[Bug tree-optimization/95663] static_cast checks for null even when the pointer is dereferenced

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95663

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-01-07
             Status|UNCONFIRMED                 |NEW

--- Comment #17 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Copied from the PR 98501 dup:

Consider this code:

struct base1 { int a; };
struct base2 { int b; };
struct derived : base1, base2 {};

derived& to_derived_bad(base2* b)
{
    return *static_cast<derived*>(b);
}

derived& to_derived_good(base2* b)
{
    return static_cast<derived&>(*b);
}

I believe both of these functions are functionally equivalent and should
generate the same code. Both functions cast pointer from base to derived if it
is not nullptr and both cause undefined behavior if it is nullptr.

GCC optimizes to_derived_good() to a single subtraction, but it inserts
nullptr-check into to_derived_bad():

to_derived_good(base2*):
    lea rax, [rdi-4]
    ret
to_derived_bad(base2*):
    lea rax, [rdi-4]
    test rdi, rdi
    mov edx, 0
    cmove rax, rdx
    ret

Could GCC omit the nullptr-check in to_derived_bad?