[PATCH 2/2] [OpenACC] Detect pointer updates for attach operations (PR95590)

[Thomas Schwinge]

Hi Julian!

On 2020-06-22T05:14:44-0700, Julian Brown <julian@codesourcery.com> wrote:
> As mentioned in the parent email, this is a fix for PR95590 that detects
> updates of attached pointers in blocks, and rewrites the attached pointer
> and resets its attachment counter appropriately. I am however not entirely
> sure this is desirable or required by the spec: points against are:
>
>  - To avoid expensive copies from the device to the host and/or "wrong
>    way" device-to-host splay tree lookups, it requires keeping an extra
>    shadow copy of mapped blocks on the host in order to detect if a
>    host pointer with attachments in the block has been changed between
>    attach operations.

I haven't spent too much time trying, but I too have not yet seen a way
to avoid keeping this state ("shadow copy"), or looking it up on demand
("expensive copies from the device to the host").

I suppose we cannot get the necessary information/state from the
host-side pointer (value) alone, and/or other state kept in the
'splay_tree_key n' etc.?

>    We incur this overhead unconditionally if
>    attach/detach are in use for what's not likely to be a common use case

Is the overhead so bad, though?  As soon as there's an 'attach', we have
to 'malloc' anyway (can combine the two, as you've done), and the
checking overhead doesn't seem so bad either?

Should we reach out to other OpenACC compiler implementors, and ask for
their understanding/approach to this aspect?

>    (it's slightly tricky to write a test case to exercise the behaviour,
>    even -- Thomas's unmodified original for the PR raises an error after
>    the previous patch in this series).

Challange accepted!  ;-P (..., but not right now.)

>  - From a user perspective, I think it's going to be quite easy to get
>    confused wrt. the hidden attachment counter state

(Indeed that "hidden" aspect is a bit confusing.  I've even thought
whether we should add some 'gomp_get_attach_count' function just for our
own testing purposes.)

>    with this kind of
>    reset-on-host-pointer-modification behaviour.  Mind you, silently *not*
>    doing the update is likewise going to be confusing (the stale device
>    pointer would be updated at present).  Maybe this should be detected
>    as an error instead?

I don't understand that, I'm afraid, because as I have quoted in
<https://gcc.gnu.org/PR95590> "OpenACC 'attach' behavior if already
attached to different data", OpenACC explicitly mandates the
"reset-on-host-pointer-modification" behavior, so I don't see a way to
avoid implementing that?

>  - The text in "2.6.8. Attachment Counter" *might* contribute to the
>    argument that this kind of pointer-update detection is not required.

Do you think these texts are in conflict in some way (that's not obvious
to me)?


Conceptually we're talking about making this behave:

    int a[];
    #pragma acc enter data create(a)

    int b[];
    #pragma acc enter data create(b)

    int *p;

    #pragma acc data create(p) // create long-lived device copy of 'p'
      {
        p = a;
        #pragma acc enter data attach(p) // explicit, or implicit
        #pragma acc parallel present(p)
          { fill_array(p); } // writes to device copy of 'a'

        p = b;
        #pragma acc enter data attach(p) // explicit, or implicit
        #pragma acc parallel present(p)
          { fill_array(p); } // writes to device copy of 'b'
      }

    #pragma acc parallel present(a, b)
      { [use device copies of 'a', 'b'] }

..., or this:

    int a[];
    #pragma acc enter data create(a)

    int b[];
    #pragma acc enter data create(b)

    struct {
      int data;
      int *p;
    } s s;

    #pragma acc data create(s) // create long-lived device copy of 's'
      {
        s.data = [...];
        s.p = a;
        #pragma acc update device(s) // invokes ("expected") undefined behavior w.r.t. 's.p'
        #pragma acc parallel // implicit 'attach(s.p)'
          { fill_array(s.data, s.p); } // writes to device copy of 'a'

        s.data = [...];
        s.p = a;
        #pragma acc update device(s) // invokes ("expected") undefined behavior w.r.t. 's.p'
        #pragma acc parallel // implicit 'attach(s.p)'
          { fill_array(s.data, s.p); } // writes to device copy of 'b'
      }

    #pragma acc parallel
      { [use device copies of 'a', 'b'] }

Please verify these conceptually -- and, is there any other, different
scenario to consider here?

> OK for mainline?

I think the implementation is fine, but I'd like to think about all this
a bit more.


