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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Invalid optimization in the presence of shift instructions?"
href="https://bugs.llvm.org/show_bug.cgi?id=34133">34133</a>
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<th>Summary</th>
<td>Invalid optimization in the presence of shift instructions?
</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>
</tr>
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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>c.cadar@imperial.ac.uk
</td>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Consider the program below:
$ cat shift.c
int a = 0;
int foo(int x) { return (a < 2) || (a >> x) ; }
int main() {
return foo(100);
}
$ clang -O1 -emit-llvm -c shift.c
$ llvm-dis shift.bc
$ cat shift.ll
...
define i32 @foo(i32) local_unnamed_addr #0 {
%2 = load i32, i32* @a, align 4, !tbaa !1
%3 = icmp slt i32 %2, 2
%4 = ashr i32 %2, %0
%5 = icmp ne i32 %4, 0
%6 = or i1 %3, %5
%7 = zext i1 %6 to i32
ret i32 %7
}
...
define i32 @main() local_unnamed_addr #0 {
%1 = tail call i32 @foo(i32 100)
ret i32 %1
}
...
Note that in the C code, the || operator has short-circuiting
behaviour, so "a >> x" should not be evaluated since the first clause
is true. However, the optimization generates code that always
executes the corresponding ashr instruction, which has undefined
behaviour in this case, as the shift amount is 100. So is this
optimization valid?
We discovered this while working with KLEE, which generates an
overshift error when the code is compiled with -O1, but not with -O0,
so we'd like to understand whether KLEE's behaviour is correct here.
Note that UBSan does not complain here, as its instrumentation
disables the optimization.
Thank you,
Cristian</pre>
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