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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missing optimization, elimination of empty data dependant loops"
href="https://bugs.llvm.org/show_bug.cgi?id=41497">41497</a>
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<th>Summary</th>
<td>Missing optimization, elimination of empty data dependant loops
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
</td>
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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>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>gareth@ignition-web.co.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
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<p>
<div>
<pre>void * x;
void test(){ for(void * p = x; p; p=*(void**)p); }
With -O3 gives:
test(): # @test()
mov rax, qword ptr [rip + x]
test rax, rax
je .LBB0_3
.LBB0_1: # =>This Inner Loop Header: Depth=1
mov rax, qword ptr [rax]
test rax, rax
jne .LBB0_1
.LBB0_3:
ret
As you can see it currently traverses pointers even though it could be
optimized away.
6.8.2.2 Forward progress
The implementation may assume that any thread will eventually do one of the
following:
(1.1) terminate,
(1.2) make a call to a library I/O function,
(1.3) perform an access through a volatile glvalue, or
(1.4) perform a synchronization operation or an atomic operation.
[ Note: This is intended to allow compiler transformations such as removal of
empty loops, even when termination cannot be proven. — end note ]</pre>
</div>
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