beam_emu.c: Avoid triggering an assertion for the wrong reason

Before 2d2e78ad6e66 that introduced tail-recursive calls of BIFs, the
stack was guaranteed not to be empty when `erlang:raise/3` was called
from the `catch` block of a `try` (because the `try` had set up a
stack frame that would be deallocated after the `raise` call).

Now the stack can be empty, so the ASSERT() call in next_catch()
that checks that there is a continuation pointer at the top of
the stack may fail.

Move the ASSERT() call to after check for empty stack.

While at it, also add a comment of the reason for the assertion.

1 files changed, 7 insertions, 1 deletions

diff --git a/erts/emulator/beam/beam_emu.c b/erts/emulator/beam/beam_emu.c
index d68d021679..73bf443372 100644
--- a/erts/emulator/beam/beam_emu.c
+++ b/erts/emulator/beam/beam_emu.c
@@ -1479,9 +1479,15 @@ next_catch(Process* c_p, Eterm *reg) {
     BeamInstr i_return_time_trace = beam_return_time_trace[0];
 
     ptr = prev = c_p->stop;
-    ASSERT(is_CP(*ptr));
     ASSERT(ptr <= STACK_START(c_p));
     if (ptr == STACK_START(c_p)) return NULL;
+
+    /*
+     * Better safe than sorry here. In debug builds, produce a core
+     * dump if the top of the stack doesn't point to a continuation
+     * pointer. In other builds, ignore a non-CP at the top of stack.
+     */
+    ASSERT(is_CP(*ptr));
     if ((is_not_CP(*ptr) || (*cp_val(*ptr) != i_return_trace &&
 			     *cp_val(*ptr) != i_return_to_trace &&
 			     *cp_val(*ptr) != i_return_time_trace ))