[Bug c++/98501] New: potential optimization for base<->derived pointer casts

[Bug c++/98501] New: potential optimization for base<->derived pointer casts

[vanyacpp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98501

            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?

[Bug c++/98501] potential optimization for base<->derived pointer casts

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98501

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think there's a duplicate of this PR.

[Bug c++/98501] potential optimization for base<->derived pointer casts

[vanyacpp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98501

--- Comment #2 from Ivan Sorokin <vanyacpp at gmail dot com> ---
(In reply to Richard Biener from comment #1)
> I think there's a duplicate of this PR.

I searched the list of bugs and I found PR95663. Is it it?

[Bug c++/98501] potential optimization for base<->derived pointer casts

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98501

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, that looks like the same thing. Thanks for finding it.

*** This bug has been marked as a duplicate of bug 95663 ***