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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 - incorrect optimization with signaling NaN and implicit conversion in fabs: missing exception"
href="https://bugs.llvm.org/show_bug.cgi?id=39112">39112</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>incorrect optimization with signaling NaN and implicit conversion in fabs: missing exception
</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>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>vincent-llvm@vinc17.net
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>With a code that does something like
float f = fabs (double_snan)
one does not get the FE_INVALID exception with -O.
Details:
#include <stdio.h>
#include <inttypes.h>
#include <math.h>
#include <fenv.h>
typedef union { float f; uint32_t i; } ieee_float_t;
int main (void)
{
volatile ieee_float_t x, y;
volatile double d;
x.i = 0x7f800001;
printf ("0. %x (%g)\n", x.i, x.f);
feclearexcept (FE_INVALID);
y.f = fabs (x.f);
if (fetestexcept (FE_INVALID))
printf ("2. FE_INVALID\n");
printf ("2. %x (%g)\n", y.i, y.f);
return 0;
}
yields:
cventin% clang-8 snan.c -o snan -lm
cventin% ./snan
0. 7f800001 (nan)
2. FE_INVALID
2. 7fc00001 (nan)
which is correct (due to the float-to-double conversion of sNaN), but
cventin% clang-8 snan.c -o snan -lm -O
cventin% ./snan
0. 7f800001 (nan)
2. 7f800001 (nan)
The FE_INVALID exception is missing, and the result is a signaling NaN instead
of a quiet NaN.
Tested with the clang-8 1:8~svn342638-1 Debian package (x86_64 architecture).</pre>
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