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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - x86-64: __builtin_bit_cast(anything, long double) wrongly errors with "indeterminate value""
href="https://bugs.llvm.org/show_bug.cgi?id=51759">51759</a>
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<th>Summary</th>
<td>x86-64: __builtin_bit_cast(anything, long double) wrongly errors with "indeterminate value"
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<th>Product</th>
<td>clang
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<th>Version</th>
<td>trunk
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>arthur.j.odwyer@gmail.com
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>On a plain old x86-64 Linux or OSX system, where `long double` is 128-bit and
IEC559 standard as far as I know:
constexpr int f() {
long double ld = 1.0;
__uint128_t u = __builtin_bit_cast(__uint128_t, ld);
return 4;
}
int a[f()];
GCC is happy with this program. Clang complains:
<source>:1:15: error: constexpr function never produces a constant expression
[-Winvalid-constexpr]
constexpr int f() {
^
<source>:3:21: note: indeterminate value can only initialize an object of type
'unsigned char' or 'std::byte'; 'unsigned __int128' is invalid
__uint128_t u = __builtin_bit_cast(__uint128_t, ld);
^
And it's not only when converting to __uint128_t; converting to a trivial
16-byte struct type gives the same diagnostic. It seems that Clang flatly
doesn't support __builtin_bit_cast from a `long double` source operand.
<a href="https://godbolt.org/z/EYraxf9aY">https://godbolt.org/z/EYraxf9aY</a></pre>
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