[gcc r14-6869] C: Fix type compatibility for structs with variable sized fields.

[Martin Uecker]

https://gcc.gnu.org/g:38c33fd2b5967f52144226a19f761fac8f85ea32

commit r14-6869-g38c33fd2b5967f52144226a19f761fac8f85ea32
Author: Martin Uecker <uecker@tugraz.at>
Date:   Fri Dec 22 17:32:34 2023 +0100

    C: Fix type compatibility for structs with variable sized fields.
    
    This fixes the test gcc.dg/gnu23-tag-4.c introduced by commit 23fee88f
    which fails for -march=... because the DECL_FIELD_BIT_OFFSET are set
    inconsistently for types with and without variable-sized field.  This
    is fixed by testing for DECL_ALIGN instead.  The code is further
    simplified by removing some unnecessary conditions, i.e. anon_field is
    set unconditionaly and all fields are assumed to be DECL_FIELDs.
    
    gcc/c:
            * c-typeck.cc (tagged_types_tu_compatible_p): Revise.
    
    gcc/testsuite:
            * gcc.dg/c23-tag-9.c: New test.

Diff:
---
 gcc/c/c-typeck.cc                | 19 ++++++++-----------
 gcc/testsuite/gcc.dg/c23-tag-9.c |  8 ++++++++
 2 files changed, 16 insertions(+), 11 deletions(-)

diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
index 2d9139d09d2..84ddda1ebab 100644
--- a/gcc/c/c-typeck.cc
+++ b/gcc/c/c-typeck.cc
@@ -1511,8 +1511,6 @@ tagged_types_tu_compatible_p (const_tree t1, const_tree t2,
   if (!data->anon_field && TYPE_STUB_DECL (t1) != TYPE_STUB_DECL (t2))
     data->different_types_p = true;
 
-  data->anon_field = false;
-
   /* Incomplete types are incompatible inside a TU.  */
   if (TYPE_SIZE (t1) == NULL || TYPE_SIZE (t2) == NULL)
     return false;
@@ -1592,22 +1590,21 @@ tagged_types_tu_compatible_p (const_tree t1, const_tree t2,
 	     s1 && s2;
 	     s1 = DECL_CHAIN (s1), s2 = DECL_CHAIN (s2))
 	  {
-	    if (TREE_CODE (s1) != TREE_CODE (s2)
-		|| DECL_NAME (s1) != DECL_NAME (s2))
+	    gcc_assert (TREE_CODE (s1) == FIELD_DECL);
+	    gcc_assert (TREE_CODE (s2) == FIELD_DECL);
+
+	    if (DECL_NAME (s1) != DECL_NAME (s2))
+	      return false;
+
+	    if (DECL_ALIGN (s1) != DECL_ALIGN (s2))
 	      return false;
 
-	    if (!DECL_NAME (s1) && RECORD_OR_UNION_TYPE_P (TREE_TYPE (s1)))
-	      data->anon_field = true;
+	    data->anon_field = !DECL_NAME (s1);
 
 	    data->cache = &entry;
 	    if (!comptypes_internal (TREE_TYPE (s1), TREE_TYPE (s2), data))
 	      return false;
 
-	    if (TREE_CODE (s1) == FIELD_DECL
-		&& simple_cst_equal (DECL_FIELD_BIT_OFFSET (s1),
-				     DECL_FIELD_BIT_OFFSET (s2)) != 1)
-	      return false;
-
 	    tree st1 = TYPE_SIZE (TREE_TYPE (s1));
 	    tree st2 = TYPE_SIZE (TREE_TYPE (s2));
 
diff --git a/gcc/testsuite/gcc.dg/c23-tag-9.c b/gcc/testsuite/gcc.dg/c23-tag-9.c
new file mode 100644
index 00000000000..1d32560ec23
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-tag-9.c
@@ -0,0 +1,8 @@
+/* { dg-do compile } */
+/* { dg-options "-std=c23" } */
+
+struct foo { int x; } x;
+struct foo { alignas(128) int x; } y;	/* { dg-error "redefinition" } */
+static_assert(alignof(y) == 128);
+
+