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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 - Optimizer for __builtin_bswap32 fails"
href="https://bugs.llvm.org/show_bug.cgi?id=44110">44110</a>
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<th>Summary</th>
<td>Optimizer for __builtin_bswap32 fails
</td>
</tr>
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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>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<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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</tr>
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<th>Reporter</th>
<td>ditlef.martens@gmail.com
</td>
</tr>
<tr>
<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 code runs OK in Compiler Explorer with opts: -std=c++11 -O1
Fails running: -std=c++11 -O3
Byte swapping is omitted.
#include <cstdint>
#include <cstdlib>
#include <iostream>
inline
static void swap(uint32_t & value)
{
value = __builtin_bswap32(value);
}
inline
void swap(float & value)
{
swap(*(reinterpret_cast<uint32_t*>(&value)));
}
// Swap an array of values
template<typename PtrType>
inline void swap(PtrType * data, size_t length)
{
for (size_t i = 0; i < length; ++i)
swap(data[i]);
}
template<typename T>
void output(const char * s, T v) // __attribute__((noinline))
{
std::cout << s << ": " << v << std::endl;
}
int main()
{
float vec1[2];
vec1[0] = rand(); vec1[1] = rand();
const float vec1Orig = vec1[0];
output("vec1[0] value before swap", vec1[0]);
swap(vec1, 2);
if (vec1[0] == vec1Orig)
output("Failed: vec1[0] value after swap", vec1[0]);
else
output("OK: vec1[0] value after swap", vec1[0]);
}</pre>
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