[PATCH RFC] c++: fix broken conversion in coroutines

[PATCH RFC] c++: fix broken conversion in coroutines

[Jason Merrill]

You can't use CONVERT_EXPR to convert between two class types, and it was
breaking copy elision.

Unfortunately, this patch breaks symmetric-transfer-00-basic.C, where
susp_type is Loopy<int>::handle_type.  How is this supposed to work?

gcc/cp/ChangeLog:

	* coroutines.cc (expand_one_await_expression): Change conversion
	to assert.
---
 gcc/cp/coroutines.cc | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
index eca01abcb7a..568f2edf67d 100644
--- a/gcc/cp/coroutines.cc
+++ b/gcc/cp/coroutines.cc
@@ -1728,7 +1728,9 @@ expand_one_await_expression (tree *stmt, tree *await_expr, void *d)
     }
   else
     {
-      r = build1_loc (loc, CONVERT_EXPR, void_coro_handle_type, suspend);
+      gcc_checking_assert (same_type_ignoring_top_level_qualifiers_p
+			   (void_coro_handle_type, susp_type));
+      r = suspend;
       r = build2_loc (loc, INIT_EXPR, void_coro_handle_type, data->conthand, r);
       r = build1 (CONVERT_EXPR, void_type_node, r);
       append_to_statement_list (r, &body_list);

base-commit: 43faf3e5445b571731e52faa1be085ecd0a09323
-- 
2.31.1

Re: [PATCH RFC] c++: fix broken conversion in coroutines

[Iain Sandoe]

Hi Jason,

> On 30 Sep 2022, at 23:06, Jason Merrill <jason@redhat.com> wrote:
> 
> You can't use CONVERT_EXPR to convert between two class types, and it was
> breaking copy elision.
> 
> Unfortunately, this patch breaks symmetric-transfer-00-basic.C, where
> susp_type is Loopy<int>::handle_type.  How is this supposed to work?

We are trying to save a type-erased handle (which the symmetric transfer makes
and indirect call through, nothing else).

so, I suppose the equivalent could be:

conthand = coroutine_handle::from_address (suspend.address())

or, is there some cast version that would be valid here?
Iain

> 
> gcc/cp/ChangeLog:
> 
> 	* coroutines.cc (expand_one_await_expression): Change conversion
> 	to assert.
> ---
> gcc/cp/coroutines.cc | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
> index eca01abcb7a..568f2edf67d 100644
> --- a/gcc/cp/coroutines.cc
> +++ b/gcc/cp/coroutines.cc
> @@ -1728,7 +1728,9 @@ expand_one_await_expression (tree *stmt, tree *await_expr, void *d)
>     }
>   else
>     {
> -      r = build1_loc (loc, CONVERT_EXPR, void_coro_handle_type, suspend);
> +      gcc_checking_assert (same_type_ignoring_top_level_qualifiers_p
> +			   (void_coro_handle_type, susp_type));
> +      r = suspend;
>       r = build2_loc (loc, INIT_EXPR, void_coro_handle_type, data->conthand, r);
>       r = build1 (CONVERT_EXPR, void_type_node, r);
>       append_to_statement_list (r, &body_list);
> 
> base-commit: 43faf3e5445b571731e52faa1be085ecd0a09323
> -- 
> 2.31.1
> 

Re: [PATCH RFC] c++: fix broken conversion in coroutines

[Jason Merrill]

On 9/30/22 18:50, Iain Sandoe wrote:
> Hi Jason,
> 
>> On 30 Sep 2022, at 23:06, Jason Merrill <jason@redhat.com> wrote:
>>
>> You can't use CONVERT_EXPR to convert between two class types, and it was
>> breaking copy elision.
>>
>> Unfortunately, this patch breaks symmetric-transfer-00-basic.C, where
>> susp_type is Loopy<int>::handle_type.  How is this supposed to work?
> 
> We are trying to save a type-erased handle (which the symmetric transfer makes
> and indirect call through, nothing else).

The problem is you're treating one class directly as another class here, 
without the indirection involved in usual type-erasure idioms.

It does seem that the gimplifier handles this fine, but it doesn't 
correspond to anything in the language and much of the front end assumes 
that CONVERT_EXPR is only used for scalars.  VIEW_CONVERT_EXPR would 
better express that we're not doing anything to the value, just cheating 
the type system.  That's still dodgy from a language perspective, but 
probably safe enough in this case.

Note that I was wrong to mention copy elision above; it's irrelevant to 
codegen here since the handle type returns in a register.

> so, I suppose the equivalent could be:
> 
> conthand = coroutine_handle::from_address (suspend.address())

That sounds more correct, yes.

