https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116518
Bug ID: 116518
Summary: GCC does not optimize-out useless operations. Clang
does.
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: socketpair at gmail dot com
Target Milestone: ---
https://godbolt.org/z/d46448vqa
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-std=c++23 -O3 -fno-exceptions
--------------------------------
#include <algorithm>
#include <array>
#include <ranges>
#include <string>
#include <vector>
using namespace std;
namespace {
consteval string gen() { return "xxx"; }
class qwe {
public:
consteval qwe(const string& str) : s(str.size()) { str.copy(x.data(), s); }
constexpr operator const vector<char>() const {
return {x.cbegin(), x.cbegin() + s};
}
private:
// 1048576 is a clang limit
array<char, 10000> x{};
size_t s;
};
} // namespace
int fun1() {
const vector<char> v2{qwe(gen())};
return v2.size();
}
-----------------
fun1():
subq $10024, %rsp
movl $10008, %edx
xorl %esi, %esi
movq %rsp, %rdi
call memset
movl $3, %edi
movl $7895160, (%rsp)
call operator new(unsigned long)
movzwl (%rsp), %edx
movl $3, %esi
movq %rax, %rdi
movw %dx, (%rax)
movzbl 2(%rsp), %edx
movb %dl, 2(%rax)
call operator delete(void*, unsigned long)
movl $3, %eax
addq $10024, %rsp
ret
-----------------
Clang:
----------------
fun1():
mov eax, 3
ret
----------------
