c++: Update DECL_*SIZE for objects with flexible array members with initializers...

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.

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.

(cherry picked from commit 818c505188ff5cd8eb048eb0e614c4ef732225bd)

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index e0c603aaab6c2b71f49f051e589759332faf78a8..9a6e3641aacc031af03eaf628d810f211cd3f223 100644 (file)
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -6006,6 +6006,38 @@ 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));
+           }
        }
     }
 }

diff --git a/gcc/testsuite/g++.target/i386/pr102295.C b/gcc/testsuite/g++.target/i386/pr102295.C
new file mode 100644 (file)
index 0000000..09efc3c
--- /dev/null
+++ b/gcc/testsuite/g++.target/i386/pr102295.C
@@ -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" } } */