From: Jason Merrill <jason@redhat.com>
To: Marek Polacek <polacek@redhat.com>,
GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH] c++: Extend std::is_constant_evaluated in if warning [PR100995]
Date: Thu, 10 Jun 2021 10:27:44 -0400 [thread overview]
Message-ID: <a226fb87-2244-7a45-f7f9-2a73b73b81da@redhat.com> (raw)
In-Reply-To: <20210610014628.2194915-1-polacek@redhat.com>
On 6/9/21 9:46 PM, Marek Polacek wrote:
> Jakub pointed me at
> <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1938r3.html#compiler-warnings>
> which shows that our existing warning could be extended to handle more
> cases. This patch implements that.
>
> A minor annoyance was handling macros, in libstdc++ we have
>
> reference operator[](size_type __pos) {
> __glibcxx_assert(__pos <= size());
> ...
> }
>
> wherein __glibcxx_assert expands to
>
> if (__builtin_is_constant_evaluated() && !bool(__pos <= size())
> ...
>
> but I'm of a mind to not warn on that.
>
> Possible tweaks: merge the "always true" warnings and say something
> about a manifestly evaluated context, and perhaps add an early exit
> for TREE_CONSTANT trees because for constexpr if we are going to call
> maybe_warn_for_constant_evaluated twice.
>
> Once consteval if makes it in, we should tweak this warning one more
> time.
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> PR c++/100995
>
> gcc/cp/ChangeLog:
>
> * semantics.c (find_std_constant_evaluated_r): New.
> (maybe_warn_for_constant_evaluated): New.
> (finish_if_stmt_cond): Call it.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp2a/is-constant-evaluated9.C: Add dg-warning.
> * g++.dg/cpp2a/is-constant-evaluated12.C: New test.
> ---
> gcc/cp/semantics.c | 85 ++++++++++++++++---
> .../g++.dg/cpp2a/is-constant-evaluated12.C | 79 +++++++++++++++++
> .../g++.dg/cpp2a/is-constant-evaluated9.C | 4 +-
> 3 files changed, 152 insertions(+), 16 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated12.C
>
> diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
> index f506a239864..07459a357e2 100644
> --- a/gcc/cp/semantics.c
> +++ b/gcc/cp/semantics.c
> @@ -927,6 +927,75 @@ is_std_constant_evaluated_p (tree fn)
> return name && id_equal (name, "is_constant_evaluated");
> }
>
> +/* Callback function for maybe_warn_for_constant_evaluated that looks
> + for calls to std::is_constant_evaluated in TP. */
> +
> +static tree
> +find_std_constant_evaluated_r (tree *tp, int *walk_subtrees, void *)
> +{
> + tree t = *tp;
> +
> + if (TYPE_P (t) || TREE_CONSTANT (t))
> + {
> + *walk_subtrees = false;
> + return NULL_TREE;
> + }
> +
> + switch (TREE_CODE (t))
> + {
> + case CALL_EXPR:
> + if (is_std_constant_evaluated_p (t))
> + return t;
> + break;
> + case EXPR_STMT:
> + /* Don't warn in statement expressions. */
> + *walk_subtrees = false;
> + return NULL_TREE;
> + default:
> + break;
> + }
> +
> + return NULL_TREE;
> +}
> +
> +/* In certain contexts, std::is_constant_evaluated() is always true (for
> + instance, in a consteval function or in a constexpr if), or always false
> + (e.g., in a non-constexpr non-consteval function) so give the user a clue. */
> +
> +static void
> +maybe_warn_for_constant_evaluated (tree cond, bool constexpr_if)
> +{
> + if (!warn_tautological_compare)
> + return;
> +
> + /* Suppress warning for std::is_constant_evaluated if the conditional
> + comes from a macro. */
> + if (from_macro_expansion_at (EXPR_LOCATION (cond)))
> + return;
> +
> + cond = cp_walk_tree_without_duplicates (&cond, find_std_constant_evaluated_r,
> + NULL);
> + if (cond)
> + {
> + if (constexpr_if)
> + warning_at (EXPR_LOCATION (cond), OPT_Wtautological_compare,
> + "%<std::is_constant_evaluated%> always evaluates to "
> + "true in %<if constexpr%>");
> + else if (!DECL_DECLARED_CONSTEXPR_P (current_function_decl)
> + /* C++17 lambda op() is implicitly constexpr but finish_function
> + may not have marked it as such. */
> + && !(cxx_dialect >= cxx17
> + && LAMBDA_TYPE_P (CP_DECL_CONTEXT (current_function_decl))))
Let's factor a new maybe_constexpr_fn out of var_in_maybe_constexpr_fn.
