Re: Handle CONSTRUCTOR in operand_equal_p

[Jan Hubicka <hubicka at ucw dot cz>]

Hi,
this is updated patch I am going to commit.  As discussed, we also need to match
non-empty CONSTRUCTOR of vectors, but those should never be having CONSTANT flags
set, so they need care in the other path trhough operand_equal_p, so I will post
separate patch for that.

I updated the comment per Richard's comment and added a testcase.  Vuriously
enoug we fold "val? (struct a){} : (struct a){}" only in PRE (tailmerging).
W/o the patch even .optimized dump has the test and we fold it away only after
lowering to RTL:
ret (int val)
{
  struct a D.1764;

  <bb 2>:
  if (val_2(D) != 0)
    goto <bb 3>;
  else
    goto <bb 4>;

  <bb 3>:
  D.1764 = {};
  goto <bb 5>;

  <bb 4>:
  D.1764 = {};

  <bb 5>:
  return D.1764;

}

ret:
.LFB0:
        .cfi_startproc
        xorl    %eax, %eax
        ret
        .cfi_endproc

With the patch we get:

ret (int val)
{
  struct a D.1764;

  <bb 2>:
  D.1764 = {};
  return D.1764;

}

Honza

	* fold-const.c (operand_equal_p): Add code matching empty
	constructors.
	* gcc.dg/tree-ssa/operand-equal-1.c: Verify that empty constructors
	are matched.

Index: fold-const.c
===================================================================
--- fold-const.c	(revision 229133)
+++ fold-const.c	(working copy)
@@ -2892,6 +2892,11 @@ operand_equal_p (const_tree arg0, const_
 	return operand_equal_p (TREE_OPERAND (arg0, 0), TREE_OPERAND (arg1, 0),
 				flags | OEP_ADDRESS_OF
 				| OEP_CONSTANT_ADDRESS_OF);
+      case CONSTRUCTOR:
+	/* In GIMPLE empty constructors are allowed in initializers of
+	   aggregates.  */
+	return (!vec_safe_length (CONSTRUCTOR_ELTS (arg0))
+		&& !vec_safe_length (CONSTRUCTOR_ELTS (arg1)));
       default:
 	break;
       }
Index: testsuite/gcc.dg/tree-ssa/operand-equal-1.c
===================================================================
--- testsuite/gcc.dg/tree-ssa/operand-equal-1.c	(revision 0)
+++ testsuite/gcc.dg/tree-ssa/operand-equal-1.c	(revision 0)
@@ -0,0 +1,8 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-pre" } */
+struct a {int a,b;};
+struct a ret(int val)
+{
+   return val? (struct a){} : (struct a){};
+}
+/* { dg-final { scan-tree-dump-not "if " "pre"} } */