[Bug c++/111388] New: std:.get_if variant, unnecessary branch when outside of if statement

[Bug c++/111388] New: std:.get_if variant, unnecessary branch when outside of if statement

[federico at kircheis dot it]

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".

[Bug tree-optimization/111388] std:.get_if variant, unnecessary branch when outside of if statement

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111388

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note variant can still hold neither ...
You might need to add a check for valueless_by_exception here.

But variant::index could be improved to say the only values that are valid is
[0,N],[-1]. and that will remove the check.


Note clang even produces the similar code even with libc++:
        xorl    %eax, %eax
        cmpl    $2, 8(%rdi)
        cmovaeq %rax, %rdi

Just GCC isolates the null pointer access.

[Bug tree-optimization/111388] std:.get_if variant, unnecessary branch when outside of if statement

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111388

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Adding:
```
        if (ab.index()>=2)
          __builtin_unreachable();
```
to operator->

Also fixes the issue.
C++23 would be:
[[assume(ab.index()<2)]];
(Except that does not optimize currently).