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From: "vanyacpp at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/98501] New: potential optimization for base<->derived
 pointer casts
Date: Sat, 02 Jan 2021 22:56:36 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98501

            Bug ID: 98501
           Summary: potential optimization for base<->derived pointer
                    casts
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vanyacpp at gmail dot com
  Target Milestone: ---

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?=