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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 - Miscompile with -ffast-math and -ftree-vectorize on arithmetic involving doubles and unsigned longs"
href="https://bugs.llvm.org/show_bug.cgi?id=43609">43609</a>
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<th>Summary</th>
<td>Miscompile with -ffast-math and -ftree-vectorize on arithmetic involving doubles and unsigned longs
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>8.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>rlcamp.pdx@gmail.com
</td>
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>The following program:
#include <stdlib.h>
#include <stdio.h>
__attribute((noinline)) void fill_vector(double * x, const double dx, const
size_t X) {
/* fill vector with uniformly spaced values */
for (size_t ix = 0; ix < X; ix++)
x[ix] = (ix + 0.5) * dx;
}
int main(void) {
const size_t X = 19;
double * x = malloc(sizeof(double) * X);
fill_vector(x, 1.0, X);
/* print the values */
for (size_t ix = 0; ix < X; ix++)
fprintf(stdout, "x[%zu] = %g\n", ix, x[ix]);
}
produces the following output in Clang 8 and newer with -Ofast (or -O1
-ffast-math -ftree-vectorize):
x[0] = 0
x[1] = 2
x[2] = 2
x[3] = 4
x[4] = 4
x[5] = 6
x[6] = 6
x[7] = 8
x[8] = 8
x[9] = 10
x[10] = 10
x[11] = 12
x[12] = 12
x[13] = 14
x[14] = 14
x[15] = 16
x[16] = 16.5
x[17] = 17.5
x[18] = 18.5
Note the change from incorrect to correct behaviour between the groups-of-4 and
the last three entries.
Changing the loop variable from "size_t" to "unsigned int" makes the problem go
away. Compiling with "-fno-associative-math" or, strangely,
"-fno-reciprocal-math" also makes it go away.</pre>
</div>
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