https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79578
Bug ID: 79578
Summary: Unnecessary instructions in generated code
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: maxim.yegorushkin at gmail dot com
Target Milestone: ---
When the following code is compiled with gcc-7 and earlier versions with flags
--std=c++14 -O3 -pthread:
#include <new>
#include <cstdint>
struct A
{
std::uint16_t a, b;
};
A* f(char* b) __attribute__((noinline));
A* f(char* b) {
auto a = new(b) A{};
a->a = 1;
a->b = 2;
return a;
}
int main() {
char b[sizeof(A)] alignas(A);
f(b);
}
The generated code for f contains unnecessary instructions:
f(char*):
testq %rdi, %rdi <--- unnecessary code begin
movq %rdi, %rax
je .L2
movl $0, (%rdi) <--- unnecessary code end
.L2:
movl $131073, (%rax)
ret
I expect the generated code to be just:
f(char*):
movl $131073, (%rdi)
ret
Because:
1. operator new(size_t, void*) is an inline function, its definition is
available in this translation unit.
2. The code assigns through that pointer, so the pointer must be valid.
Therefore there is no need to test the result of new for 0.
3. Static single assignment form must notice that the assignment of 0 is
redundant.
