public inbox for gcc-patches@gcc.gnu.org
 help / color / mirror / Atom feed
* [PATCH] c++: Extend std::is_constant_evaluated in if warning [PR100995]
@ 2021-06-10  1:46 Marek Polacek
  2021-06-10 14:27 ` Jason Merrill
  0 siblings, 1 reply; 4+ messages in thread
From: Marek Polacek @ 2021-06-10  1:46 UTC (permalink / raw)
  To: Jason Merrill, GCC Patches

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))))
+	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);
     }
+  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
-- 
2.31.1


^ permalink raw reply	[flat|nested] 4+ messages in thread
end of thread, other threads:[~2021-06-11  2:25 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-06-10  1:46 [PATCH] c++: Extend std::is_constant_evaluated in if warning [PR100995] Marek Polacek
2021-06-10 14:27 ` Jason Merrill
2021-06-10 23:04   ` [PATCH v2] " Marek Polacek
2021-06-11  2:25     ` Jason Merrill

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).