c: Don't drop vector attributes that affect type identity [PR98852]

<arm_neon.h> types are distinct from GNU vector types in at least
their mangling.  However, there used to be nothing explicit in the
VECTOR_TYPE itself to indicate the difference: we simply treated them
as distinct TYPE_MAIN_VARIANTs.  This caused problems like the ones
reported in PR95726.

The fix for that PR was to add type attributes to the <arm_neon.h>
types, in order to maintain the distinction between them and GNU
vectors.  However, this in turn caused PR98852, where c_common_type
would unconditionally drop the attributes on the source types.
This meant that:

   <arm_neon.h> vector + <arm_neon.h> vector

had a GNU vector type rather than an <arm_neon.h> vector type.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96377#c2 for
Jakub's analysis of the history of this c_common_type code.
TBH I'm not sure which case the build_type_attribute_variant
code is handling, but I think we should at least avoid dropping
attributes that affect type identity.

I've tried to audit the C and target-specific attributes to look
for other types that might be affected by this, but I couldn't
see any.  We are only dealing with:

  gcc_assert (code1 == VECTOR_TYPE || code1 == COMPLEX_TYPE
	      || code1 == FIXED_POINT_TYPE || code1 == REAL_TYPE
	      || code1 == INTEGER_TYPE);

which excludes most affects_type_identity attributes.  The closest
was s390_vector_bool, but the handler for that attribute changes
the type node and drops the attribute itself (*no_add_attrs = true).

I put the main list handling into a separate function
(remove_attributes_matching) because a later patch will need it
for something else.

gcc/
	PR c/98852
	* attribs.h (affects_type_identity_attributes): Declare.
	* attribs.c (remove_attributes_matching): New function.
	(affects_type_identity_attributes): Likewise.

gcc/c/
	PR c/98852
	* c-typeck.c (c_common_type): Do not drop attributes that
	affect type identity.

gcc/testsuite/
	PR c/98852
	* gcc.target/aarch64/advsimd-intrinsics/pr98852.c: New test.

4 files changed, 193 insertions, 2 deletions

diff --git a/gcc/attribs.c b/gcc/attribs.c
index 16c6b12..2fb2954 100644
--- a/gcc/attribs.c
+++ b/gcc/attribs.c
@@ -1366,6 +1366,60 @@ comp_type_attributes (const_tree type1, const_tree type2)
   return targetm.comp_type_attributes (type1, type2);
 }
 
+/* PREDICATE acts as a function of type:
+
+     (const_tree attr, const attribute_spec *as) -> bool
+
+   where ATTR is an attribute and AS is its possibly-null specification.
+   Return a list of every attribute in attribute list ATTRS for which
+   PREDICATE is true.  Return ATTRS itself if PREDICATE returns true
+   for every attribute.  */
+
+template<typename Predicate>
+tree
+remove_attributes_matching (tree attrs, Predicate predicate)
+{
+  tree new_attrs = NULL_TREE;
+  tree *ptr = &new_attrs;
+  const_tree start = attrs;
+  for (const_tree attr = attrs; attr; attr = TREE_CHAIN (attr))
+    {
+      tree name = get_attribute_name (attr);
+      const attribute_spec *as = lookup_attribute_spec (name);
+      const_tree end;
+      if (!predicate (attr, as))
+	end = attr;
+      else if (start == attrs)
+	continue;
+      else
+	end = TREE_CHAIN (attr);
+
+      for (; start != end; start = TREE_CHAIN (start))
+	{
+	  *ptr = tree_cons (TREE_PURPOSE (start),
+			    TREE_VALUE (start), NULL_TREE);
+	  TREE_CHAIN (*ptr) = NULL_TREE;
+	  ptr = &TREE_CHAIN (*ptr);
+	}
+      start = TREE_CHAIN (attr);
+    }
+  gcc_assert (!start || start == attrs);
+  return start ? attrs : new_attrs;
+}
+
+/* If VALUE is true, return the subset of ATTRS that affect type identity,
+   otherwise return the subset of ATTRS that don't affect type identity.  */
+
+tree
+affects_type_identity_attributes (tree attrs, bool value)
+{
+  auto predicate = [value](const_tree, const attribute_spec *as) -> bool
+    {
+      return bool (as && as->affects_type_identity) == value;
+    };
+  return remove_attributes_matching (attrs, predicate);
+}
+
 /* Return a type like TTYPE except that its TYPE_ATTRIBUTE
    is ATTRIBUTE.
 
