c++: decl_constant_value and unsharing [PR96197]

In the testcase from the PR we're seeing excessive memory use (> 5GB)
during constexpr evaluation, almost all of which is due to the call to
decl_constant_value in the VAR_DECL/CONST_DECL branch of
cxx_eval_constant_expression.  We reach here every time we evaluate an
ARRAY_REF of a constexpr VAR_DECL, and from there decl_constant_value
makes an unshared copy of the VAR_DECL's initializer.  But unsharing
here is unnecessary because callers of cxx_eval_constant_expression
already unshare its result when necessary.

To fix this excessive unsharing, this patch adds a new defaulted
parameter unshare_p to decl_really_constant_value and
decl_constant_value so that callers can control whether to unshare.

As a simplification, we can also move the call to unshare_expr in
constant_value_1 outside of the loop, since doing unshare_expr on a
DECL_P is a no-op.

Now that we no longer unshare the result of decl_constant_value and
decl_really_constant_value from cxx_eval_constant_expression, memory use
during constexpr evaluation for the testcase from the PR falls from ~5GB
to 15MB according to -ftime-report.

gcc/cp/ChangeLog:

	PR c++/96197
	* constexpr.c (cxx_eval_constant_expression) <case CONST_DECL>:
	Pass false to decl_constant_value and decl_really_constant_value
	so that they don't unshare their result.
	* cp-tree.h (decl_constant_value): New declaration with an added
	bool parameter.
	(decl_really_constant_value): Add bool parameter defaulting to
	true to existing declaration.
	* init.c (constant_value_1): Add bool parameter which controls
	whether to unshare the initializer before returning.  Call
	unshare_expr at most once.
	(scalar_constant_value): Pass true to constant_value_1's new
	bool parameter.
	(decl_really_constant_value): Add bool parameter and forward it
	to constant_value_1.
	(decl_constant_value): Likewise, but instead define a new
	overload with an added bool parameter.

gcc/testsuite/ChangeLog:

	PR c++/96197
	* g++.dg/cpp1y/constexpr-array8.C: New test.

gcc/cp/constexpr.c

@@ -5695,9 +5695,9 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	  TREE_CONSTANT (r) = true;
 	}
       else if (ctx->strict)
-	r = decl_really_constant_value (t);
+	r = decl_really_constant_value (t, /*unshare_p=*/false);
       else
-	r = decl_constant_value (t);
+	r = decl_constant_value (t, /*unshare_p=*/false);
       if (TREE_CODE (r) == TARGET_EXPR
 	  && TREE_CODE (TARGET_EXPR_INITIAL (r)) == CONSTRUCTOR)
 	r = TARGET_EXPR_INITIAL (r);

gcc/cp/cp-tree.h

@@ -6773,7 +6773,8 @@ extern tree build_vec_delete			(location_t, tree, tree,
 extern tree create_temporary_var		(tree);
 extern void initialize_vtbl_ptrs		(tree);
 extern tree scalar_constant_value		(tree);
-extern tree decl_really_constant_value		(tree);
+extern tree decl_constant_value			(tree, bool);
+extern tree decl_really_constant_value		(tree, bool = true);
 extern int diagnose_uninitialized_cst_or_ref_member (tree, bool, bool);
 extern tree build_vtbl_address                  (tree);
 extern bool maybe_reject_flexarray_init		(tree, tree);

gcc/cp/init.c

@@ -2272,10 +2272,12 @@ build_offset_ref (tree type, tree member, bool address_p,
    recursively); otherwise, return DECL.  If STRICT_P, the
    initializer is only returned if DECL is a
    constant-expression.  If RETURN_AGGREGATE_CST_OK_P, it is ok to
-   return an aggregate constant.  */
+   return an aggregate constant.  If UNSHARE_P, return an unshared
+   copy of the initializer.  */
 
 static tree
-constant_value_1 (tree decl, bool strict_p, bool return_aggregate_cst_ok_p)
+constant_value_1 (tree decl, bool strict_p, bool return_aggregate_cst_ok_p,
+		  bool unshare_p)
 {
   while (TREE_CODE (decl) == CONST_DECL
 	 || decl_constant_var_p (decl)
@@ -2343,9 +2345,9 @@ constant_value_1 (tree decl, bool strict_p, bool return_aggregate_cst_ok_p)
 	  && !DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (decl)
 	  && DECL_NONTRIVIALLY_INITIALIZED_P (decl))
 	break;
-      decl = unshare_expr (init);
+      decl = init;
     }
-  return decl;
+  return unshare_p ? unshare_expr (decl) : decl;
 }
 
 /* If DECL is a CONST_DECL, or a constant VAR_DECL initialized by constant
@@ -2357,26 +2359,36 @@ tree
 scalar_constant_value (tree decl)
 {
   return constant_value_1 (decl, /*strict_p=*/true,
-			   /*return_aggregate_cst_ok_p=*/false);
+			   /*return_aggregate_cst_ok_p=*/false,
+			   /*unshare_p=*/true);
 }
 
-/* Like scalar_constant_value, but can also return aggregate initializers.  */
+/* Like scalar_constant_value, but can also return aggregate initializers.
+   If UNSHARE_P, return an unshared copy of the initializer.  */
 
 tree
-decl_really_constant_value (tree decl)
+decl_really_constant_value (tree decl, bool unshare_p /*= true*/)
 {
   return constant_value_1 (decl, /*strict_p=*/true,
-			   /*return_aggregate_cst_ok_p=*/true);
+			   /*return_aggregate_cst_ok_p=*/true,
+			   /*unshare_p=*/unshare_p);
 }
 
-/* A more relaxed version of scalar_constant_value, used by the
+/* A more relaxed version of decl_really_constant_value, used by the
    common C/C++ code.  */
 
+tree
+decl_constant_value (tree decl, bool unshare_p)
+{
+  return constant_value_1 (decl, /*strict_p=*/processing_template_decl,
+			   /*return_aggregate_cst_ok_p=*/true,
+			   /*unshare_p=*/unshare_p);
+}
+
 tree
 decl_constant_value (tree decl)
 {
-  return constant_value_1 (decl, /*strict_p=*/processing_template_decl,
-			   /*return_aggregate_cst_ok_p=*/true);
+  return decl_constant_value (decl, /*unshare_p=*/true);
 }
 
 /* Common subroutines of build_new and build_vec_delete.  */

gcc/testsuite/g++.dg/cpp1y/constexpr-array8.C

@@ -0,0 +1,18 @@
+// PR c++/96197
+// { dg-do compile { target c++14 } }
+
+struct S {
+  S* p = this;
+};
+
+constexpr S ary[5000] = {};
+
+constexpr int foo() {
+  int count = 0;
+  for (int i = 0; i < 5000; i++)
+    if (ary[i].p != nullptr)
+      count++;
+  return count;
+}
+
+constexpr int bar = foo();