[Bug target/113779] Very inefficient m68k code generated for simple copy loop

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113779

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-02-06
     Ever confirmed|0                           |1

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
It's already visible with a simple

void f(const long* src, long* dst)
{
  *dst++ = *src++;
  *dst = *src;
}

where we expand to RTL from

  _1 = *src_3(D);
  *dst_4(D) = _1;
  _2 = MEM[(const long int *)src_3(D) + 4B];
  MEM[(long int *)dst_4(D) + 4B] = _2;

there's nothing on GIMPLE that would split the add and RTLs auto-inc-dec
pass doesn't do anything either.  We'd need a form of "strength-reduction"
or maybe targets prefering auto-inc/dec should not legitimize constant
offsets before reload ...

Note with one more copy you then see

  _1 = *src_4(D);
  *dst_5(D) = _1;
  _2 = MEM[(const long int *)src_4(D) + 4B];
  MEM[(long int *)dst_5(D) + 4B] = _2;
  _3 = MEM[(const long int *)src_4(D) + 8B];
  MEM[(long int *)dst_5(D) + 8B] = _3;

and naiively splitting gives you

  src_6 = src_4(D) + 4;
  src_7 = src_4(D) + 8;

that said, it's really sth for RTL since it's going to be highly target
dependent which form is more efficient.  The auto-inc pass is well
structured, so it should be possible to extend it.