> + warning_at (EXPR_LOCATION (cond), OPT_Wtautological_compare,
> + "%<std::is_constant_evaluated%> always evaluates to "
> + "false in a non-%<constexpr%> function");
> + else if (DECL_IMMEDIATE_FUNCTION_P (current_function_decl))
> + warning_at (EXPR_LOCATION (cond), OPT_Wtautological_compare,
> + "%<std::is_constant_evaluated%> always evaluates to "
> + "true in a %<consteval%> function");
> + }
> +}
> +
> /* Process the COND of an if-statement, which may be given by
> IF_STMT. */
>
> @@ -942,23 +1011,11 @@ finish_if_stmt_cond (tree cond, tree if_stmt)
> converted to bool. */
> && TYPE_MAIN_VARIANT (TREE_TYPE (cond)) == boolean_type_node)
> {
> - /* if constexpr (std::is_constant_evaluated()) is always true,
> - so give the user a clue. */
> - if (warn_tautological_compare)
> - {
> - tree t = cond;
> - if (TREE_CODE (t) == CLEANUP_POINT_EXPR)
> - t = TREE_OPERAND (t, 0);
> - if (TREE_CODE (t) == CALL_EXPR
> - && is_std_constant_evaluated_p (t))
> - warning_at (EXPR_LOCATION (cond), OPT_Wtautological_compare,
> - "%qs always evaluates to true in %<if constexpr%>",
> - "std::is_constant_evaluated");
> - }
> -
> + maybe_warn_for_constant_evaluated (cond, /*constexpr_if=*/true);
> cond = instantiate_non_dependent_expr (cond);
> cond = cxx_constant_value (cond, NULL_TREE);
> }
Missing an 'else' here? In your intro you mention calling the function
twice for constexpr if, but why is that necessary?
> + maybe_warn_for_constant_evaluated (cond, /*constexpr_if=*/false);
> finish_cond (&IF_COND (if_stmt), cond);
> add_stmt (if_stmt);
> THEN_CLAUSE (if_stmt) = push_stmt_list ();
> diff --git a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated12.C b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated12.C
> new file mode 100644
> index 00000000000..0de6bf74f4a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated12.C
> @@ -0,0 +1,79 @@
> +// PR c++/100995
> +// { dg-do compile { target c++20 } }
> +// { dg-options "-Wtautological-compare" }
> +
> +namespace std {
> + constexpr inline bool
> + is_constant_evaluated () noexcept
> + {
> + return __builtin_is_constant_evaluated ();
> + }
> +}
> +
> +template <int I = []() constexpr { if (std::is_constant_evaluated()) return 1; return 0; }()>
> +struct X { };
> +
> +template <int I = [](){ if (std::is_constant_evaluated()) return 1; return 0; }()>
> +struct Y { };
> +
> +template <int I = [](){ if constexpr (std::is_constant_evaluated()) return 1; return 0; }()> // { dg-warning "always evaluates to true" }
> +struct Z { };
> +
> +constexpr bool b = true;
> +
> +#define __glibcxx_assert(cond) \
> + if (__builtin_is_constant_evaluated() && !bool(cond)) \
> +__builtin_unreachable()
> +#define CHECK __builtin_is_constant_evaluated() // { dg-warning "always evaluates to false" }
> +#define CHECK2 __builtin_is_constant_evaluated()
> +
> +int
> +foo ()
> +{
> + if (std::is_constant_evaluated ()) // { dg-warning "always evaluates to false" }
> + return 1;
> + __glibcxx_assert(b);
> + if (CHECK && b)
> + return 2;
> + if (CHECK2)
> + return 3;
> + return 0;
> +}
> +
> +constexpr int
> +bar ()
> +{
> + if (std::is_constant_evaluated ())
> + return 1;
> + if constexpr (std::is_constant_evaluated ()) // { dg-warning "always evaluates to true" }
> + return 2;
> + if constexpr (std::is_constant_evaluated () && b) // { dg-warning "always evaluates to true" }
> + return 3;
> + if constexpr (!std::is_constant_evaluated ()) // { dg-warning "always evaluates to true" }
> + return 4;
> + return 0;
> +}
> +
> +consteval int
> +baz ()
> +{
> + if (std::is_constant_evaluated ()) // { dg-warning "always evaluates to true" }
> + return 1;
> + return 0;
> +}
> +
> +int
> +qux ()
> +{
> + if (({ static bool a = std::is_constant_evaluated (); a; }))
> + return 1;
> + if (({ bool a = std::is_constant_evaluated (); a; }))
> + return 2;
> + if (static bool a = std::is_constant_evaluated (); a)
> + return 3;
> + if (bool a = std::is_constant_evaluated (); a)
> + return 4;
> + if constexpr (constexpr bool a = std::is_constant_evaluated (); a)
> + return 5;
> + return 0;
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated9.C b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated9.C
> index 0e96a1a3729..5e405e71cc0 100644
> --- a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated9.C
> +++ b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated9.C
> @@ -32,7 +32,7 @@ constexpr int
> foo3(int i)
> {
> // I is not a constant expression but we short-circuit it.
> - if constexpr (__builtin_is_constant_evaluated () || i)
> + if constexpr (__builtin_is_constant_evaluated () || i) // { dg-warning ".std::is_constant_evaluated. always evaluates to true in .if constexpr." }
> return 42;
> else
> return i;
> @@ -42,7 +42,7 @@ constexpr int
> foo4(int i)
> {
> const int j = 0;
> - if constexpr (j && __builtin_is_constant_evaluated ())
> + if constexpr (j && __builtin_is_constant_evaluated ()) // { dg-warning ".std::is_constant_evaluated. always evaluates to true in .if constexpr." }
> return 42;
> else
> return i;
>
> base-commit: 4f625f47b4456e5c05a025fca4d072831e59126c
>
next prev parent reply other threads:[~2021-06-10 14:27 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-10 1:46 Marek Polacek
2021-06-10 14:27 ` Jason Merrill [this message]
2021-06-10 23:04 ` [PATCH v2] " Marek Polacek
2021-06-11 2:25 ` Jason Merrill
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