument lists are used.</li>
<li>On x86-64 when generating GOT/PIC code only module-local calls (visibility = hidden or protected) are supported.</li>
</ul>
</p>
<p>PowerPC constraints:
<ul>
<li>No variable argument lists are used.</li>
<li>No byval parameters are used.</li>
<li>On ppc32/64 GOT/PIC only module-local calls (visibility = hidden or protected) are supported.</li>
</ul>
</p>
<p>Example:</p>
<p>Call as <tt>llc -tailcallopt test.ll</tt>.
<div class="doc_code">
<pre>
declare fastcc i32 @tailcallee(i32 inreg %a1, i32 inreg %a2, i32 %a3, i32 %a4)
define fastcc i32 @tailcaller(i32 %in1, i32 %in2) {
%l1 = add i32 %in1, %in2
%tmp = tail call fastcc i32 @tailcallee(i32 %in1 inreg, i32 %in2 inreg, i32 %in1, i32 %l1)
ret i32 %tmp
}</pre>
</div>
</p>
<p>Implications of <tt>-tailcallopt</tt>:</p>
<p>To support tail call optimization in situations where the callee has more arguments than the caller a 'callee pops arguments' convention is used. This currently causes each <tt>fastcc</tt> call that is not tail call optimized (because one or more of above constraints are not met) to be followed by a readjustment of the stack. So performance might be worse in such cases.</p>
<p>On x86 and x86-64 one register is reserved for indirect tail calls (e.g via a function pointer). So there is one less register for integer argument passing. For x86 this means 2 registers (if <tt>inreg</tt> parameter attribute is used) and for x86-64 this means 5 register are used.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="x86">The X86 backend</a>

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@ -586,9 +586,11 @@ the future:</p>
(e.g. by passing things in registers). This calling convention allows the
target to use whatever tricks it wants to produce fast code for the target,
without having to conform to an externally specified ABI. Implementations of
this convention should allow arbitrary tail call optimization to be supported.
This calling convention does not support varargs and requires the prototype of
all callees to exactly match the prototype of the function definition.
this convention should allow arbitrary
<a href="CodeGenerator.html#tailcallopt">tail call optimization</a> to be
supported. This calling convention does not support varargs and requires the
prototype of all callees to exactly match the prototype of the function
definition.
</dd>
<dt><b>"<tt>coldcc</tt>" - The cold calling convention</b>:</dt>