[PATCH] c++: Update DECL_*SIZE for objects with flexible array members with initializers [PR102295]

[Jakub Jelinek <jakub@redhat.com>]

Hi!

The C FE updates DECL_*SIZE for vars which have initializers for flexible
array members for many years, but C++ FE kept DECL_*SIZE the same as the
type size (i.e. as if there were zero elements in the flexible array
member).  This results e.g. in ELF symbol sizes being too small.

The following patch fixes that, bootstrapped/regtested on x86_64-linux and
i686-linux, ok for trunk?

Note, if the flexible array member is initialized only with non-constant
initializers, we have a worse bug that this patch doesn't solve, the
splitting of initializers into constant and dynamic initialization removes
the initializer and we don't have just wrong DECL_*SIZE, but nothing is
emitted when emitting those vars into assembly either and so the dynamic
initialization clobbers other vars that may overlap the variable.
I think we need keep an empty CONSTRUCTOR elt in DECL_INITIAL for the
flexible array member in that case.

2021-09-14  Jakub Jelinek  <jakub@redhat.com>

	PR c++/102295
	* decl.c (layout_var_decl): For aggregates ending with a flexible
	array member, add the size of the initializer for that member to
	DECL_SIZE and DECL_SIZE_UNIT.

	* g++.target/i386/pr102295.C: New test.

--- gcc/cp/decl.c.jj	2021-09-09 10:40:26.063176136 +0200
+++ gcc/cp/decl.c	2021-09-13 18:23:01.131587057 +0200
@@ -6091,6 +6091,38 @@ layout_var_decl (tree decl)
 	  error_at (DECL_SOURCE_LOCATION (decl),
 		    "storage size of %qD isn%'t constant", decl);
 	  TREE_TYPE (decl) = error_mark_node;
+	  type = error_mark_node;
+	}
+    }
+
+  /* If the final element initializes a flexible array field, add the size of
+     that initializer to DECL's size.  */
+  if (type != error_mark_node
+      && DECL_INITIAL (decl)
+      && TREE_CODE (DECL_INITIAL (decl)) == CONSTRUCTOR
+      && !vec_safe_is_empty (CONSTRUCTOR_ELTS (DECL_INITIAL (decl)))
+      && DECL_SIZE (decl) != NULL_TREE
+      && TREE_CODE (DECL_SIZE (decl)) == INTEGER_CST
+      && TYPE_SIZE (type) != NULL_TREE
+      && TREE_CODE (TYPE_SIZE (type)) == INTEGER_CST
+      && tree_int_cst_equal (DECL_SIZE (decl), TYPE_SIZE (type)))
+    {
+      constructor_elt &elt = CONSTRUCTOR_ELTS (DECL_INITIAL (decl))->last ();
+      if (elt.index)
+	{
+	  tree itype = TREE_TYPE (elt.index);
+	  tree vtype = TREE_TYPE (elt.value);
+	  if (TREE_CODE (itype) == ARRAY_TYPE
+	      && TYPE_DOMAIN (itype) == NULL
+	      && TREE_CODE (vtype) == ARRAY_TYPE
+	      && COMPLETE_TYPE_P (vtype))
+	    {
+	      DECL_SIZE (decl)
+		= size_binop (PLUS_EXPR, DECL_SIZE (decl), TYPE_SIZE (vtype));
+	      DECL_SIZE_UNIT (decl)
+		= size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl),
+			      TYPE_SIZE_UNIT (vtype));
+	    }
 	}
     }
 }
--- gcc/testsuite/g++.target/i386/pr102295.C.jj	2021-09-13 18:28:08.406370163 +0200
+++ gcc/testsuite/g++.target/i386/pr102295.C	2021-09-13 18:28:26.846117106 +0200
@@ -0,0 +1,12 @@
+// PR c++/102295
+// { dg-do compile { target *-*-linux* } }
+// { dg-options "-Wno-pedantic" }
+
+struct S {
+  int a;
+  int b[];
+} S;
+
+struct S s = { 1, { 2, 3 } };
+
+/* { dg-final { scan-assembler ".size\[\t \]*s, 12" } } */

	Jakub