https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111388
Bug ID: 111388
Summary: std:.get_if variant, unnecessary branch when outside
of if statement
Product: gcc
Version: 13.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: federico at kircheis dot it
Target Milestone: ---
Example of code (https://godbolt.org/z/M1bWf5sz3)
----
#include <variant>
struct interface{
virtual ~interface()= default;
virtual int foo() const = 0;
};
struct a : interface{
int foo() const override;
};
struct b : interface{
int foo() const override;
};
class a_or_b{
std::variant<a,b> ab;
public:
a_or_b() = delete;
a_or_b(a _) : ab(_){}
a_or_b(b _) : ab(_){}
interface* operator->() noexcept {
if (interface* ptr = std::get_if<0>(&ab); ptr) {
return ptr;
}
#if 0
else if (interface* ptr = std::get_if<1>(&ab); ptr) {
return ptr;
}
#else
return std::get_if<1>(&ab);
#endif
}
};
int bar3(a_or_b& ab){
return ab->foo()+1;
}
----
With `#if 1`, the generated code looks like
----
bar3(a_or_b&):
sub rsp, 8
mov rax, QWORD PTR [rdi]
call [QWORD PTR [rax+16]]
add rsp, 8
add eax, 1
ret
----
while with `#if 0`, the assembly looks like
----
bar3(a_or_b&):
cmp BYTE PTR [rdi+8], 0
jne .L2
sub rsp, 8
mov rax, QWORD PTR [rdi]
call [QWORD PTR [rax+16]]
add rsp, 8
add eax, 1
ret
bar3(a_or_b&) [clone .cold]:
.L2:
mov rax, QWORD PTR ds:0
ud2
----
If I'm not mistake, with `return std::get_if<1>(&ab);` the compiler verifies if
the return of get_if is nullptr, and if it is, then sets the return value to
nullptr, which is unnecessary.
With `if 1`, the result is passed as is.
AFAIK the generated assembly is functionally equivalant, but the "more safe"(1)
version less optimal
1) more safe as in "there is no UB if the class changes and the variant could
be empty or hold another type".
NOTE: replacing "std::get_if<0>"/"std::get_if<1>" with
"std::get_if<a>/std::get_if<b>" does not make a relevante difference, the
generated code is the same for both "if 0" and "if 1".
For what is worth, clang generates the same code for "if 0" and "if 1".
