Re: [V4] [PATCH 3/4] c23: aliasing of compatible tagged types
On Mon, 27 Nov 2023, Martin Uecker wrote:
> diff --git a/gcc/c/c-tree.h b/gcc/c/c-tree.h
> index a5dd9a37944..ece5b6a5d26 100644
> --- a/gcc/c/c-tree.h
> +++ b/gcc/c/c-tree.h
> @@ -758,6 +758,7 @@ extern tree require_complete_type (location_t, tree);
> extern bool same_translation_unit_p (const_tree, const_tree);
> extern int comptypes (tree, tree);
> extern bool comptypes_same_p (tree, tree);
> +extern int comptypes_equiv_p (tree, tree);
This function should return bool.
> @@ -1250,6 +1266,9 @@ comptypes_internal (const_tree type1, const_tree type2,
>
> if ((d1 == NULL_TREE) != (d2 == NULL_TREE))
> data->different_types_p = true;
> + /* Ignore size mismatches. */
> + if (data->equiv)
> + return 1;
> /* Sizes must match unless one is missing or variable. */
> if (d1 == NULL_TREE || d2 == NULL_TREE || d1 == d2)
> return true;
> @@ -1467,6 +1486,9 @@ tagged_types_tu_compatible_p (const_tree t1, const_tree
> t2,
> if (list_length (TYPE_FIELDS (t1)) != list_length (TYPE_FIELDS (t2)))
> return false;
>
> + if (data->equiv && (C_TYPE_VARIABLE_SIZE (t1) || C_TYPE_VARIABLE_SIZE
> (t2)))
> + return 0;
> +
> for (s1 = TYPE_FIELDS (t1), s2 = TYPE_FIELDS (t2);
>s1 && s2;
>s1 = DECL_CHAIN (s1), s2 = DECL_CHAIN (s2))
> @@ -1486,6 +1508,15 @@ tagged_types_tu_compatible_p (const_tree t1,
> const_tree t2,
> && 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));
> +
> + if (data->equiv
> + && st1 && TREE_CODE (st1) == INTEGER_CST
> + && st2 && TREE_CODE (st2) == INTEGER_CST
> + && !tree_int_cst_equal (st1, st2))
> + return 0;
And these functions do return bool, so you should use true and false
instead of 1 and 0.
> +/* The structs are incompatible so can be assumed not to
> + * alias, but this is not exploited. So do not check for
> + * this below but check the warning about incompatibility. */
> +
> +int test_bar3(struct bar* a, void* b)
> +{
> + a->x = 1;
> +
> + struct bar { int x; int f[1]; }* p = b;
> + struct bar* q = a; /* { dg-warning "incompatible"
> } */
I expect you'll now need -fpermissive or
-Wno-error=incompatible-pointer-types (this is an execution test so you
need to stop this being an error, but see below).
> + // allow both results here
> + int r = test_bar3(, );
> + if ((r != 2) && (r != 1))
> + __builtin_abort();
I don't think you should really be executing this call at all (aliasing
not allowed means undefined behavior at runtime); better to put this in a
separate compile-only test (which would also avoid the need for
-fpermissive or -Wno-error=incompatible-pointer-types because once it's no
longer an execution test, having an error is OK).
--
Joseph S. Myers
jos...@codesourcery.com
[V4] [PATCH 3/4] c23: aliasing of compatible tagged types
(this mostly got an extended description and more
comments, also tests were reorganized)
c23: aliasing of compatible tagged types
Tell the backend which types are equivalent by setting
TYPE_CANONICAL to one struct in the set of equivalent
structs. Structs are considered equivalent by ignoring
all sizes of arrays nested in types below field level.
The following two structs are incompatible and lvalues
with these types can be assumed not to alias:
struct foo { int a[3]; };
struct foo { int a[4]; };
The following two structs are also incompatible, but
will get the same TYPE_CANONICAL and it is then not
exploited that lvalues with those types can not alias:
struct bar { int (*p)[3]; };
struct bar { int (*p)[4]; };
The reason is that both are compatible to
struct bar { int (*p)[]; };
and therefore are in the same equivalence class. For
the same reason all enums with the same underyling type
are in the same equivalence class. Tests are added
for the expected aliasing behavior with optimization.
gcc/c:
* c-decl.cc (c_struct_hasher): Hash stable for struct
types.
(c_struct_hasher::hash, c_struct_hasher::equal): New
functions.
(finish_struct): Set TYPE_CANONICAL to first struct in
equivalence class.
* c-objc-common.cc (c_get_alias_set): Let structs or
unions with variable size alias anything.
* c-tree.h (comptypes_equiv): New prototype.