Grüße
 Thomas


>       PR libgomp/95590
>
>       libgomp/
>       * target.c (gomp_attach_pointer): Initialise shadow copy of block with
>       attached pointers, and use to detect modifications of those pointers.
>       * testsuite/libgomp.oacc-c-c++-common/attach-ptr-change-1.c: New test.
> ---
>  libgomp/target.c                              | 29 +++++++-
>  .../attach-ptr-change-1.c                     | 74 +++++++++++++++++++
>  2 files changed, 100 insertions(+), 3 deletions(-)
>  create mode 100644 libgomp/testsuite/libgomp.oacc-c-c++-common/attach-ptr-change-1.c
>
> diff --git a/libgomp/target.c b/libgomp/target.c
> index db6f56a8ff8..076cc2bbbcb 100644
> --- a/libgomp/target.c
> +++ b/libgomp/target.c
> @@ -691,6 +691,8 @@ gomp_attach_pointer (struct gomp_device_descr *devicep,
>  {
>    struct splay_tree_key_s s;
>    size_t size, idx;
> +  char *shadow_block;
> +  size_t shadow_size = n->host_end - n->host_start;
>
>    if (n == NULL)
>      {
> @@ -707,9 +709,31 @@ gomp_attach_pointer (struct gomp_device_descr *devicep,
>    if (!n->aux)
>      n->aux = gomp_malloc_cleared (sizeof (struct splay_tree_aux));
>
> +  bool first = false;
> +
>    if (!n->aux->attach_count)
> -    n->aux->attach_count
> -      = gomp_malloc_cleared (sizeof (*n->aux->attach_count) * size);
> +    {
> +      n->aux->attach_count
> +     = gomp_malloc_cleared (sizeof (*n->aux->attach_count) * size
> +                            + shadow_size);
> +      first = true;
> +    }
> +
> +  shadow_block = ((char *) n->aux->attach_count)
> +              + sizeof (*n->aux->attach_count) * size;
> +
> +  if (first)
> +    memcpy (shadow_block, (const void *) n->host_start, shadow_size);
> +
> +  uintptr_t target = (uintptr_t) *(void **) attach_to;
> +  uintptr_t shadow_target
> +    = (uintptr_t) *(void **) (shadow_block + attach_to - n->host_start);
> +  if (target != shadow_target)
> +    {
> +      n->aux->attach_count[idx] = 0;
> +      memcpy ((char *) shadow_block + attach_to - n->host_start,
> +           (const void *) target, sizeof (void *));
> +    }
>
>    if (n->aux->attach_count[idx] < UINTPTR_MAX)
>      n->aux->attach_count[idx]++;
> @@ -723,7 +747,6 @@ gomp_attach_pointer (struct gomp_device_descr *devicep,
>      {
>        uintptr_t devptr = n->tgt->tgt_start + n->tgt_offset + attach_to
>                        - n->host_start;
> -      uintptr_t target = (uintptr_t) *(void **) attach_to;
>        splay_tree_key tn;
>        uintptr_t data;
>
> diff --git a/libgomp/testsuite/libgomp.oacc-c-c++-common/attach-ptr-change-1.c b/libgomp/testsuite/libgomp.oacc-c-c++-common/attach-ptr-change-1.c
> new file mode 100644
> index 00000000000..d4d84fdb092
> --- /dev/null
> +++ b/libgomp/testsuite/libgomp.oacc-c-c++-common/attach-ptr-change-1.c
> @@ -0,0 +1,74 @@
> +#include <assert.h>
> +#include <stdlib.h>
> +#include <openacc.h>
> +
> +struct str {
> +  unsigned char *c;
> +};
> +
> +int main()
> +{
> +  const int size_1 = sizeof (void *);
> +  unsigned char *data_1 = (unsigned char *) malloc(sizeof (void *));
> +  assert(data_1);
> +  void *data_1_d = acc_create(data_1, size_1);
> +  assert(data_1_d);
> +  assert(acc_is_present(data_1, size_1));
> +
> +  const int size_2 = sizeof (void *);
> +  unsigned char *data_2 = (unsigned char *) malloc(size_2);
> +  assert(data_2);
> +  void *data_2_d = acc_create(data_2, size_2);
> +  assert(data_2_d);
> +  assert(acc_is_present(data_2, size_2));
> +
> +  struct str data_work;
> +  data_work.c = data_1;
> +
> +  acc_copyin(&data_work, sizeof data_work);
> +  assert(acc_is_present(&data_work, sizeof data_work));
> +  assert(data_work.c == data_1);
> +
> +  /* No attach has taken place so far.  We can still do a self-update.  */
> +  acc_update_self(&data_work, sizeof data_work);
> +  assert(data_work.c == data_1);
> +
> +  data_1[0] = 'a';
> +  data_2[0] = 'b';
> +
> +  acc_update_device (data_1, size_1);
> +  acc_update_device (data_2, size_2);
> +
> +  acc_attach((void **) &data_work.c);
> +  #pragma acc serial /* { dg-warning "using vector_length \\(32\\), ignoring 1" "" { target openacc_nvidia_accel_selected } } */
> +  {
> +    data_work.c[0] = 'c';
> +  }
> +
> +  acc_update_self (data_1, size_1);
> +  acc_update_self (data_2, size_2);
> +
> +  assert (data_1[0] == 'c');
> +  assert (data_2[0] == 'b');
> +
> +  data_1[0] = 'a';
> +  data_2[0] = 'b';
> +
> +  acc_update_device (data_1, size_1);
> +  acc_update_device (data_2, size_2);
> +
> +  data_work.c = data_2;
> +  acc_attach((void **) &data_work.c);
> +  #pragma acc serial /* { dg-warning "using vector_length \\(32\\), ignoring 1" "" { target openacc_nvidia_accel_selected } } */
> +  {
> +    data_work.c[0] = 'd';
> +  }
> +
> +  acc_update_self (data_1, size_1);
> +  acc_update_self (data_2, size_2);
> +
> +  assert (data_1[0] == 'a');
> +  assert (data_2[0] == 'd');
> +
> +  return 0;
> +}
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