c++: top level bind when rewriting coroutines [PR106188]

In the edge case of a coroutine not containing any locals, the ifcd/switch
temporaries would get added to the coroutine frame, corrupting its
layout. To prevent this, we can make sure there is always a BIND_EXPR at
the top of the function body, and thus, always a place for our new
temporaries to go without interfering with the coroutine frame.

PR c++/106188 - Incorrect frame layout after transforming conditional statement without top-level bind expression
PR c++/106713 - if (co_await ...) crashes with a jump to ud2

	PR c++/106188
	PR c++/106713

gcc/cp/ChangeLog:

	* coroutines.cc (coro_rewrite_function_body): Ensure we have a
	BIND_EXPR wrapping the function body.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr106188.C: New test.

Signed-off-by: Arsen Arsenović <arsen@aarsen.me>

2 files changed, 43 insertions, 0 deletions

diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
index edb3b70..eca01ab 100644
--- a/gcc/cp/coroutines.cc
+++ b/gcc/cp/coroutines.cc
@@ -4095,6 +4095,15 @@ coro_rewrite_function_body (location_t fn_start, tree fnbody, tree orig,
       BLOCK_SUPERCONTEXT (replace_blk) = top_block;
       BLOCK_SUBBLOCKS (top_block) = replace_blk;
     }
+  else
+    {
+      /* We are missing a top level BIND_EXPR. We need one to ensure that we
+	 don't shuffle around the coroutine frame and corrupt it.  */
+      tree bind_wrap = build3_loc (fn_start, BIND_EXPR, void_type_node,
+				   NULL, NULL, NULL);
+      BIND_EXPR_BODY (bind_wrap) = fnbody;
+      fnbody = bind_wrap;
+    }
 
   /* Wrap the function body in a try {} catch (...) {} block, if exceptions
      are enabled.  */
diff --git a/gcc/testsuite/g++.dg/coroutines/pr106188.C b/gcc/testsuite/g++.dg/coroutines/pr106188.C
new file mode 100644
index 0000000..9db3778
--- /dev/null
+++ b/gcc/testsuite/g++.dg/coroutines/pr106188.C
@@ -0,0 +1,34 @@
+// { dg-do run { target c++20 } }
+// test case from pr106188, w/o workaround
+#include <coroutine>
+
+struct task {
+  struct promise_type {
+    task get_return_object() { return task{}; }
+    void return_void() {}
+    void unhandled_exception() {}
+    auto initial_suspend() noexcept { return std::suspend_never{}; }
+    auto final_suspend() noexcept { return std::suspend_never{}; }
+  };
+};
+
+struct suspend_and_resume {
+  bool await_ready() const { return false; }
+  void await_suspend(std::coroutine_handle<> h) { h.resume(); }
+  void await_resume() {}
+};
+
+task f() {
+  if (co_await suspend_and_resume{}, false) {}
+}
+
+task g() {
+  switch (co_await suspend_and_resume{}, 0) {
+    default: break;
+  }
+}
+
+int main() {
+  f();
+  g();
+}