Handle constant CONSTRUCTORs in operand_compare

This teaches operand_compare to compare constant CONSTRUCTORs, which is
quite helpful for so-called fat pointers in Ada, i.e. objects that are
semantically pointers but are represented by structures made up of two
pointers.  This is modeled on the implementation present in the ICF pass.

gcc/
	* fold-const.cc (operand_compare::operand_equal_p) <CONSTRUCTOR>:
	Deal with nonempty constant CONSTRUCTORs.
	(operand_compare::hash_operand) <CONSTRUCTOR>: Hash DECL_FIELD_OFFSET
	and DECL_FIELD_BIT_OFFSET for FIELD_DECLs.

gcc/testsuite/
	* gnat.dg/opt103.ads, gnat.dg/opt103.adb: New test.

1 files changed, 67 insertions, 7 deletions

diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
index 4076773..332bc8a 100644
--- a/gcc/fold-const.cc
+++ b/gcc/fold-const.cc
@@ -3315,9 +3315,65 @@ operand_compare::operand_equal_p (const_tree arg0, const_tree arg1,
 				flags | OEP_ADDRESS_OF
 				| OEP_MATCH_SIDE_EFFECTS);
       case CONSTRUCTOR:
-	/* In GIMPLE empty constructors are allowed in initializers of
-	   aggregates.  */
-	return !CONSTRUCTOR_NELTS (arg0) && !CONSTRUCTOR_NELTS (arg1);
+	{
+	  /* In GIMPLE empty constructors are allowed in initializers of
+	     aggregates.  */
+	  if (!CONSTRUCTOR_NELTS (arg0) && !CONSTRUCTOR_NELTS (arg1))
+	    return true;
+
+	  /* See sem_variable::equals in ipa-icf for a similar approach.  */
+	  tree typ0 = TREE_TYPE (arg0);
+	  tree typ1 = TREE_TYPE (arg1);
+
+	  if (TREE_CODE (typ0) != TREE_CODE (typ1))
+	    return false;
+	  else if (TREE_CODE (typ0) == ARRAY_TYPE)
+	    {
+	      /* For arrays, check that the sizes all match.  */
+	      const HOST_WIDE_INT siz0 = int_size_in_bytes (typ0);
+	      if (TYPE_MODE (typ0) != TYPE_MODE (typ1)
+		  || siz0 < 0
+		  || siz0 != int_size_in_bytes (typ1))
+		return false;
+	    }
+	  else if (!types_compatible_p (typ0, typ1))
+	    return false;
+
+	  vec<constructor_elt, va_gc> *v0 = CONSTRUCTOR_ELTS (arg0);
+	  vec<constructor_elt, va_gc> *v1 = CONSTRUCTOR_ELTS (arg1);
+	  if (vec_safe_length (v0) != vec_safe_length (v1))
+	    return false;
+
+	  /* Address of CONSTRUCTOR is defined in GENERIC to mean the value
+	     of the CONSTRUCTOR referenced indirectly.  */
+	  flags &= ~OEP_ADDRESS_OF;
+
+	  for (unsigned idx = 0; idx < vec_safe_length (v0); ++idx)
+	    {
+	      constructor_elt *c0 = &(*v0)[idx];
+	      constructor_elt *c1 = &(*v1)[idx];
+
+	      /* Check that the values are the same...  */
+	      if (c0->value != c1->value
+		  && !operand_equal_p (c0->value, c1->value, flags))
+		return false;
+
+	      /* ... and that they apply to the same field!  */
+	      if (c0->index != c1->index
+		  && (TREE_CODE (typ0) == ARRAY_TYPE
+		      ? !operand_equal_p (c0->index, c1->index, flags)
+		      : !operand_equal_p (DECL_FIELD_OFFSET (c0->index),
+					  DECL_FIELD_OFFSET (c1->index),
+					  flags)
+			|| !operand_equal_p (DECL_FIELD_BIT_OFFSET (c0->index),
+					     DECL_FIELD_BIT_OFFSET (c1->index),
+					     flags)))
+		return false;
+	    }
+
+	  return true;
+	}
+
       default:
 	break;
       }
@@ -3703,9 +3759,7 @@ operand_compare::operand_equal_p (const_tree arg0, const_tree arg1,
 	     elements.  Individual elements in the constructor must be
 	     indexed in increasing order and form an initial sequence.
 
-	     We make no effort to compare constructors in generic.
-	     (see sem_variable::equals in ipa-icf which can do so for
-	      constants).  */
+	     We make no effort to compare nonconstant ones in GENERIC.  */
 	  if (!VECTOR_TYPE_P (TREE_TYPE (arg0))
 	      || !VECTOR_TYPE_P (TREE_TYPE (arg1)))
 	    return false;
@@ -3887,7 +3941,13 @@ operand_compare::hash_operand (const_tree t, inchash::hash &hstate,
 	    /* In GIMPLE the indexes can be either NULL or matching i.  */
 	    if (field == NULL_TREE)
 	      field = bitsize_int (idx);
-	    hash_operand (field, hstate, flags);
+	    if (TREE_CODE (field) == FIELD_DECL)
+	      {
+		hash_operand (DECL_FIELD_OFFSET (field), hstate, flags);
+		hash_operand (DECL_FIELD_BIT_OFFSET (field), hstate, flags);
+	      }
+	    else
+	      hash_operand (field, hstate, flags);
 	    hash_operand (value, hstate, flags);
 	  }
 	return;