[PATCH] c++, v2: Don't shortcut TREE_CONSTANT vector type CONSTRUCTORs in cxx_eval_constant_expression [PR107295]

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Oct 20, 2022 at 10:51:14AM -0400, Jason Merrill wrote:
> That seems like a bug; for VECTOR_TYPE we should fold even if !changed.
> 
> > Also, the reason for the short-cutting is I think trying to avoid
> > allocating a new CONSTRUCTOR when nothing changes and we just create
> > GC garbage by it.
> 
> We might limit the shortcut to non-vector types by hoisting the vector check
> in reduced_constant_expression_p out of the CONSTRUCTOR_NO_CLEARING
> condition:
> 
> >       if (CONSTRUCTOR_NO_CLEARING (t))
> >         {
> >           if (TREE_CODE (TREE_TYPE (t)) == VECTOR_TYPE)
> >             /* An initialized vector would have a VECTOR_CST.  */
> >             return false;
> 
> then we could remove the fold in the shortcut.

Ok, so like this?
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2022-10-21  Jakub Jelinek  <jakub@redhat.com>

	PR c++/107295
	* constexpr.cc (reduced_constant_expression_p) <case CONSTRUCTOR>:
	Return false for VECTOR_TYPE CONSTRUCTORs even without
	CONSTRUCTOR_NO_CLEARING set on them.
	(cxx_eval_bare_aggregate): If constant but !changed, fold before
	returning VECTOR_TYPE_P CONSTRUCTOR.
	(cxx_eval_constant_expression) <case CONSTRUCTOR>: Don't fold
	TREE_CONSTANT CONSTRUCTOR, just return it.

	* g++.dg/ext/vector42.C: New test.

--- gcc/cp/constexpr.cc.jj	2022-10-19 11:20:28.960225787 +0200
+++ gcc/cp/constexpr.cc	2022-10-20 18:43:42.952440364 +0200
@@ -3104,12 +3104,12 @@ reduced_constant_expression_p (tree t)
     case CONSTRUCTOR:
       /* And we need to handle PTRMEM_CST wrapped in a CONSTRUCTOR.  */
       tree field;
+      if (TREE_CODE (TREE_TYPE (t)) == VECTOR_TYPE)
+	/* An initialized vector would have a VECTOR_CST.  */
+	return false;
       if (CONSTRUCTOR_NO_CLEARING (t))
 	{
-	  if (TREE_CODE (TREE_TYPE (t)) == VECTOR_TYPE)
-	    /* An initialized vector would have a VECTOR_CST.  */
-	    return false;
-	  else if (TREE_CODE (TREE_TYPE (t)) == ARRAY_TYPE)
+	  if (TREE_CODE (TREE_TYPE (t)) == ARRAY_TYPE)
 	    {
 	      /* There must be a valid constant initializer at every array
 		 index.  */
@@ -4956,8 +4956,14 @@ cxx_eval_bare_aggregate (const constexpr
 	  TREE_SIDE_EFFECTS (ctx->ctor) = side_effects_p;
 	}
     }
-  if (*non_constant_p || !changed)
+  if (*non_constant_p)
     return t;
+  if (!changed)
+    {
+      if (VECTOR_TYPE_P (type))
+	t = fold (t);
+      return t;
+    }
   t = ctx->ctor;
   if (!t)
     t = build_constructor (type, NULL);
@@ -7387,11 +7393,10 @@ cxx_eval_constant_expression (const cons
     case CONSTRUCTOR:
       if (TREE_CONSTANT (t) && reduced_constant_expression_p (t))
 	{
-	  /* Don't re-process a constant CONSTRUCTOR, but do fold it to
-	     VECTOR_CST if applicable.  */
+	  /* Don't re-process a constant CONSTRUCTOR.  */
 	  verify_constructor_flags (t);
 	  if (TREE_CONSTANT (t))
-	    return fold (t);
+	    return t;
 	}
       r = cxx_eval_bare_aggregate (ctx, t, lval,
 				   non_constant_p, overflow_p);
--- gcc/testsuite/g++.dg/ext/vector42.C.jj	2022-10-20 17:57:42.767848544 +0200
+++ gcc/testsuite/g++.dg/ext/vector42.C	2022-10-20 17:57:42.767848544 +0200
@@ -0,0 +1,12 @@
+// PR c++/107295
+// { dg-do compile { target c++11 } }
+
+template <typename T> struct A {
+  typedef T __attribute__((vector_size (sizeof (int)))) V;
+};
+template <int, typename T> using B = typename A<T>::V;
+template <typename T> using V = B<4, T>;
+using F = V<float>;
+constexpr F a = F () + 0.0f;
+constexpr F b = F () + (float) 0.0;
+constexpr F c = F () + (float) 0.0L;


	Jakub