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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Last case of switch generated as a comparison"
href="https://bugs.llvm.org/show_bug.cgi?id=52482">52482</a>
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<th>Summary</th>
<td>Last case of switch generated as a comparison
</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>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>llvm@rifkin.dev
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Switch conversion fires on the following code:
void a(), b(), c(), d(), e();
void foo(int x) {
switch(x) {
case 0: a(); break;
case 1: b(); break;
case 2: c(); break;
case 3: d(); break;
case 4: e(); break;
default: __builtin_unreachable();
}
}
void bar(int x) {
if (x == 0) a();
else if (x == 1) b();
else if (x == 2) c();
else if (x == 3) d();
else if (x == 4) e();
else __builtin_unreachable();
}
But bar ends up being implemented with a switch, something like:
void bar(int x) {
if((unsigned)x > 3) {
e();
} else {
switch(x) {
case 0: a(); break;
case 1: b(); break;
case 2: c(); break;
case 3: d(); break;
default: __builtin_unreachable();
}
}
}
Case 4 is in its own branch rather than in the switch.
This appears to be due to SimplifyCFG transforming the last if-else in bar to
if(x == c) {
e();
} else {
__builtin_unreachable();
}
// into:
__builtin_assume(x == c);
e();
Then the if-else chain as a whole ends up being transformed to:
void bar(int x) {
switch(x) {
case 0: a(); break;
case 1: b(); break;
case 2: c(); break;
case 3: d(); break;
default:
__builtin_assume(x == 4);
e();
}
}
<a href="https://godbolt.org/z/qrYb1hnh7">https://godbolt.org/z/qrYb1hnh7</a>
I'm not sure how GCC deals with this (it does transform bar() correctly), I'm
interested what the best way to address this in LLVM will be.</pre>
</div>
</p>
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