Jason

Re: [PATCH RFC] c++: fix broken conversion in coroutines

[Jason Merrill]

[-- Attachment #1: Type: text/plain, Size: 1139 bytes --]

On 10/3/22 23:53, Jason Merrill wrote:
> On 9/30/22 18:50, Iain Sandoe wrote:
>> Hi Jason,
>>
>>> On 30 Sep 2022, at 23:06, Jason Merrill <jason@redhat.com> wrote:
>>>
>>> You can't use CONVERT_EXPR to convert between two class types, and it 
>>> was
>>> breaking copy elision.
>>>
>>> Unfortunately, this patch breaks symmetric-transfer-00-basic.C, where
>>> susp_type is Loopy<int>::handle_type.  How is this supposed to work?
>>
>> We are trying to save a type-erased handle (which the symmetric 
>> transfer makes
>> and indirect call through, nothing else).
> 
> The problem is you're treating one class directly as another class here, 
> without the indirection involved in usual type-erasure idioms.
> 
> It does seem that the gimplifier handles this fine, but it doesn't 
> correspond to anything in the language and much of the front end assumes 
> that CONVERT_EXPR is only used for scalars.  VIEW_CONVERT_EXPR would 
> better express that we're not doing anything to the value, just cheating 
> the type system.  That's still dodgy from a language perspective, but 
> probably safe enough in this case.

So I'm applying this:

[-- Attachment #2: 0001-c-fix-broken-conversion-in-coroutines.patch --]
[-- Type: text/x-patch, Size: 1891 bytes --]

From 0cd9dafcfa8cf69b3a1eae91bea82d388c5328d2 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Fri, 30 Sep 2022 10:04:22 -0400
Subject: [PATCH] c++: fix broken conversion in coroutines
To: gcc-patches@gcc.gnu.org

You can't use CONVERT_EXPR to convert between two class types.

VIEW_CONVERT_EXPR takes liberties with the C++ type system, but is probably
safe in this context.  Let's also only use it when the type isn't already
what we want.

gcc/cp/ChangeLog:

	* coroutines.cc (expand_one_await_expression): Change conversion
	to VIEW_CONVERT_EXPR.
	* cp-gimplify.cc (cp_genericize_r) [CONVERT_EXPR]: Add assert.
---
 gcc/cp/coroutines.cc  | 5 ++++-
 gcc/cp/cp-gimplify.cc | 1 +
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
index eca01abcb7a..60b846600b9 100644
--- a/gcc/cp/coroutines.cc
+++ b/gcc/cp/coroutines.cc
@@ -1728,7 +1728,10 @@ expand_one_await_expression (tree *stmt, tree *await_expr, void *d)
     }
   else
     {
-      r = build1_loc (loc, CONVERT_EXPR, void_coro_handle_type, suspend);
+      r = suspend;
+      if (!same_type_ignoring_top_level_qualifiers_p (susp_type,
+						      void_coro_handle_type))
+	r = build1_loc (loc, VIEW_CONVERT_EXPR, void_coro_handle_type, r);
       r = build2_loc (loc, INIT_EXPR, void_coro_handle_type, data->conthand, r);
       r = build1 (CONVERT_EXPR, void_type_node, r);
       append_to_statement_list (r, &body_list);
diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index b4599fc34d8..5d26e59d098 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -1589,6 +1589,7 @@ cp_genericize_r (tree *stmt_p, int *walk_subtrees, void *data)
       break;
 
     case CONVERT_EXPR:
+      gcc_checking_assert (!AGGREGATE_TYPE_P (TREE_TYPE (stmt)));
       gcc_assert (!CONVERT_EXPR_VBASE_PATH (stmt));
       break;
 
-- 
2.31.1

Re: [PATCH RFC] c++: fix broken conversion in coroutines

[Iain Sandoe]

> On 6 Oct 2022, at 22:44, Jason Merrill <jason@redhat.com> wrote:
> 
> On 10/3/22 23:53, Jason Merrill wrote:
>> On 9/30/22 18:50, Iain Sandoe wrote:
>>> Hi Jason,
>>> 
>>>> On 30 Sep 2022, at 23:06, Jason Merrill <jason@redhat.com> wrote:
>>>> 
>>>> You can't use CONVERT_EXPR to convert between two class types, and it was
>>>> breaking copy elision.
>>>> 
>>>> Unfortunately, this patch breaks symmetric-transfer-00-basic.C, where
>>>> susp_type is Loopy<int>::handle_type.  How is this supposed to work?
>>> 
>>> We are trying to save a type-erased handle (which the symmetric transfer makes
>>> and indirect call through, nothing else).
>> The problem is you're treating one class directly as another class here, without the indirection involved in usual type-erasure idioms.
>> It does seem that the gimplifier handles this fine, but it doesn't correspond to anything in the language and much of the front end assumes that CONVERT_EXPR is only used for scalars.  VIEW_CONVERT_EXPR would better express that we're not doing anything to the value, just cheating the type system.  That's still dodgy from a language perspective, but probably safe enough in this case.
> 
> So I'm applying this:<0001-c-fix-broken-conversion-in-coroutines.patch>

thanks, I have not had any cycles to look at this.

however, when I next do - was planning on looking at the:
cont = handle.from_address(await_suspend().address())
approach, since both .address() and .from_address() are constexpr, cp_fold_function should turn that into essentially a NOP.
Iain