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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Missed optimization is switch"
href="http://llvm.org/bugs/show_bug.cgi?id=18737">18737</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed optimization is switch
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>chfast@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=12012" name="attach_12012" title="C++ source">attachment 12012</a> <a href="attachment.cgi?id=12012&action=edit" title="C++ source">[details]</a></span>
C++ source
Example code in C++:
enum class Enum {
A, B, C, D, E
};
int enum2int(Enum e) {
switch (e) {
case Enum::A: return 0;
case Enum::B: return 1;
case Enum::C: return 2;
case Enum::D: return 3;
case Enum::E: return 4;
}
}
In this case clang and opt with option -O3 used jump tables as an optimization.
@switch.table3 = private unnamed_addr constant [5 x i32] [i32 0, i32 1, i32 2,
i32 3, i32 4]
; Function Attrs: nounwind
define i32 @"\01?enum2int@@YAHW4Enum@@@Z"(i32 %e) #0 {
entry:
%0 = icmp ult i32 %e, 5
br i1 %0, label %switch.lookup, label %sw.epilog
sw.epilog: ; preds = %entry
tail call void @llvm.trap()
unreachable
switch.lookup: ; preds = %entry
%switch.gep = getelementptr inbounds [5 x i32]* @switch.table3, i32 0, i32 %e
%switch.load = load i32* %switch.gep, align 4
ret i32 %switch.load
}
I think it could be optimized further as the input value is the same as output
value. @switch.table3 looks a bit silly.</pre>
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