Re: [PATCH RFC] Fix ICE due to shared BLOCK node in coroutine generation [PR103328]

[Iain Sandoe <iain@sandoe.co.uk>]

Hi Folks

> On 27 Mar 2022, at 02:33, Jason Merrill <jason@redhat.com> wrote:
> 
> On 3/17/22 07:37, Benno Evers via Gcc-patches wrote:
>> The coroutine transformation moves the original function body into a
>> newly created actor function, but the block of the
>> `current_binding_level` still points into the original function,
>> causing the block to be shared between the two functions if it is
>> subsequently used. This may cause havoc later on, as subsequent
>> compiler passes for one function will also implicitly modify the
>> other. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103328#c19 for
>> a more detailed writeup.
>> This patch fixes the issue locally, but I'm not familiar with the GCC
>> code base so I'm not sure if this is the right way to go about it or
>> if there's a cleaner way to reset the current binding level. If this
>> is the way to go I'm happy to extend the patch with a testcase and
>> changelog entry.
> 
> Please do, it looks like a good fix to me.  Iain, does it make sense to you as well?

Yes, LGTM, thanks for the patch,

I have a testcase for this already locally (diff attatched)

The patch needs a changelog entry and to reference the PR that’s fixed.

Benno, do you have write access?

If not I can take care of this for you if you like?

thanks
Iain


diff --git a/gcc/testsuite/g++.dg/coroutines/pr103328.C b/gcc/testsuite/g++.dg/coroutines/pr103328.C
new file mode 100644
index 00000000000..56fb54ab316
--- /dev/null
+++ b/gcc/testsuite/g++.dg/coroutines/pr103328.C
@@ -0,0 +1,32 @@
+// { dg-additional-options "-g" }
+
+#include <coroutine>
+
+struct task {
+  struct promise_type {
+    task get_return_object() { return {}; }
+    std::suspend_never initial_suspend() { return {}; }
+    std::suspend_never final_suspend() noexcept { return {}; }
+    void unhandled_exception() {}
+  };
+  bool await_ready() { return false; }
+  void await_suspend(std::coroutine_handle<> h) {}
+  void await_resume() {}
+};
+
+template <typename Func>
+void call(Func func) { func(); }
+
+class foo {
+  void f();
+  task g();
+};
+
+void foo::f() {
+  auto lambda = [this]() noexcept -> task {
+      co_await g();
+  };
+  (void)call<decltype(lambda)>;
+}
+
+int main() {}

> 
>> ---
>>  gcc/cp/coroutines.cc | 2 ++
>>  1 file changed, 2 insertions(+)
>> diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
>> index b1bfdc767a4..eb5f80f499b 100644
>> --- a/gcc/cp/coroutines.cc
>> +++ b/gcc/cp/coroutines.cc
>> @@ -4541,6 +4541,8 @@ morph_fn_to_coro (tree orig, tree *resumer, tree
>> *destroyer)
>>    BLOCK_VARS (top_block) = BIND_EXPR_VARS (ramp_bind);
>>    BLOCK_SUBBLOCKS (top_block) = NULL_TREE;
>> +  current_binding_level->blocks = top_block;
>> +
>>    /* The decl_expr for the coro frame pointer, initialize to zero so that we
>>       can pass it to the IFN_CO_FRAME (since there's no way to pass a type,
>>       directly apparently).  This avoids a "used uninitialized" warning.  */