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Update information about the switch instruction

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@6037 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2003-05-08 05:08:48 +00:00
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@ -684,76 +684,48 @@ IfUnequal:
<h5>Syntax:</h5>
<pre>
<i>; Definitions for lookup indirect branch</i>
%switchtype = type [&lt;anysize&gt; x { uint, label }]
switch int &lt;value&gt;, label &lt;defaultdest&gt; [ int &lt;val&gt;, label &dest&gt;, ... ]
<i>; Lookup indirect branch</i>
switch uint &lt;value&gt;, label &lt;defaultdest&gt;, %switchtype &lt;switchtable&gt;
<!--
<i>; Indexed indirect branch</i>
switch uint &lt;idxvalue&gt;, label &lt;defaultdest&gt;, [&lt;anysize&gt; x label] &lt;desttable&gt;
-->
</pre>
<h5>Overview:</h5>
<b>NOTE:</b> The switch instruction may go away in the future. It is not very
well supported in LLVM anyway, so don't go to great lengths to support it. Talk
to <a href="mailto:sabre@nondot.org">Chris</a> for more info if this concerns
you.<p>
The '<tt>switch</tt>' instruction is used to transfer control flow to one of
several different places. It is a generalization of the '<tt>br</tt>'
instruction, allowing a branch to occur to one of many possible destinations.<p>
The '<tt>switch</tt>' statement supports two different styles of indirect
branching: lookup branching and indexed branching. Lookup branching is
generally useful if the values to switch on are spread far appart, where index
branching is useful if the values to switch on are generally dense.<p>
The two different forms of the '<tt>switch</tt>' statement are simple hints to
the underlying implementation. For example, the compiler may choose to
implement a small indirect branch table as a series of predicated comparisons:
if it is faster for the target architecture.<p>
<h5>Arguments:</h5>
The lookup form of the '<tt>switch</tt>' instruction uses three parameters: a
'<tt>uint</tt>' comparison value '<tt>value</tt>', a default '<tt>label</tt>'
destination, and an array of pairs of comparison value constants and
'<tt>label</tt>'s. The sized array must be a constant value.<p>
The indexed form of the '<tt>switch</tt>' instruction uses three parameters: an
'<tt>uint</tt>' index value, a default '<tt>label</tt>' and a sized array of
'<tt>label</tt>'s. The '<tt>dests</tt>' array must be a constant array.
The '<tt>switch</tt>' instruction uses three parameters: a '<tt>uint</tt>'
comparison value '<tt>value</tt>', a default '<tt>label</tt>' destination, and
an array of pairs of comparison value constants and '<tt>label</tt>'s.<p>
<h5>Semantics:</h5>
The lookup style switch statement specifies a table of values and destinations.
The <tt>switch</tt> instruction specifies a table of values and destinations.
When the '<tt>switch</tt>' instruction is executed, this table is searched for
the given value. If the value is found, the corresponding destination is
branched to. <p>
branched to, otherwise the default value it transfered to.<p>
The index branch form simply looks up a label element directly in a table and
branches to it.<p>
<h5>Implementation:</h5>
In either case, the compiler knows the static size of the array, because it is
provided as part of the constant values type.<p>
Depending on properties of the target machine and the particular <tt>switch</tt>
instruction, this instruction may be code generated as a series of chained
conditional branches, or with a lookup table.<p>
<h5>Example:</h5>
<pre>
<i>; Emulate a conditional br instruction</i>
%Val = <a href="#i_cast">cast</a> bool %value to uint
switch uint %Val, label %truedest, [1 x label] [label %falsedest ]
switch int %Val, label %truedest [int 0, label %falsedest ]
<i>; Emulate an unconditional br instruction</i>
switch uint 0, label %dest, [ 0 x label] [ ]
switch int 0, label %dest [ ]
<i>; Implement a jump table:</i>
switch uint %val, label %otherwise, [3 x label] [ label %onzero,
label %onone,
label %ontwo ]
switch int %val, label %otherwise [ int 0, label %onzero,
int 1, label %onone,
int 2, label %ontwo ]
</pre>
@ -1840,7 +1812,7 @@ arbitrarily complex and require memory allocation, for example.<p>
<address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
<!-- Created: Tue Jan 23 15:19:28 CST 2001 -->
<!-- hhmts start -->
Last modified: Wed May 7 23:56:16 CDT 2003
Last modified: Thu May 8 00:06:36 CDT 2003
<!-- hhmts end -->
</font>
</body></html>