diff --git a/gcc/attribs.h b/gcc/attribs.h
index 898e73d..eadb1d0 100644
--- a/gcc/attribs.h
+++ b/gcc/attribs.h
@@ -65,6 +65,8 @@ extern bool attribute_value_equal (const_tree, const_tree);
    warning to be generated).  */
 extern int comp_type_attributes (const_tree, const_tree);
 
+extern tree affects_type_identity_attributes (tree, bool = true);
+
 /* Default versions of target-overridable functions.  */
 extern tree merge_decl_attributes (tree, tree);
 extern tree merge_type_attributes (tree, tree);
diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index 21eab00..51a62c8 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -740,10 +740,16 @@ c_common_type (tree t1, tree t2)
     t2 = TYPE_MAIN_VARIANT (t2);
 
   if (TYPE_ATTRIBUTES (t1) != NULL_TREE)
-    t1 = build_type_attribute_variant (t1, NULL_TREE);
+    {
+      tree attrs = affects_type_identity_attributes (TYPE_ATTRIBUTES (t1));
+      t1 = build_type_attribute_variant (t1, attrs);
+    }
 
   if (TYPE_ATTRIBUTES (t2) != NULL_TREE)
-    t2 = build_type_attribute_variant (t2, NULL_TREE);
+    {
+      tree attrs = affects_type_identity_attributes (TYPE_ATTRIBUTES (t2));
+      t2 = build_type_attribute_variant (t2, attrs);
+    }
 
