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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Conversion from uint64_t to double produces wrong result for rounding downward"
href="https://bugs.llvm.org/show_bug.cgi?id=47393">47393</a>
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<th>Summary</th>
<td>Conversion from uint64_t to double produces wrong result for rounding downward
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>trunk
</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>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>LLVM Codegen
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>chfast@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>When floating-point rounding direction is set to "towards negative infinity"
(FE_DOWNWARD) and conversion from uint64_t(0) to double is performed the result
is -0.0 instead of +0.0.
Architecture: x86_64
Clang version: reproducible in all known versions, including trunk
Options: happens for all combinations of -O0, -O2, -frounding-math,
-fsigned-zeros.
Code (-lm is needed)
enum { FE_DOWNWARD = 0x400 };
extern int fesetround(int rounding_direction);
__attribute__((noinline))
double ui64_to_f64(unsigned long x)
{
return (double)x;
}
int main()
{
fesetround(FE_DOWNWARD);
double z = ui64_to_f64(0);
return __builtin_signbit(z) != 0; // expected 0
}
Can be played with at <a href="https://godbolt.org/z/jvPGqW">https://godbolt.org/z/jvPGqW</a>.
I believe the main issue is in LLVM.
Without -frounding-math the Clang uses uitofp instruction. The reference says
this should be fine in this context as "it is rounded using the default
rounding mode".
With -frounding-math the
call double @llvm.experimental.constrained.uitofp.f64.i64(i64 %3, metadata
!"round.dynamic", metadata !"fpexcept.ignore")
is used, but the result is wrong nevertheless.</pre>
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