[Bug tree-optimization/111779] Fail to vectorize the struct include struct

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111779

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |tree-optimization
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
                 CC|                            |jamborm at gcc dot gnu.org
   Last reconfirmed|                            |2023-10-12

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
The issue is the aggregate copy:

t.c:26:22: missed:  not vectorized: more than one data ref in stmt: a = *_3;

which SRA fails to scalarize:

  <bb 3> [local count: 955630224]:
  # s_23 = PHI <s_18(3), 0(2)>
  # i_25 = PHI <i_20(3), 0(2)>
  _1 = (long unsigned int) i_25;
  _2 = _1 * 24;
  _3 = x_16(D) + _2;
  a = *_3;
  _4 = BIT_FIELD_REF <a.b4, 8, 64>;
  _12 = _4 & 1;
  _6 = (int) _12;
  s_18 = _6 + s_23;
  a ={v} {CLOBBER(eol)};
  i_20 = i_25 + 1;
  if (y_14(D) > i_20)

Candidate (2778): a
...
! Disqualifying a - No scalar replacements to be created.

the BIT_FIELD_REF is already created by early folding in
optimize_bit_field_compare folding (int) a.b4.f != 0

    s = ((int) NON_LVALUE_EXPR <BIT_FIELD_REF <a.b4, 8, 64>> & 1) + s;

SRA could handle BIT_FIELD_REFs just fine - esp. quantities with
a byte size.  And then this folding is just premature...

Removing the folding that handles BF != CST fixes it.  I know removing
all of it, esp. BF != BF will regress some stuff.  I'll put this half-way
patch through testing.

diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
index 82299bb7f1d..3db383360d6 100644
--- a/gcc/fold-const.cc
+++ b/gcc/fold-const.cc
@@ -4695,7 +4695,7 @@ optimize_bit_field_compare (location_t loc, enum
tree_code code,
     return 0;

   if (const_p)
-    rreversep = lreversep;
+    return 0;
   else
    {
      /* If this is not a constant, we can only do something if bit positions,