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Clarify the requirements for a "tail call" to actually be optimized into a

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jump.  People clearly weren't finding the extra requirements in
CodeGenerator.html.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93070 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jeffrey Yasskin 2010-01-09 19:44:16 +00:00
parent aa38865b6b
commit 95fa80af6f
1 changed files with 25 additions and 9 deletions
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34
docs/LangRef.html
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@ -678,9 +678,9 @@ define i32 @main() { <i>; i32()* </i>
(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
(Application Binary Interface). Implementations of this convention should
allow arbitrary <a href="CodeGenerator.html#tailcallopt">tail call
optimization</a> to be supported. This calling convention does not
(Application Binary Interface).
<a href="CodeGenerator.html#tailcallopt">Tail calls can only be optimized
when this convention is used.</a> This calling convention does not
support varargs and requires the prototype of all callees to exactly match
the prototype of the function definition.</dd>
@ -4993,15 +4993,31 @@ Loop: ; Infinite loop that counts from 0 on up...
<p>This instruction requires several arguments:</p>
<ol>
<li>The optional "tail" marker indicates whether the callee function accesses
any allocas or varargs in the caller. If the "tail" marker is present,
the function call is eligible for tail call optimization. Note that calls
may be marked "tail" even if they do not occur before
a <a href="#i_ret"><tt>ret</tt></a> instruction.</li>
<li>The optional "tail" marker indicates that the callee function does not
access any allocas or varargs in the caller. Note that calls may be
marked "tail" even if they do not occur before
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:
<ul>
<li>Caller and callee both have the calling
convention <tt>fastcc</tt>.</li>
<li>The call is in tail position (ret immediately follows call and ret
uses value of call or is void).</li>
<li>Option <tt>-tailcallopt</tt> is enabled,
or <code>llvm::PerformTailCallOpt</code> is <code>true</code>.</li>
<li><a href="CodeGenerator.html#tailcallopt">Platform specific
constraints are met.</a></li>
</ul>
</li>
<li>The optional "cconv" marker indicates which <a href="#callingconv">calling
convention</a> the call should use. If none is specified, the call
defaults to using C calling conventions.</li>
defaults to using C calling conventions. The calling convention of the
call must match the calling convention of the target function, or else the
behavior is undefined.</li>
<li>The optional <a href="#paramattrs">Parameter Attributes</a> list for
return values. Only '<tt>zeroext</tt>', '<tt>signext</tt>', and