[Bug c++/101765] New: ICE when using a VLA inside of a coroutine

[Bug c++/101765] New: ICE when using a VLA inside of a coroutine

[kacper.slominski72 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101765

            Bug ID: 101765
           Summary: ICE when using a VLA inside of a coroutine
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kacper.slominski72 at gmail dot com
  Target Milestone: ---

The following code, when compiled with "-std=c++20 -fcoroutines" (on GCC
versions starting from 10.1 and including trunk) causes an ICE:
#include <coroutine>

struct dummy_coro {
        using promise_type = dummy_coro;
        bool await_ready() noexcept { return false; }
        void await_suspend(std::coroutine_handle<>) noexcept { }
        void await_resume() noexcept { }
        dummy_coro get_return_object() { return {}; }
        dummy_coro initial_suspend() { return {}; }
        dummy_coro final_suspend() noexcept { return {}; }
        void return_void() { }
        void unhandled_exception() { }
};

dummy_coro foo(int arg) {
        char arr[arg];
        co_return;
}

Compiler output from GCC 11:
<source>: In function 'dummy_coro foo(int)':
<source>:18:1: internal compiler error: Segmentation fault
   18 | }
      | ^
0x1786229 internal_error(char const*, ...)
        ???:0
0x100ff23 wi::lts_p_large(long const*, unsigned int, unsigned int, long const*,
unsigned int)
        ???:0
0x6a7c06 finish_struct_1(tree_node*)
        ???:0
0x6a9c8c finish_struct(tree_node*, tree_node*)
        ???:0
0x6cd958 morph_fn_to_coro(tree_node*, tree_node**, tree_node**)
        ???:0
0x6fd221 finish_function(bool)
        ???:0
0x7c47db c_parse_file()
        ???:0
0x896762 c_common_parse_file()
        ???:0

Compiler output from GCC trunk:
<source>: In function 'dummy_coro foo(int)':
<source>:18:1: internal compiler error: tree check: expected integer_cst, have
plus_expr in get_len, at tree.h:6066
   18 | }
      | ^
0x1dadf09 internal_error(char const*, ...)
        ???:0
0x69f111 tree_check_failed(tree_node const*, char const*, int, char const*,
...)
        ???:0
0x739efa finish_struct_1(tree_node*)
        ???:0
0x73ca54 finish_struct(tree_node*, tree_node*)
        ???:0
0x77d64c morph_fn_to_coro(tree_node*, tree_node**, tree_node**)
        ???:0
0x7de6df finish_function(bool)
        ???:0
0x914885 c_parse_file()
        ???:0
0xa97472 c_common_parse_file()
        ???:0

[Bug c++/101765] ICE when using a VLA inside of a coroutine

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101765

--- Comment #1 from Iain Sandoe <iains at gcc dot gnu.org> ---
I am not sure that a VLA can be used in a coroutine (neither can alloca, if I
remember correctly) [so not sure that this is ICE on valid, it could be ICE on
invalid]

Either way, we should not ICE from it.

[Bug c++/101765] ICE when using a VLA inside of a coroutine

[kacper.slominski72 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101765

--- Comment #2 from Kacper Słomiński <kacper.slominski72 at gmail dot com> ---
(In reply to Iain Sandoe from comment #1)
> I am not sure that a VLA can be used in a coroutine (neither can alloca, if
> I remember correctly) [so not sure that this is ICE on valid, it could be
> ICE on invalid]
> 
> Either way, we should not ICE from it.

Using alloca or __builtin_alloca inside of coroutines compiles fine, but
despite that I think this should be ICE-on-invalid, as I don't see how stack
allocations would work inside of coroutines, since AFAIK the coro frame size
must be known up-front.

[Bug c++/101765] ICE when using a VLA inside of a coroutine

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101765

--- Comment #3 from Iain Sandoe <iains at gcc dot gnu.org> ---
(In reply to Kacper Słomiński from comment #2)
> (In reply to Iain Sandoe from comment #1)
> > I am not sure that a VLA can be used in a coroutine (neither can alloca, if
> > I remember correctly) [so not sure that this is ICE on valid, it could be
> > ICE on invalid]
> > 
> > Either way, we should not ICE from it.
> 
> Using alloca or __builtin_alloca inside of coroutines compiles fine,

That's probably an error ;-) ... I recall discussing this with Gor after Jakub
raised the question at a GNU cauldron - with the outcome that the original
implementation (MSVC) errors-out on alloca.  

> but
> despite that I think this should be ICE-on-invalid,

yeah, a VLA would mean a (potentially) different frame layout for each call -
which the current implementations do not support.

> as I don't see how stack
> allocations would work inside of coroutines, since AFAIK the coro frame size
> must be known up-front.

It must be a compile-time constant, known before lowering to IR - but we should
not ICE  in any case.

( I have not had a chance yet to process this further )

[Bug c++/101765] ICE when using a VLA inside of a coroutine

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101765

Iain Sandoe <iains at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-10-02
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #4 from Iain Sandoe <iains at gcc dot gnu.org> ---
For the record, general VLAs are not supported by clang coroutines either.
the following coroutine fails to compile there as well.

//using test-suite convenience headers.
#include "coro.h"
#include "coro1-ret-int-yield-int.h"

struct coro1
foo (int arg) noexcept
{
  PRINTF ("foo arg = %d\n", arg);
  char arr[arg];
  if (arg < 4)
    co_return -6174;
  else
    for (int i = 0; i < arg; ++i) arr[i] = (char) i;
  co_yield (int) arr[2];
  co_return (int) arr[3];
}

[Bug c++/101765] ICE when using a VLA inside of a coroutine

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101765

Iain Sandoe <iains at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.4

[Bug c++/101765] ICE when using a VLA inside of a coroutine

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101765

Iain Sandoe <iains at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |iains at gcc dot gnu.org

[Bug c++/101765] ICE when using a VLA inside of a coroutine

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101765

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Iain D Sandoe <iains@gcc.gnu.org>:

https://gcc.gnu.org/g:fdf0b6ce6c1cfa1c328c0c40473c71ca11fd8303

commit r12-4103-gfdf0b6ce6c1cfa1c328c0c40473c71ca11fd8303
Author: Iain Sandoe <iain@sandoe.co.uk>
Date:   Sat Oct 2 16:15:38 2021 +0100

    coroutines: Fail with a sorry when presented with a VLA [PR 101765].

    We do not support this yet.

    Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

            PR c++/101765

    gcc/cp/ChangeLog:

            * coroutines.cc (register_local_var_uses): Emit a sorry if
            we encounter a VLA in the coroutine local variables.

    gcc/testsuite/ChangeLog:

            * g++.dg/coroutines/pr101765.C: New test.

[Bug c++/101765] ICE when using a VLA inside of a coroutine

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101765

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.4                        |10.5

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 10.4 is being released, retargeting bugs to GCC 10.5.

[Bug c++/101765] ICE when using a VLA inside of a coroutine

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101765

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.5                        |---