[Bug c/71538] New: Obvious optimization related to arrays aren't performed.

[sasho648 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71538

            Bug ID: 71538
           Summary: Obvious optimization related to arrays aren't
                    performed.
           Product: gcc
           Version: 6.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sasho648 at gmail dot com
  Target Milestone: ---

Consider this code:

#include <stdio.h>

extern void f(int *p) 
{
        p = *((int (*)[6])p);

        if(p == NULL)
                printf("NULL");
}


It's obvious (at least for me) that p can't be possibly NULL because it's
assigned the value of pointer to the first element of array with 6 elements.
However the assembly output of this code for my native machine compiled on
linux (x86-64 - dumped using ida pro with general assembler for intel) is:

;f function

                test    rdi, rdi
                jz      short loc_400550
                rep retn
; ---------------------------------------------------------------------------
                align 10h

loc_400550:                             
                mov     edi, (offset format+4) ; "NULL"
                xor     eax, eax
                jmp     _printf
;f function end

As you see the branch where printf is called with "NULL" is still present in
the code although on theory it should never be reached.

I don't think there is requirement by the standard to disallow evaluation of
this expression (as opposed to for example *&p).