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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 - Semantic error - - different output are observed on changing flags"
href="https://bugs.llvm.org/show_bug.cgi?id=51411">51411</a>
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<th>Summary</th>
<td>Semantic error - - different output are observed on changing flags
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>12.0
</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>
</tr>
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<th>Severity</th>
<td>release blocker
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>Rohit.Aggarwal@amd.com
</td>
</tr>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>The issue is present in main stream LLVM. I tired the example code on clang-12
release.
## Sample Code (inline_ex.c)
int main() {
double dbl;
unsigned i[1000], j[1000], k[1000], l[1000];
int index;
for (index = 0; index < 1000; index++) {
i[index] = j[index] = k[index] = l[index] = 10000;
}
unsigned long r1=0, r2=0, r3=0, r4=0;
for (int a = 0; a < 1000; a++) {
r1 += i[a];
r2 += j[a];
r3 += k[a];
r4 += l[a];
}
double R1 = DBL_MAX + (double)r1;
double R2 = DBL_MAX + (double)r2;
double R3 = DBL_MAX + (double)r3;
double R4 = DBL_MAX + (double)r4;
// printf("%lf\n%lf\n%lf\n%lf\n", R1, R2, R3, R4);
dbl = R1 + R2 + R3 + R4;
printf("Result: %lf\n", dbl);
return 0;
}
## GCC
(gcc inline_ex.c -o a.out && ./a.out )
Result: inf
(gcc inline_ex.c -O3 -ffast-math -o a.out && ./a.out )
Result: inf
## CLANG 12
clang-12 -O3 -flto -ffast-math -fno-unroll-loops inline_ex.c -o a.out &&
./a.out )
Result: 0.000000
(clang-12 -O3 -ffast-math inline_ex.c -o a.out && ./a.out )
Result: 0.000000
(clang-12 -O3 -flto -fno-unroll-loops inline_ex.c -o a.out && ./a.out )
Result: inf
(clang-12 -O3 inline_ex.c -o a.out && ./a.out )
Result: inf
(clang-12 inline_ex.c -o a.out && ./a.out )
Result: inf
Observation
The problem occurs when we use fast-math flag with O3. This flag enables
certain transformation in instCombine pass which leads to creation of poison
value. The poison value is not handle properly and is propagated to it's uses.
In the end, a poison value remains as the operand in printf. The value of
poison depends upon type and for double the value is 0.0000.</pre>
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