[Bug fortran/92698] Unnecessary copy in overlapping array assignment

[tkoenig at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92698

Thomas Koenig <tkoenig at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2019-12-01
     Ever confirmed|0                           |1
           Severity|normal                      |enhancement

--- Comment #3 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
So, here's a C test case, also functionally equivalent to
a memmove.

#include <stdlib.h>

void foo (double *a, int n, int len, int offset)
{
  int i;
  double *p;
  p = malloc(sizeof(*p) * len);
  for (i=0; i<len; i++)
    p[i] = a[n+i];

  for (i=0; i<len; i++)
    a[n+offset+i] = p[i];

  free(p);
}

So, this is something the middle end could possibly catch, it doesn't
do so at the moment.

Regarding loop versioning wrt unity / non-unity strides: We've had that
discussion for a more general case, I cannot find the PR at the moment,
it may have been closed as WONTFIX.  One problem that we saw was that
the time overhead and growth of code size could be too high, especially
with short arrays.

With the CONTIGOUS attribute and the IS_CONTIGUOUS function, the user
now can control what needs to happen. Older code, by the way, usually
has

    real a(n)

style arguments, and these are always contiguous (and may need to
be converted on entry / exit).

So, what's left? We could indeed translate

  a(n:n+len-1) = a(n+offset:n+offset+len-1)

into pseudo-code

  if (is_contiguous(a)) then
      i = n + offset + len - 1 - (n + offset) + 1
      if (bounds_checking) then
        if (n + len - 1 /=i) stop "out of bounds error"
      end if
      if (i>0) then
          call memcpy (c_loc(a(n)), c_loc(a(n+offset), i*c_sizeof(a(1))
      end if
  else
      a(n:n+len-1) = a(n+offset:n+offset+len-1)

(this is pseudo-code because we cannot use the real c_loc because
a is usually not a target).

So, confirmed as an enhancement request.