Out-of-bounds array access (?)

[nicknytko]

Hi all 👋🏻

I am trying to translate this following kernel with loopy:

===========================================================================
---------------------------------------------------------------------------
KERNEL: bmg2_symstd_restrict_kernel_98
---------------------------------------------------------------------------
ARGUMENTS:
nx: ValueArg, type: np:dtype('int64')
q: type: np:dtype('float64'), shape: (nx, ny), dim_tags: (N0:stride:1, N1:stride:nx) in/out aspace: global
ny: ValueArg, type: np:dtype('int64')
---------------------------------------------------------------------------
DOMAINS:
[nx] -> { [i] : 0 <= i < nx }
---------------------------------------------------------------------------
INAME TAGS:
i: None
---------------------------------------------------------------------------
INSTRUCTIONS:
  for i
↱   q[i, 0] = q[i, -2 + ny]  {id=insn0}
└   q[i, -1 + ny] = q[i, 1]  {id=insn1}
  end i
---------------------------------------------------------------------------
===========================================================================

(it is interchanging values for boundary conditions along the top and bottom of the domain), and am running into the issue that:

'q[i, 0]' in instruction 'insn0' accesses out-of-bounds array element (could not establish '[nx, ny] -> { [i0, 0] : 0 <= i0 < nx }' is a subset of '[nx, ny] -> { [i0, i1] : 0 <= i0 < nx and 0 <= i1 < ny }').

I was originally thinking that it was not liking that there are no assumptions on the size of nx nor ny, however when I try to assume that ny>=3, I get an ISL syntax error:

call to isl_basic_set_read_from_str failed: syntax error in isl/isl_stream.c:149

Am I overlooking something obvious here to get this kernel working? Thanks in advance for any insights.

[inducer]

The substance of the first error is indeed that loopy can't establish that ny > 0, and your thinking is correct that an assumption is needed.

I was originally thinking that it was not liking that there are no assumptions on the size of nx nor ny, however when I try to assume that ny>=3, I get an ISL syntax error:

call to isl_basic_set_read_from_str failed: syntax error in isl/isl_stream.c:149

Am I overlooking something obvious here to get this kernel working? Thanks in advance for any insights.

Show the code that you're using. Probably a simple typo thing.

[nicknytko]

Here is a minimum working example to get the ISL error:

import loopy

kernel = loopy.make_kernel(
    "{ [i] : 0 <= i < nx }",
    """
    q[i, 0] = q[i, ny - 2] {id=insn0}
    q[i, ny - 1] = q[i, 1] {id=insn1}
    """,
    assumptions="ny>=3")

which triggers the syntax error at this line:

loopy/loopy/kernel/creation.py

Line 2429 in e855c21

assumptions = isl.BasicSet.read_from_str(domains[0].get_ctx(),

[inducer]

As it stands, loopy isn't aware that ny is supposed to be a domain size parameter, which prevents assumptions on it from working. This:

import loopy

kernel = loopy.make_kernel(
    "[nx,ny] -> { [i] : 0 <= i < nx }",
    """
    q[i, 0] = q[i, ny - 2] {id=insn0}
    q[i, ny - 1] = q[i, 1] {id=insn1}
    """,
    assumptions="ny>=3")

does work.