https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115693
Bug ID: 115693
Summary: 8 std::byte std::array comparison potential missed
optimization
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: paulhaile3 at gmail dot com
Target Milestone: ---
It seems to be inconsistent with char array comparison and with std::bit_cast
Code Snippet: compiled with x86-64 gcc 14.1 with flags --std=c++20 -O3
#include <array>
#include <bit>
#include <cstdint>
bool compare1(const std::array<std::byte, 8> &p, std::array<std::byte, 8> r)
{
return p == r;
}
bool compare2(const std::array<char, 8> &p, std::array<char, 8> r)
{
return p == r;
}
// same assembly if you use char instead of byte
bool compare3(const std::array<std::byte, 8> &p, std::array<std::byte, 8> r)
{
return std::bit_cast<uint64_t>(p) == std::bit_cast<uint64_t>(r);
}
generated assembly:
compare1(std::array<std::byte, 8ul> const&, std::array<std::byte, 8ul>):
mov rax, rsi
cmp BYTE PTR [rdi], sil
jne .L9
cmp ah, BYTE PTR [rdi+1]
jne .L9
mov rdx, rsi
shr rdx, 16
cmp BYTE PTR [rdi+2], dl
jne .L9
mov rdx, rsi
shr rdx, 24
cmp BYTE PTR [rdi+3], dl
jne .L9
mov rdx, rsi
shr rdx, 32
cmp BYTE PTR [rdi+4], dl
jne .L9
mov rdx, rsi
shr rdx, 40
cmp BYTE PTR [rdi+5], dl
jne .L9
mov rdx, rsi
shr rdx, 48
cmp BYTE PTR [rdi+6], dl
jne .L9
shr rax, 56
cmp BYTE PTR [rdi+7], al
sete al
ret
.L9:
xor eax, eax
ret
compare2(std::array<char, 8ul> const&, std::array<char, 8ul>):
cmp QWORD PTR [rdi], rsi
sete al
ret
compare3(std::array<std::byte, 8ul> const&, std::array<std::byte, 8ul>):
cmp QWORD PTR [rdi], rsi
sete al
ret
link to compiler explorer:
https://godbolt.org/#g:!((g:!((g:!((h:codeEditor,i:(filename:'1',fontScale:14,fontUsePx:'0',j:1,lang:c%2B%2B,selection:(endColumn:1,endLineNumber:4,positionColumn:1,positionLineNumber:4,selectionStartColumn:1,selectionStartLineNumber:4,startColumn:1,startLineNumber:4),source:'%23include+%3Carray%3E%0A%23include+%3Cbit%3E%0A%23include+%3Ccstdint%3E%0A%0Abool+compare1(const+std::array%3Cstd::byte,+8%3E+%26p,+std::array%3Cstd::byte,+8%3E+r)+%7Breturn+p+%3D%3D+r%3B%7D%0Abool+compare2(const+std::array%3Cchar,+8%3E+%26p,+std::array%3Cchar,+8%3E+r)+%7B+return+p+%3D%3D+r%3B+%7D%0Abool+compare3(const+std::array%3Cstd::byte,+8%3E+%26p,+std::array%3Cstd::byte,+8%3E+r)+%7Breturn+std::bit_cast%3Cuint64_t%3E(p)+%3D%3D+std::bit_cast%3Cuint64_t%3E(r)%3B%7D'),l:'5',n:'1',o:'C%2B%2B+source+%231',t:'0')),k:63.98956975228162,l:'4',n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:g141,filters:(b:'0',binary:'1',binaryObject:'1',commentOnly:'0',debugCalls:'1',demangle:'0',directives:'0',execute:'1',intel:'0',libraryCode:'0',trim:'1',verboseDemangling:'0'),flagsViewOpen:'1',fontScale:14,fontUsePx:'0',j:2,lang:c%2B%2B,libs:!(),options:'--std%3Dc%2B%2B20+-O3',overrides:!(),selection:(endColumn:1,endLineNumber:1,positionColumn:1,positionLineNumber:1,selectionStartColumn:1,selectionStartLineNumber:1,startColumn:1,startLineNumber:1),source:1),l:'5',n:'0',o:'+x86-64+gcc+14.1+(Editor+%231)',t:'0')),k:36.01043024771838,l:'4',n:'0',o:'',s:0,t:'0')),l:'2',n:'0',o:'',t:'0')),version:4
