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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 - GVN must not perform substitution based on pointer equality"
href="https://bugs.llvm.org/show_bug.cgi?id=35229">35229</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>GVN must not perform substitution based on pointer equality
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>5.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>post+llvm@ralfj.de
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Clang/LLVM currently miscompiles the following program:
// gvnbug.c
#include <stdio.h>
int foo();
void test(int* gp1, int* gp2)
{
int g = 0;
int a = 0, b = 0;
int x = 7777, y = 6666; // also try swapping these
int* p = &g;
int* q = &g;
int* r = &g;
if (foo()) {
a = 1;
p = &y+1;
q = &x;
}
*gp1 = (int)p+1;
if (q == p) {
b = 1;
*gp2 = (int)q+1;
r = q;
}
*r = 42;
printf("a = %d, b = %d, x = %d\n", a, b, x);
}
int main() {
int gp1 = 0;
int gp2 = 0;
test(&gp1, &gp2);
return 0;
}
// aux.c
int foo() { return 1; }
$ clang-5.0 aux.c gvnbug.c -o gvnbug -O3 && ./gvnbug
a = 1, b = 1, x = 7777
This result is not allowed. If a and b are both 1, the branch "q == p" must
have been taken, so r was set to &x (via q), so x cannot be 7777.
I think this issue has already come up in
<<a class="bz_bug_link
bz_status_NEW "
title="NEW - InstCombine cannot blindly assume that inttoptr(ptrtoint x) -> x"
href="show_bug.cgi?id=34548">https://bugs.llvm.org/show_bug.cgi?id=34548</a>>, but so far there was no example
showing that the bug arises independent of the incorrect
inttoptr-simplification.
What is happening here (if my analysis is correct) is that GVN sees the
equality "q == p" and uses that to replace "q" by "p" in the then-branch.
Next, LLVM notices that because p is derived from y, writing to r (which will
either have value &g or p in the line where the assignment happens) cannot
possibly affect x, and hence the initial value of x can be propagated into the
printf. GVN is wrong to perform this kind of replacement; just because the bit
representations of two pointers are equal, that doesn't mean that their
provenance information is equal.
Test case by Gil Hur.</pre>
</div>
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