Commit 406d24b5 authored by Tanya Lattner's avatar Tanya Lattner
Browse files

Merge 97980 from mainline. 

Add documentation on sibling call optimization. Rename tailcall2.ll test to sibcall.ll.

llvm-svn: 98313
parent 6fdd7771

llvm/docs/CodeGenerator.html

+45 −0
Original line number Diff line number Diff line
@@ -86,6 +86,7 @@ 
  <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>
@@ -1732,6 +1733,50 @@ define fastcc i32 @tailcaller(i32 %in1, i32 %in2) { 
   (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>



</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">

llvm/docs/LangRef.html

+5 −2
Original line number Diff line number Diff line
@@ -5169,8 +5169,11 @@ Loop: ; Infinite loop that counts from 0 on up... 
      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>