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Subject: [Bug tree-optimization/95663] static_cast checks for null even when
 the pointer is dereferenced
Date: Thu, 07 Jan 2021 13:14:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95663

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?=