   /* Save time if the two types are the same.  */
 
diff --git a/gcc/testsuite/gcc.target/aarch64/advsimd-intrinsics/pr98852.c b/gcc/testsuite/gcc.target/aarch64/advsimd-intrinsics/pr98852.c
new file mode 100644
index 0000000..31e51b0d
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/advsimd-intrinsics/pr98852.c
@@ -0,0 +1,129 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-std=c99" } */
+
+#include <arm_neon.h>
+
+typedef __typeof(((int32x4_t *) 0)[0][0]) int32_elt;
+typedef __typeof(((uint32x4_t *) 0)[0][0]) uint32_elt;
+
+typedef int32_elt gnu_int32x4_t __attribute__((vector_size(16)));
+typedef uint32_elt gnu_uint32x4_t __attribute__((vector_size(16)));
+
+#define X_gnu_int32x4_t 1
+#define X_gnu_uint32x4_t 2
+#define X_int32x4_t 3
+#define X_uint32x4_t 4
+
+#define CHECK(T) T: X_##T
+
+#define CHECK_TYPE(EXPR, TYPE) \
+  do { \
+    int x[_Generic (EXPR, \
+		    CHECK (gnu_int32x4_t), \
+		    CHECK (gnu_uint32x4_t), \
+		    CHECK (int32x4_t), \
+		    CHECK (uint32x4_t), \
+		    default : 0) == X_##TYPE ? 1 : -1]; \
+  } while (0)
+
+void
+f (gnu_int32x4_t sg, gnu_uint32x4_t ug, int32x4_t sn, uint32x4_t un, int c)
+{
+  CHECK_TYPE (sg, gnu_int32x4_t);
+  CHECK_TYPE (ug, gnu_uint32x4_t);
+  CHECK_TYPE (sn, int32x4_t);
+  CHECK_TYPE (un, uint32x4_t);
+
+  CHECK_TYPE (sg + 1, gnu_int32x4_t);
+  CHECK_TYPE (ug + 1, gnu_uint32x4_t);
+  CHECK_TYPE (sn + 1, int32x4_t);
+  CHECK_TYPE (un + 1, uint32x4_t);
+
+  CHECK_TYPE (1 + sg, gnu_int32x4_t);
+  CHECK_TYPE (1 + ug, gnu_uint32x4_t);
+  CHECK_TYPE (1 + sn, int32x4_t);
+  CHECK_TYPE (1 + un, uint32x4_t);
+
+  CHECK_TYPE (sg + sg, gnu_int32x4_t);
+  CHECK_TYPE (ug + ug, gnu_uint32x4_t);
+  CHECK_TYPE (sn + sn, int32x4_t);
+  CHECK_TYPE (un + un, uint32x4_t);
+
+  /* Traditional behavior for mixed signs is to pick the signedness of the
+     first operand.  We don't have any Arm-specific reason for preferring that
+     behavior, but including the tests helps to demonstrate the points in the
+     comments below.  */
+  CHECK_TYPE (sg + ug, gnu_int32x4_t);
+  CHECK_TYPE (ug + sg, gnu_uint32x4_t);
+  CHECK_TYPE (sn + un, int32x4_t);
+  CHECK_TYPE (un + sn, uint32x4_t);
+
+  /* Nothing specifies the type of mixed GNU and arm_neon.h operations, but:
+
+     - it would be surprising if sg + un had a different signedness from
+       sg + ug
+
+     - it would also be mildly surprising if sg + un had a different type from
+       both of its operands
+
+     So in cases where the operands differ in both signedness and ABI, it seems
+     more consistent to ignore the ABI difference and apply the usual rules for
+     differences in sign.  */
+  CHECK_TYPE (sg + un, gnu_int32x4_t);
+  CHECK_TYPE (ug + sn, gnu_uint32x4_t);
+  CHECK_TYPE (sn + ug, int32x4_t);
+  CHECK_TYPE (un + sg, uint32x4_t);
+
+  /* And if the first vector wins when operands differ in both signedness
+     and ABI, it seems more consistent to do the same if the operands differ
+     only in ABI.  */
+  CHECK_TYPE (sg + sn, gnu_int32x4_t);
+  CHECK_TYPE (ug + un, gnu_uint32x4_t);
+  CHECK_TYPE (sn + sg, int32x4_t);
+  CHECK_TYPE (un + ug, uint32x4_t);
+
+  CHECK_TYPE (c ? sg + sg : sg, gnu_int32x4_t);
+  CHECK_TYPE (c ? ug + ug : ug, gnu_uint32x4_t);
+  CHECK_TYPE (c ? sn + sn : sn, int32x4_t);
+  CHECK_TYPE (c ? un + un : un, uint32x4_t);
+
+  CHECK_TYPE (c ? sg + 1 : sg, gnu_int32x4_t);
+  CHECK_TYPE (c ? ug + 1 : ug, gnu_uint32x4_t);
+  CHECK_TYPE (c ? sn + 1 : sn, int32x4_t);
+  CHECK_TYPE (c ? un + 1 : un, uint32x4_t);
+
+  CHECK_TYPE (c ? 1 + sg : sg, gnu_int32x4_t);
+  CHECK_TYPE (c ? 1 + ug : ug, gnu_uint32x4_t);
+  CHECK_TYPE (c ? 1 + sn : sn, int32x4_t);
+  CHECK_TYPE (c ? 1 + un : un, uint32x4_t);
+
+  CHECK_TYPE (c ? sg : sg + sg, gnu_int32x4_t);
+  CHECK_TYPE (c ? ug : ug + ug, gnu_uint32x4_t);
+  CHECK_TYPE (c ? sn : sn + sn, int32x4_t);
+  CHECK_TYPE (c ? un : un + un, uint32x4_t);
+
+  CHECK_TYPE (c ? sg : sg + 1, gnu_int32x4_t);
+  CHECK_TYPE (c ? ug : ug + 1, gnu_uint32x4_t);
+  CHECK_TYPE (c ? sn : sn + 1, int32x4_t);
+  CHECK_TYPE (c ? un : un + 1, uint32x4_t);
+
+  CHECK_TYPE (c ? sg : 1 + sg, gnu_int32x4_t);
+  CHECK_TYPE (c ? ug : 1 + ug, gnu_uint32x4_t);
+  CHECK_TYPE (c ? sn : 1 + sn, int32x4_t);
+  CHECK_TYPE (c ? un : 1 + un, uint32x4_t);
+
+  CHECK_TYPE (c ? sg + sg : sg + sg, gnu_int32x4_t);
+  CHECK_TYPE (c ? ug + ug : ug + ug, gnu_uint32x4_t);
+  CHECK_TYPE (c ? sn + sn : sn + sn, int32x4_t);
+  CHECK_TYPE (c ? un + un : un + un, uint32x4_t);
+
+  CHECK_TYPE (c ? sg + sg : sg + 1, gnu_int32x4_t);
+  CHECK_TYPE (c ? ug + ug : ug + 1, gnu_uint32x4_t);
+  CHECK_TYPE (c ? sn + sn : sn + 1, int32x4_t);
+  CHECK_TYPE (c ? un + un : un + 1, uint32x4_t);
+
+  CHECK_TYPE (c ? 1 + sg : sg + sg, gnu_int32x4_t);
+  CHECK_TYPE (c ? 1 + ug : ug + ug, gnu_uint32x4_t);
+  CHECK_TYPE (c ? 1 + sn : sn + sn, int32x4_t);
+  CHECK_TYPE (c ? 1 + un : un + un, uint32x4_t);
+}