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

The excess precision support broke building skia (dependency of firefox)
on ia32 (it has something like the a constexpr variable), but as the other
cases show, it is actually a preexisting problem if one uses casts from
constants with wider floating point types.
The problem is that cxx_eval_constant_expression tries to short-cut
processing of TREE_CONSTANT CONSTRUCTORs if they satisfy
reduced_constant_expression_p - instead of calling cxx_eval_bare_aggregate
on them it just verifies flags and if they are TREE_CONSTANT even after
that, just fold.
Now, on the testcase we have a TREE_CONSTANT CONSTRUCTOR containing
TREE_CONSTANT NOP_EXPR of REAL_CST.  And, fold, which isn't recursive,
doesn't optimize that into VECTOR_CST, while later on we are only able
to optimize VECTOR_CST arithmetics, not arithmetics with vector
CONSTRUCTORs.
The following patch fixes that by rejecting CONSTRUCTORs with vector type
in reduced_constant_expression_p regardless of whether they have
CONSTRUCTOR_NO_CLEARING set or not, folding result in cxx_eval_bare_aggregate
even if nothing has changed but it wasn't non-constant and removing folding
from the TREE_CONSTANT reduced_constant_expression_p short-cut.

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.

1 files changed, 13 insertions, 8 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 0366396..c3ee970 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -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_ctx *ctx, tree t,
 	  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 constexpr_ctx *ctx, tree t,
     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);