c++: Don't cache calls which rethrow etc. [PR126508]

The first 3 testcase below are miscompiled, we happily cache
calls during constant evaluation which don't depend just on their
arguments, but also on the current exceptions (uncaught or caught).
If we decide to cache such functions and then try to evaluate them
with different uncaught/caught exceptions (or none), we can get wrong
results.
We already don't cache calls which allocate and don't free all heap
allocations, or free some heap allocations they haven't allocated,
or which call (right now any) metafunctions, or have exited through
exception, or aren't constant.
This patch just adds the rethrow/__builtin_uncaught_exceptions/
__builtin_current_exception calls to the set of non-cacheable operations
(to be precise, e.g. rethrow would be safe to cache if we can prove
that the current exception was always thrown from within that function,
ditto __builtin_current_exception, but it is hard to figure out).
The last testcase attempts to check if we don't need something similar
also for __builtin_eh_ptr_adjust_ref, but the call to foo for some reason
isn't cached and so I don't have a proof we need to handle it too.

2026-07-31  Jakub Jelinek  <jakub@redhat.com>

	PR c++/126508
	* constexpr.cc (cxx_eval_cxa_builtin_fn): Avoid caching
	calls which rethrow or call __builtin_uncaught_exceptions
	or __builtin_current_exception.

	* g++.dg/cpp26/constexpr-eh20.C: New test.
	* g++.dg/cpp26/constexpr-eh21.C: New test.
	* g++.dg/cpp26/constexpr-eh22.C: New test.
	* g++.dg/cpp26/constexpr-eh23.C: New test.

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index b4db9fd2bae42663ae1f13de742b2e1b459b29c1..105f3c6d8149336c43e3cc00e14d218d2a364f65 100644 (file)
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -2118,6 +2118,9 @@ cxx_eval_cxa_builtin_fn (const constexpr_ctx *ctx, tree call,
       DECL_EXCEPTION_REFCOUNT (arg)
        = size_binop (PLUS_EXPR, DECL_EXCEPTION_REFCOUNT (arg), size_one_node);
       ++ctx->global->uncaught_exceptions;
+      /* Don't cache calls which rethrow, they depend on the current
+        exception which might be caught in the caller.  */
+      ctx->global->metafns_called = true;
       *jump_target = arg;
       return void_node;
     case CXA_BAD_CAST:
@@ -2196,6 +2199,10 @@ cxx_eval_cxa_builtin_fn (const constexpr_ctx *ctx, tree call,
          *non_constant_p = true;
          return call;
        }
+      /* Don't cache calls which call __builtin_uncaught_exceptions (),
+        they depend on the current uncaught exceptions which might
+        be the state from their caller.  */
+      ctx->global->metafns_called = true;
       return build_int_cst (integer_type_node,
                            ctx->global->uncaught_exceptions);
     case BUILTIN_CURRENT_EXCEPTION:
@@ -2247,6 +2254,10 @@ cxx_eval_cxa_builtin_fn (const constexpr_ctx *ctx, tree call,
                              size_one_node);
              arg = fold_convert (ptr_type_node, build_address (arg));
            }
+         /* Don't cache calls which call __builtin_current_exception (),
+            they depend on the current exception which might be caught
+            in the caller.  */
+         ctx->global->metafns_called = true;
          return build_constructor_single (TREE_TYPE (decl), fld, arg);
        }
     case STD_RETHROW_EXCEPTION:

diff --git a/gcc/testsuite/g++.dg/cpp26/constexpr-eh20.C b/gcc/testsuite/g++.dg/cpp26/constexpr-eh20.C
new file mode 100644 (file)
index 0000000..4384698
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/constexpr-eh20.C
@@ -0,0 +1,31 @@
+// PR c++/126508
+// { dg-do compile { target c++26 } }
+
+constexpr int
+foo ()
+{
+  try
+    {
+      throw;
+    }
+  catch (const int x)
+    {
+      return x;
+    }
+}
+
+constexpr int
+bar (int x)
+{
+  try
+    {
+      throw x;
+    }
+  catch (...)
+    {
+      return foo ();
+    }
+}
+
+static_assert (bar (42) == 42);
+static_assert (bar (43) == 43);

diff --git a/gcc/testsuite/g++.dg/cpp26/constexpr-eh21.C b/gcc/testsuite/g++.dg/cpp26/constexpr-eh21.C
new file mode 100644 (file)
index 0000000..30fd6dc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/constexpr-eh21.C
@@ -0,0 +1,43 @@
+// PR c++/126508
+// { dg-do compile { target c++26 } }
+
+constexpr int
+foo ()
+{
+  return __builtin_uncaught_exceptions ();
+}
+
+constexpr int
+bar ()
+{
+  return __builtin_uncaught_exceptions ();
+}
+
+struct A { constexpr A () : a (0) {} constexpr ~A () { if (foo () != a) asm (""); } int a; };
+struct B { constexpr B () : b (0) {} constexpr ~B () { if (bar () != b) asm (""); } int b; };
+
+constexpr bool
+baz ()
+{
+  {
+    A a;
+  }
+  try
+    {
+      A a;
+      B b;
+      a.a = 1;
+      b.b = 1;
+      throw 42;
+    }
+  catch (...)
+    {
+    }
+  {
+    A a;
+    B b;
+  }
+  return true;
+}
+
+static_assert (baz ());

diff --git a/gcc/testsuite/g++.dg/cpp26/constexpr-eh22.C b/gcc/testsuite/g++.dg/cpp26/constexpr-eh22.C
new file mode 100644 (file)
index 0000000..291ec33
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/constexpr-eh22.C
@@ -0,0 +1,41 @@
+// PR c++/126508
+// { dg-do compile { target c++26 } }
+
+#include <exception>
+
+constexpr bool
+foo ()
+{
+  return __builtin_current_exception () != nullptr;
+}
+
+constexpr int
+bar ()
+{
+  return __builtin_current_exception () != nullptr;
+}
+
+constexpr bool
+baz ()
+{
+  if (foo ())
+    return false;
+  try
+    {
+      throw 42;
+    }
+  catch (...)
+    {
+      if (!foo ())
+       return false;
+      if (!bar ())
+       return false;
+    }
+  if (foo ())
+    return false;
+  if (bar ())
+    return false;
+  return true;
+}
+
+static_assert (baz ());

diff --git a/gcc/testsuite/g++.dg/cpp26/constexpr-eh23.C b/gcc/testsuite/g++.dg/cpp26/constexpr-eh23.C
new file mode 100644 (file)
index 0000000..1b1a77d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/constexpr-eh23.C
@@ -0,0 +1,22 @@
+// PR c++/126508
+// { dg-do compile { target c++26 } }
+
+#include <exception>
+
+constexpr std::exception_ptr
+foo (const std::exception_ptr &x)
+{
+  return x;
+}
+
+constexpr bool
+baz ()
+{
+  std::exception_ptr a = std::make_exception_ptr (42);
+  auto b = foo (a);
+  auto c = foo (a);
+  auto d = foo (a);
+  return true;
+}
+
+static_assert (baz ());