https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93059
Bug ID: 93059
Summary: 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
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: euloanty at live dot com
Target Milestone: ---
https://godbolt.org/z/SPktTz
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
