This patch addresses a difference between the hash function and the equality
function for canonical types of template parameters (ctp_hasher). The equality
function uses comptypes (typeck.cc) (with COMPARE_STRUCTURAL) and checks
constraint equality for two auto nodes (typeck.cc:1586), while the hash
function ignores it (pt.cc:4528). This leads to hash collisions that can be
avoided by using `hash_placeholder_constraint` (constraint.cc:1150).
* constraint.cc (hash_placeholder_constraint): Rename to
iterative_hash_placeholder_constraint.
(iterative_hash_placeholder_constraint): Rename from
hash_placeholder_constraint and add the initial val argument.
* cp-tree.h (hash_placeholder_constraint): Rename to
iterative_hash_placeholder_constraint.
(iterative_hash_placeholder_constraint): Renamed from
hash_placeholder_constraint and add the initial val argument.
* pt.cc (struct ctp_hasher): Updated to use
iterative_hash_placeholder_constraint in the case of a valid placeholder
constraint.
(auto_hash::hash): Reflect the renaming of hash_placeholder_constraint to
iterative_hash_placeholder_constraint.
---
gcc/cp/constraint.cc | 4 ++--
gcc/cp/cp-tree.h | 2 +-
gcc/cp/pt.cc | 9 +++++++--
3 files changed, 10 insertions(+), 5 deletions(-)
diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index ebf4255e5..78aacb77a 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -1971,13 +1971,13 @@ equivalent_placeholder_constraints (tree c1, tree c2)
/* Return a hash value for the placeholder ATOMIC_CONSTR C. */
hashval_t
-hash_placeholder_constraint (tree c)
+iterative_hash_placeholder_constraint (tree c, hashval_t val)
{
tree t, a;
placeholder_extract_concept_and_args (c, t, a);
/* Like hash_tmpl_and_args, but skip the first argument. */
- hashval_t val = iterative_hash_object (DECL_UID (t), 0);
+ val = iterative_hash_object (DECL_UID (t), val);
for (int i = TREE_VEC_LENGTH (a)-1; i > 0; --i)
val = iterative_hash_template_arg (TREE_VEC_ELT (a, i), val);
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 4bb3e9c49..294e88f75 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8588,7 +8588,7 @@ extern tree_pair finish_type_constraints (tree,
tree, tsubst_flags_t);
extern tree build_constrained_parameter (tree, tree, tree = NULL_TREE);
extern void placeholder_extract_concept_and_args (tree, tree&, tree&);
extern bool equivalent_placeholder_constraints (tree, tree);
-extern hashval_t hash_placeholder_constraint (tree);
+extern hashval_t iterative_hash_placeholder_constraint (tree, hashval_t);
extern bool deduce_constrained_parameter (tree, tree&, tree&);
extern tree resolve_constraint_check (tree);
extern tree check_function_concept (tree);
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index d1316483e..9a80c44a5 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -4525,7 +4525,12 @@ struct ctp_hasher : ggc_ptr_hash<tree_node>
val = iterative_hash_object (TEMPLATE_TYPE_LEVEL (t), val);
val = iterative_hash_object (TEMPLATE_TYPE_IDX (t), val);
if (TREE_CODE (t) == TEMPLATE_TYPE_PARM)
- val = iterative_hash_template_arg (CLASS_PLACEHOLDER_TEMPLATE (t), val);
+ {
+ val
+ = iterative_hash_template_arg (CLASS_PLACEHOLDER_TEMPLATE (t), val);
+ if (tree c = NON_ERROR (PLACEHOLDER_TYPE_CONSTRAINTS (t)))
+ val = iterative_hash_placeholder_constraint(c, val);
+ }
if (TREE_CODE (t) == BOUND_TEMPLATE_TEMPLATE_PARM)
val = iterative_hash_template_arg (TYPE_TI_ARGS (t), val);
--comparing_specializations;
@@ -29605,7 +29610,7 @@ auto_hash::hash (tree t)
if (tree c = NON_ERROR (PLACEHOLDER_TYPE_CONSTRAINTS (t)))
/* Matching constrained-type-specifiers denote the same template
parameter, so hash the constraint. */
- return hash_placeholder_constraint (c);
+ return iterative_hash_placeholder_constraint (c, 0);
else
/* But unconstrained autos are all separate, so just hash the pointer. */
return iterative_hash_object (t, 0);
--
2.45.2
On Tue, 9 Jul 2024 at 14:41, Seyed Sajad Kahani <sska1...@gmail.com> wrote:
>
> Hi.
