[Bug tree-optimization/109667] [12/13/14 Regression] Unnecessary temporary storage used for 32-byte struct

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109667

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.3
                 CC|                            |jamborm at gcc dot gnu.org
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-04-28
           Priority|P3                          |P2

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  We end up with

void assign (struct i256 * v, long int z)
{
  struct i256 r;

  <bb 2> [local count: 536870913]:
  MEM <char[24]> [(struct i256 *)&r + 8B] = {};
  r.v[0] = z_8(D);
  *v_5(D) = r;
  r ={v} {CLOBBER(eol)};
  return;
}

and I think the issue is that DSE trims the zero-initialization.  -fno-tree-dse
fixes it - not doing DSE enables SRA:

  r = {};
  r.v[0] = z_8(D);
  *v_5(D) = r;
  r ={v} {CLOBBER(eol)};

can SRA while

  MEM <char[24]> [(struct i256 *)&r + 8B] = {};
  r.v[0] = z_8(D);
  *v_5(D) = r;
  r ={v} {CLOBBER(eol)};

FAILs:

Candidate (2756): r
Created a replacement for r offset: 0, size: 64: r$v$0D.2766
...
  MEM <char[24]> [(struct i256 *)&r + 8B] = {};
  r$v$0_13 = z_8(D);
  r.v[0] = r$v$0_13;
  *v_5(D) = r;

that was pointless - compared to

Candidate (2756): r
Will attempt to totally scalarize r (UID: 2756):
Created a replacement for r offset: 0, size: 64: r$v$0D.2766
Created a replacement for r offset: 64, size: 64: r$v$1D.2767
Created a replacement for r offset: 128, size: 64: r$v$2D.2768
Created a replacement for r offset: 192, size: 64: r$v$3D.2769
...
  r$v$0_13 = 0;
  r$v$1_2 = 0;
  r$v$2_1 = 0;
  r$v$3_11 = 0;
  r$v$0_10 = z_8(D);
  v_5(D)->v[0] = r$v$0_10;
  v_5(D)->v[1] = r$v$1_2;
  v_5(D)->v[2] = r$v$2_1;
  v_5(D)->v[3] = r$v$3_11;

possibly SRA is confused by the char[24] type.  It's going to be difficult
to do better though.