* c-typeck.cc (comptypes_equiv): New function.
(comptypes_internal): Implement equivalence mode.
(tagged_types_tu_compatible): Implement equivalence mode.
gcc/testsuite:
* gcc.dg/c23-tag-2.c: Activate.
* gcc.dg/c23-tag-6.c: Activate.
* gcc.dg/c23-tag-alias-1.c: New test.
* gcc.dg/c23-tag-alias-2.c: New test.
* gcc.dg/gnu23-tag-alias-1.c: New test.
* gcc.dg/gnu23-tag-alias-2.c: New test.
* gcc.dg/gnu23-tag-alias-3.c: New test.
* gcc.dg/gnu23-tag-alias-4.c: New test.
* gcc.dg/gnu23-tag-alias-5.c: New test.
* gcc.dg/gnu23-tag-alias-6.c: New test.
* gcc.dg/gnu23-tag-alias-7.c: New test.
---
gcc/c/c-decl.cc | 51 ++-
gcc/c/c-objc-common.cc | 5 ++
gcc/c/c-tree.h | 1 +
gcc/c/c-typeck.cc| 31 +++
gcc/testsuite/gcc.dg/c23-tag-2.c | 2 +-
gcc/testsuite/gcc.dg/c23-tag-5.c | 2 +-
gcc/testsuite/gcc.dg/c23-tag-alias-1.c | 49 +++
gcc/testsuite/gcc.dg/c23-tag-alias-2.c | 50 +++
gcc/testsuite/gcc.dg/c23-tag-alias-3.c | 32 +++
gcc/testsuite/gcc.dg/c23-tag-alias-4.c | 54
gcc/testsuite/gcc.dg/gnu23-tag-alias-1.c | 33 +++
gcc/testsuite/gcc.dg/gnu23-tag-alias-2.c | 85 ++
gcc/testsuite/gcc.dg/gnu23-tag-alias-3.c | 83 ++
gcc/testsuite/gcc.dg/gnu23-tag-alias-4.c | 36
gcc/testsuite/gcc.dg/gnu23-tag-alias-5.c | 107 +++
gcc/testsuite/gcc.dg/gnu23-tag-alias-6.c | 60 +
gcc/testsuite/gcc.dg/gnu23-tag-alias-7.c | 93
17 files changed, 771 insertions(+), 3 deletions(-)
create mode 100644 gcc/testsuite/gcc.dg/c23-tag-alias-1.c
create mode 100644 gcc/testsuite/gcc.dg/c23-tag-alias-2.c
create mode 100644 gcc/testsuite/gcc.dg/c23-tag-alias-3.c
create mode 100644 gcc/testsuite/gcc.dg/c23-tag-alias-4.c
create mode 100644 gcc/testsuite/gcc.dg/gnu23-tag-alias-1.c
create mode 100644 gcc/testsuite/gcc.dg/gnu23-tag-alias-2.c
create mode 100644 gcc/testsuite/gcc.dg/gnu23-tag-alias-3.c
create mode 100644 gcc/testsuite/gcc.dg/gnu23-tag-alias-4.c
create mode 100644 gcc/testsuite/gcc.dg/gnu23-tag-alias-5.c
create mode 100644 gcc/testsuite/gcc.dg/gnu23-tag-alias-6.c
create mode 100644 gcc/testsuite/gcc.dg/gnu23-tag-alias-7.c
diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index bcc09ba479e..68cba131704 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -634,6 +634,36 @@ public:
auto_vec typedefs_seen;
};
+
+/* Hash table for structs and unions. */
+struct c_struct_hasher : ggc_ptr_hash
+{
+ static hashval_t hash (tree t);
+ static bool equal (tree, tree);
+};
+
+/* Hash an RECORD OR UNION. */
+hashval_t
+c_struct_hasher::hash (tree type)
+{
+ inchash::hash hstate;
+
+ hstate.add_int (TREE_CODE (type));
+ hstate.add_object (TYPE_NAME (type));
+
+ return hstate.end ();
+}
+
+/* Compare two RECORD or UNION types. */
+bool
+c_struct_hasher::equal (tree t1, tree t2)
+{
+ return comptypes_equiv_p (t1, t2);
+}
+
+/* All tagged typed so that TYPE_CANONICAL can be set correctly. */
+static GTY (()) hash_table *c_struct_htab;
+
/* Information for the struct or union currently being parsed, or
NULL if not parsing a struct or union. */
static class c_struct_parse_info *struct_parse_info;
@@ -8713,7 +8743,8 @@ parser_xref_tag (location_t loc, enum tree_code code,
tree name,
ref = lookup_tag (code, name,