>
> While investigating a fix for C++/PR115030 (a bug in constrained auto
> deduction), I was wondering why we are not substituting constraint args of an
> auto node in tsubst (pt.cc:16533). Instead, this substitution is delayed until
> do_auto_deduction (pt.cc), where we attempt to find the substituted args of
> the
> enclosing scope. At that point, it becomes difficult to find partially
> specialised args, as they are erased in the process. I propose modifying
> tsubst
> to eagerly substitute the constraint args of an auto node.
>
> By making this change, we would not need to provide outer_targs for
> do_auto_deduction in cases where tsubst has been called for the type, which
> covers most scenarios. However, we still need outer_targs for cases like:
>
> template <typename T, C<T> auto V>
> struct S { };
>
> Hence, outer_targs cannot be completely removed but will be set to TREE_NULL
> in
> all calls except the one in convert_template_argument (pt.cc:8788).
> Additionally, the tmpl argument of do_auto_deduction, which helps to provide
> outer args in the scope, will no longer be necessary and can be safely
> removed.
>
> Also, the trimming hack proposed in
> https://gcc.gnu.org/pipermail/gcc-patches/2024-June/654724.html to address
> C++/PR114915 is no longer needed. We will add an assertion to ensure that the
> missing levels do not become negative.
>
> Substituting constraint arguments earlier will slightly alter error messages
> (see testsuite/g++.dg/cpp2a/concepts-placeholder3.C as an example).
>
> To summarise, I have made the following changes:
> - Modified tsubst to substitute the constraint args.
> - Fixed shallow checks for using the cache from TEMPLATE_TYPE_DESCENDANTS
> (pt.cc:16513).
> - Substituted the constraint args and the auto node itself, while retaining
> all
> other parameters as is (pt.cc:16533).
> - Removed now unnecessary code that attempted to find outer scope template
> info
> and args in various locations.
> - Updated the missing levels hack (pt.cc:31320) to work with the substituted
> constraints.
> - Used level instead of orig_level to find the missing levels.
> - Added an assertion for future safety.
>
> Details of these changes can be found in the patch "[PATCH v1] c++: Eagerly
> substitute auto constraint args in tsubst [PR115030]" that will be replied to
> this thread.
>
> This patch, in my opinion, improves code quality by removing an argument from
> do_auto_deduction and eliminating the out-of-scope task of finding outer_targs
> within do_auto_deduction. It simplifies the usage of do_auto_deduction and
> resolves C++/PR115030 without complex calculations for specialised args. It
> also enhances the consistency of tsubst behaviour by not leaving constraints
> un-substituted. However, these changes make args in constraints_satisfied_p
> (being called from do_auto_deduction) a bit misleading, as it will not carry
> the actual args of the constraint and can even be an empty vec.
>
> I have added a testsuite for C++/PR115030, and as far as I have tested (only
> c++ dg.exp on x86_64-linux), there are no regressions.
>
> Extra:
> While doing this, I also realised that the hash function in ctp_hasher (for
> canonical type of template parameters) slightly differs from its equality
> function. Specifically, the equality function uses comptypes (typeck.cc) (with
> COMPARE_STRUCTURAL) and compares placeholder constraints (typeck.cc:1586),
> while the hash function ignores them (pt.cc:4528). As a result, we can have
> two
> types with equal hashes that are unequal. For example, assuming:
>
> template <typename T, typename U>
> concept C1 = ...
>
> template <typename T, typename U>
> concept C2 = ...
>
>
> "C1<U> auto" and "C2<V> auto" have the same hash value but are unequal. This
> issue does not cause any error (it is a hash collision and it is handled), but
> it can be avoided by using hash_placeholder_constraint (constraint.cc:1150).
>
> Therefore, I have made the following changes:
> - Fixed the hash function to calculate the hash of the constraint
> (pt.cc:4528).
> - Slightly modified the existing hash_placeholder_constraint
> (constraint.cc:1150) to accept the initial hash value as an argument.
>
> Details of these changes can be found in the patch "[PATCH v1] c++: Hash
> placeholder constraint in ctp_hasher" that will be replied to this email.
>
> Thanks. I really appreciate your comments and feedback on these proposed
> changes.
