From 6550198bd8467f435959fa25b69c217a6ef75c7a Mon Sep 17 00:00:00 2001
From: Jakub Jelinek <jakub@redhat.com>
Date: Tue, 14 Sep 2021 16:56:30 +0200
Subject: [PATCH] c++: Update DECL_*SIZE for objects with flexible array
 members with initializers [PR102295]

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)
---
 gcc/cp/decl.c                            | 32 ++++++++++++++++++++++++
 gcc/testsuite/g++.target/i386/pr102295.C | 12 +++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 gcc/testsuite/g++.target/i386/pr102295.C

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index e0c603aaab6..9a6e3641aac 100644
--- 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
index 00000000000..09efc3c8a86
--- /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" } } */