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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Same issue with GCC. char and char8_t does not talk with each other with memcpy. std::copy std::copy_n, std::fill, std::fill_n, std::uninitialized_copy std::uninitialized_copy_n, std::fill, std::uninitialized_fill_n fails to convert to memxxx functions"
href="https://bugs.llvm.org/show_bug.cgi?id=45104">45104</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Same issue with GCC. char and char8_t does not talk with each other with memcpy. std::copy std::copy_n, std::fill, std::fill_n, std::uninitialized_copy std::uninitialized_copy_n, std::fill, std::uninitialized_fill_n fails to convert to memxxx functions
</td>
</tr>
<tr>
<th>Product</th>
<td>libc++
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>All Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>euloanty@live.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
</td>
</tr></table>
<p>
<div>
<pre><a href="https://godbolt.org/z/dBEAS">https://godbolt.org/z/dBEAS</a>-
Miss an optimization point.
Same issue with GCC bug 93923
<a href="https://godbolt.org/z/SPktTz">https://godbolt.org/z/SPktTz</a>
All these functions should generate exactly the same assembly but they do not.
GCC does not treat char and char8_t the same because libstdc++ does not do this
check. I did my native manually fix and it works. (does not do mul_overflow
something)
//g++ -S copy.cc -Ofast -std=c++2a
#include<cstring>
#include<algorithm>
#include<array>
#include<concepts>
#include<iterator>
auto copy_char8_t_array(char* out,std::array<char8_t,2> const& bits)
{
return std::copy_n(bits.data(),bits.size(),out);
}
auto memcpy_char8_t_array(char* out,std::array<char8_t,2> const& bits)
{
std::memcpy(out,bits.data(),bits.size());
return bits.size();
}
auto copy_char_array(char* out,std::array<char,2> const& bits)
{
return std::copy_n(bits.data(),bits.size(),out);
}
auto memcpy_char_array(char* out,std::array<char,2> const& bits)
{
std::memcpy(out,bits.data(),bits.size());
return bits.size();
}
auto copy_char_array_chars(char8_t* out,std::array<char8_t,2> const& bits)
{
return std::copy_n(bits.data(),bits.size(),out);
}
auto memcpy_char_array_array_chars(char8_t* out,std::array<char8_t,2> const&
bits)
{
std::memcpy(out,bits.data(),bits.size());
return bits.size();
}
auto copy_char_array(char8_t* out,std::array<char,2> const& bits)
{
return std::copy_n(bits.data(),bits.size(),out);
}
auto memcpy_char_array(char8_t* out,std::array<char,2> const& bits)
{
std::memcpy(out,bits.data(),bits.size());
return bits.size();
}
template<std::input_iterator input_iter,std::input_iterator output_iter>
inline constexpr output_iter my_copy_n(input_iter first,std::size_t
count,output_iter result)
{
if constexpr(std::contiguous_iterator<input_iter>&&
std::contiguous_iterator<output_iter>&&
std::is_trivially_copyable_v<typename
std::iterator_traits<input_iter>::value_type>&&
std::is_trivially_copyable_v<typename
std::iterator_traits<output_iter>::value_type>)
{
if constexpr(sizeof(std::is_trivially_copyable_v<typename
std::iterator_traits<input_iter>::value_type>)
==sizeof(std::is_trivially_copyable_v<typename
std::iterator_traits<output_iter>::value_type>))
{
memcpy(std::to_address(result),std::to_address(first),
sizeof(typename
std::iterator_traits<input_iter>::value_type)*count);
return result+count;
}
}
return std::copy_n(first,count,result);
}
auto my_copy_char_array(char8_t* out,std::array<char,2> const& bits)
{
return my_copy_n(bits.data(),bits.size(),out);
}
auto my_copy_char_array(char* out,std::array<char8_t,2> const& bits)
{
return my_copy_n(bits.data(),bits.size(),out);
}
auto uninit_copy_char_array(char8_t* out,std::array<char,2> const& bits)
{
return std::uninitialized_copy_n(bits.data(),bits.size(),out);
}
_Z29memcpy_char_array_array_charsPDuRKSt5arrayIDuLm2EE:
movzwl (%rsi), %eax
movw %ax, (%rdi)
movl $2, %eax
ret
std::copy_n generates more assembly than it should
_Z15copy_char_arrayPDuRKSt5arrayIcLm2EE:
movzbl (%rsi), %eax
movb %al, (%rdi)
movzbl 1(%rsi), %eax
movb %al, 1(%rdi)
leaq 2(%rdi), %rax
ret</pre>
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