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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - load instruction erroneously removed by GVN"
href="https://llvm.org/bugs/show_bug.cgi?id=24426">24426</a>
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<th>Summary</th>
<td>load instruction erroneously removed by GVN
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>opt
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>mikael.holmen@ericsson.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>I have the following program:
declare void @check(i8)
declare void @write(i8* %res)
define i8 @TEST1() {
%buf = alloca [10 x i8]
%_tmp30 = bitcast [10 x i8]* %buf to i8*
call void @write(i8* %_tmp30)
%_tmp33 = load i8, i8* %_tmp30
call void @check(i8 %_tmp33)
ret i8 0
}
and I run opt on it with
build-all/bin/opt -S -memcpyopt -mldst-motion -gvn -O3 ./f2.ll
and then the load is removed by GVN and %_tmp33 is replaced with undef.
GVN iteration: 0
GVN removed: %_tmp33 = load i8, i8* %_tmp30
I notice that if I change TEST1 slightly, and rewrite it as
define i8 @TEST2() {
%buf = alloca i8
call void @write(i8* %buf)
%_tmp33 = load i8, i8* %buf
call void @check(i8 %_tmp33)
ret i8 0
}
then the load is left intact
GVN iteration: 0
GVN: load i8 %_tmp33 is clobbered by call void @write(i8* %buf.17)
In this case it seems to be the code
if (llvm::PointerMayBeCapturedBefore(Object, /* ReturnCaptures */ true,
/* StoreCaptures */ true, I, DT,
/* include Object */ true,
/* OrderedBasicBlock */ OBB))
return MRI_ModRef;
in AliasAnalysis::callCapturesBefore that saves the load, which it doesn't in
the first case.</pre>
</div>
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