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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 - Strict-aliasing not noticing valid aliasing of two unions with active members"
href="https://bugs.llvm.org/show_bug.cgi?id=34632">34632</a>
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<th>Summary</th>
<td>Strict-aliasing not noticing valid aliasing of two unions with active members
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</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>Linux
</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>C++
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>myriachan@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
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</tr></table>
<p>
<div>
<pre>Consider the following C/C++ code with -O3 -fstrict-aliasing:
struct s1 {unsigned short x;};
struct s2 {unsigned short x;};
union s1s2 { struct s1 v1; struct s2 v2; };
static int read_s1x(struct s1 *p) { return p->x; }
static void write_s2x(struct s2 *p, int v) { p->x=v;}
int test(union s1s2 *p1, union s1s2 *p2, union s1s2 *p3)
{
if (read_s1x(&p1->v1))
{
unsigned short temp;
temp = p3->v1.x;
p3->v2.x = temp;
write_s2x(&p2->v2,1234);
temp = p3->v2.x;
p3->v1.x = temp;
}
return read_s1x(&p1->v1);
}
int test2(int x)
{
union s1s2 q[2];
q->v1.x = 4321;
return test(q,q+x,q+x);
}
#include <stdio.h>
int main(void)
{
printf("%d\n",test2(0));
}
Clang (and GCC) generate code that outputs 4321 instead of the expected 1234.
I don't really understand things in terms of the C standard, but in terms of
the C++ standard, it seems as if Clang and GCC are incorrect, and this code is
well-defined. (The output is 4321 in both C and C++ mode.)
According to [class.union]/5 in the C++17 draft N4659, the assignment
expression "p3->v2.x = temp;" changes the active member of the union. It's
done through a union member access expression. Thus the pointer &p2->v2 is
valid here.
Even if I switch this to "p3->v2 = { x };", avoiding the nested case, the
problem still happens.
Even if I explicitly change the active member of the union with placement new
as "new(&p3.v2) s2;", the problem still happens.
Is it possible that Clang doesn't see the possibility that p2 and p3 point to
the same object?</pre>
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</p>
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