[PATCH] tree-optimization/108522 Use COMPONENT_REF offset when available

[Siddhesh Poyarekar]

Use the offset in TREE_OPERAND(component_ref, 2) when available instead
of DECL_FIELD_OFFSET when trying to compute offset for a COMPONENT_REF.

OK for gcc 13 and gcc 12?

Co-authored-by: Jakub Jelinek <ja...@redhat.com>

gcc/ChangeLog:

        PR tree-optimization/108522
        * tree-object-size.cc (compute_object_offset): Use
        TREE_OPERAND(ref, 2) for COMPONENT_REF when available.

gcc/testsuite/ChangeLog:

        PR tree-optimization/108522
        * builtin-dynamic-object-size-0.c (test_dynarray_struct_member):
        new test.
        (main): Call it.

Signed-off-by: Siddhesh Poyarekar <siddh...@gotplt.org>
---
Testing:

- Bootstrapped on x86_64, I'm checking to confirm if a couple of
  seemingly unrelated failures are in fact unrelated.
- ubsan config bootstrap and i686 tests in progress

 .../gcc.dg/builtin-dynamic-object-size-0.c       | 16 ++++++++++++++++
 gcc/tree-object-size.cc                          |  4 +++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/gcc/testsuite/gcc.dg/builtin-dynamic-object-size-0.c 
b/gcc/testsuite/gcc.dg/builtin-dynamic-object-size-0.c
index f9047a037d9..569c0a87722 100644
--- a/gcc/testsuite/gcc.dg/builtin-dynamic-object-size-0.c
+++ b/gcc/testsuite/gcc.dg/builtin-dynamic-object-size-0.c
@@ -314,6 +314,20 @@ test_dynarray_struct_subobj2 (size_t sz, size_t off, 
size_t *objsz)
   return __builtin_dynamic_object_size (&bin.c[off], 1);
 }
 
+/* See pr #108522.  */
+size_t
+__attribute__ ((noinline))
+test_dynarray_struct_member (size_t sz)
+{
+  struct
+    {
+      char a[sz];
+      char b;
+    } s;
+
+  return __builtin_dynamic_object_size (&s.b, 0);
+}
+
 size_t
 __attribute__ ((noinline))
 test_substring (size_t sz, size_t off)
@@ -619,6 +633,8 @@ main (int argc, char **argv)
   if (test_dynarray_struct_subobj2 (42, 4, &objsz)
     != objsz - 4 - sizeof (long) - sizeof (int))
     FAIL ();
+  if (test_dynarray_struct_member (42) != sizeof (char))
+    FAIL ();
   if (test_substring_ptrplus (128, 4) != (128 - 4) * sizeof (int))
     FAIL ();
   if (test_substring_ptrplus (128, 142) != 0)
diff --git a/gcc/tree-object-size.cc b/gcc/tree-object-size.cc
index 356591c22cc..de93ffad9c9 100644
--- a/gcc/tree-object-size.cc
+++ b/gcc/tree-object-size.cc
@@ -412,7 +412,9 @@ compute_object_offset (const_tree expr, const_tree var)
        return base;
 
       t = TREE_OPERAND (expr, 1);
-      off = size_binop (PLUS_EXPR, DECL_FIELD_OFFSET (t),
+      off = size_binop (PLUS_EXPR,
+                       (TREE_OPERAND (expr, 2) ? TREE_OPERAND (expr, 2)
+                        : DECL_FIELD_OFFSET (t)),
                        size_int (tree_to_uhwi (DECL_FIELD_BIT_OFFSET (t))
                                  / BITS_PER_UNIT));
       break;
-- 
2.38.1