From dc444e9a236c9e5a9a3a5e6da75f29f7000208d3 Mon Sep 17 00:00:00 2001
From: Evan Cheng <evan.cheng@apple.com>
Date: Mon, 8 Mar 2010 21:05:02 +0000
Subject: [PATCH] Add documentation on sibling call optimization. Rename
 tailcall2.ll test to sibcall.ll.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@97980 91177308-0d34-0410-b5e6-96231b3b80d8
---
 docs/CodeGenerator.html                       | 45 +++++++++++++++++++
 docs/LangRef.html                             |  7 ++-
 test/CodeGen/X86/{tailcall2.ll => sibcall.ll} |  0
 3 files changed, 50 insertions(+), 2 deletions(-)
 rename test/CodeGen/X86/{tailcall2.ll => sibcall.ll} (100%)

diff --git a/docs/CodeGenerator.html b/docs/CodeGenerator.html
index 2be31851e36..f26d0abfad5 100644
--- a/docs/CodeGenerator.html
+++ b/docs/CodeGenerator.html
@@ -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>
@@ -1731,6 +1732,50 @@ 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">
diff --git a/docs/LangRef.html b/docs/LangRef.html
index 7cfa05205ce..eec06fcc023 100644
--- a/docs/LangRef.html
+++ b/docs/LangRef.html
@@ -5149,8 +5149,11 @@ 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>
diff --git a/test/CodeGen/X86/tailcall2.ll b/test/CodeGen/X86/sibcall.ll
similarity index 100%
rename from test/CodeGen/X86/tailcall2.ll
rename to test/CodeGen/X86/sibcall.ll
-- 
2.34.1