[Bug middle-end/83215] New: C++: struct with char-array assumed to alias with everything

[dominik.infuehr at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83215

            Bug ID: 83215
           Summary: C++: struct with char-array assumed to alias with
                    everything
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dominik.infuehr at gmail dot com
  Target Milestone: ---

TBAA seems to be more conservative for C++ in the following example since GCC
7:

struct mytest {
  float a;
#ifdef WITH_BUFFER
  char buf[256];
#endif
};

int foo(mytest *m, int *i) {
  int tmp = *i; // first load
  m->a = 10.0f;
  return tmp + *i; // second load since GCC 7
}

Since GCC 7 this code generates two loads with -DWITH_BUFFER, without buf in
the struct there is just one load. buf isn't touched at all in this function.
This only affects C++-code, not the C-frontend.

TYPE_TYPLESS_STORAGE is set for this struct, aggregates with this flag set are
assumed to alias with everything in gcc/alias.c(get_alias_set).

Seems to be introduced with revision 246866
(https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=246866).