[Bug tree-optimization/86914] New: gcc 8.1 -O2 generates wrong code with strlen() of pointers within one-element arrays of structures

[even.rouault at spatialys dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86914

Bug ID: 86914
   Summary: gcc 8.1 -O2 generates wrong code with strlen() of
pointers within one-element arrays of structures
   Product: gcc
   Version: unknown
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: tree-optimization
  Assignee: unassigned at gcc dot gnu.org
  Reporter: even.rouault at spatialys dot com
  Target Milestone: ---

The following code
{{{
#include 

struct s
{
int i;
char c[1];
};

size_t foo(struct s* p)
{
return strlen(p->c+1);
}
}}}

compiled with gcc 8.1 -O2 generates the following code
{{{
 :
   0:   31 c0   xor%eax,%eax
   2:   c3  retq   
}}}

returning 0

Previous gcc versions generate the "correct" code
{{{
 :
   0:   48 83 c7 05 add$0x5,%rdi
   4:   e9 00 00 00 00  jmpq   9   5: R_X86_64_PC32   
strlen-0x4
}}}

According to https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html , """Although
using one-element arrays this way is discouraged, GCC handles accesses to
trailing one-element array members analogously to zero-length arrays. """

If modifying c[1] to be c[0], gcc 8.1 -O2 generates the correct code. If
changing strlen(p->c+1) to strlen(p->c), the correct code is although generated

So it looks to be a too agressive optimization.

[Bug tree-optimization/86914] gcc 8.1 -O2 generates wrong code with strlen() of pointers within one-element arrays of structures

[even.rouault at spatialys dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86914

--- Comment #1 from Even Rouault  ---
Interstingly with gcc 8.2, char c[0] doesn't generate the appropriate code. The
only way of making it work is to use char c[] syntax