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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Stale variable value in optimised code"
href="https://bugs.llvm.org/show_bug.cgi?id=38753">38753</a>
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<th>Summary</th>
<td>Stale variable value in optimised code
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>jeremy.morse.llvm@gmail.com
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<th>CC</th>
<td>greg.bedwell@sony.com, international.phantom@gmail.com, llvm-bugs@lists.llvm.org, paul_robinson@playstation.sony.com
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<pre>With the (contrived) test below, the "cheese" variable has a stale value
reported inside the "if" block. Using an up-to-date toolchain (r340912), and
compiling with "clang++ test.cpp -g -O2 -o a.out" for x86_64, on the marked
line gdb and lldb report the value of "cheese" to be four, wheras if compiled
-O0 the correct value of eight is seen.
The likely cause is SimplifyCFG's tryCSEWithPredecessor replacing the duplicate
"read1 + read2" expression, however the debug value for "cheese" is either lost
or not updated, causing debuggers to report a state the program isn't in.
It should be possible to get this right or mark "cheese" optimised out, the
true-if-block isn't merged with anything else.
Found using DExTer ( <a href="https://github.com/SNSystems/dexter">https://github.com/SNSystems/dexter</a> ).
-------->8--------
int
main()
{
volatile int foo = 4;
int read1 = foo;
int read2 = foo;
int cheese = foo;
int a = read1 + read2;
a += cheese;
if (foo == 4) {
cheese = read1 + read2;
a -= cheese - 12;
a *= 20; // <------ stale value seen
a /= 3;
} else {
a = 0;
}
return a;
}
--------8<--------</pre>
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