MLIR passage 2: Graph launch execution boundary

// Inputs: modules of `dataflow.thread` definitions that each own one explicit
// `loom.spatial_region` publication candidate, optionally nested in fixed
// structured scopes (`scf.for`, `scf.if`) that Part 3 publication supports.

start: {new COUNT = random.randint(1, 3); new I = 0}
       'module {\n'
       threads
       '}\n';

threads: (I < COUNT) thread_def {I += 1} threads
       | (I == COUNT) '';

thread_def: {new WRAP = random.randint(0, 3); new FORM = random.randint(0, 4)}
       '  dataflow.thread private @thread_' idx
       ' domain(#dataflow.thread_domain<dense>)(%value: i32, %mem: memref<4xi32>, %chan: !dataflow.channel<i32>, %flag: i1) ctrl (%start: none) {\n'
       '    %c0 = arith.constant 0 : index\n'
       '    %c1 = arith.constant 1 : index\n'
       '    %c4 = arith.constant 4 : index\n'
       wrapped
       '    dataflow.thread.yield\n'
       '  }\n';

idx: [str(I)];

wrapped: (WRAP == 0) region
       | (WRAP == 1) '    scf.for %i = %c0 to %c4 step %c1 {\n' region '    }\n'
       | (WRAP == 2) '    scf.if %flag {\n' region '    }\n'
       | (WRAP == 3) '    scf.for %i = %c0 to %c4 step %c1 {\n      scf.if %flag {\n' region '      }\n    }\n';

region: (FORM == 0) form_store
      | (FORM == 1) form_empty
      | (FORM == 2 and WRAP == 0) form_stream
      | (FORM == 2 and WRAP > 0) form_empty
      | (FORM == 3) form_value
      | (FORM == 4 and WRAP == 0) form_receive
      | (FORM == 4 and WRAP > 0) form_store;

// Value input plus memory import; no boundary results.
form_store: '      "loom.spatial_region"(%value, %mem)\n'
       '          <{operandSegmentSizes = array<i32: 1, 0, 1, 0>,\n'
       '            resultSegmentSizes = array<i32: 0, 0>}> ({\n'
       '        ^bb0(%payload: i32, %memory: memref<4xi32>):\n'
       '          %zero = arith.constant 0 : index\n'
       '          memref.store %payload, %memory[%zero] : memref<4xi32>\n'
       '          "loom.spatial_yield"()\n'
       '              <{operandSegmentSizes = array<i32: 0, 0>}> : () -> ()\n'
       '      }) {graph_name = "graph_' idx '", source_maps = []} :\n'
       '          (i32, memref<4xi32>) -> ()\n';

// Empty boundary: no inputs, no results.
form_empty: '      "loom.spatial_region"()\n'
       '          <{operandSegmentSizes = array<i32: 0, 0, 0, 0>,\n'
       '            resultSegmentSizes = array<i32: 0, 0>}> ({\n'
       '        ^bb0:\n'
       '          "loom.spatial_yield"()\n'
       '              <{operandSegmentSizes = array<i32: 0, 0>}> : () -> ()\n'
       '      }) {graph_name = "graph_' idx '", source_maps = []} : () -> ()\n';

// Value input plus one stream output channel binding.
form_stream: '      "loom.spatial_region"(%value, %chan)\n'
       '          <{operandSegmentSizes = array<i32: 1, 0, 0, 1>,\n'
       '            resultSegmentSizes = array<i32: 0, 0>}> ({\n'
       '        ^bb0(%payload: i32, %output: !dataflow.channel<i32>):\n'
       '          dataflow.channel.send %output, %payload : !dataflow.channel<i32>\n'
       '          "loom.spatial_yield"()\n'
       '              <{operandSegmentSizes = array<i32: 0, 0>}> : () -> ()\n'
       '      }) {graph_name = "graph_' idx '", source_maps = []} :\n'
       '          (i32, !dataflow.channel<i32>) -> ()\n';

// Value input and one value output.
form_value: '      %sum_' idx ' = "loom.spatial_region"(%value)\n'
       '          <{operandSegmentSizes = array<i32: 1, 0, 0, 0>,\n'
       '            resultSegmentSizes = array<i32: 1, 0>}> ({\n'
       '        ^bb0(%payload: i32):\n'
       '          %doubled = arith.addi %payload, %payload : i32\n'
       '          "loom.spatial_yield"(%doubled)\n'
       '              <{operandSegmentSizes = array<i32: 1, 0>}> : (i32) -> ()\n'
       '      }) {graph_name = "graph_' idx '", source_maps = []} :\n'
       '          (i32) -> i32\n';

// One stream input channel binding with its affine `source_map`, plus a
// memory import.
form_receive: '      "loom.spatial_region"(%chan, %mem)\n'
       '          <{operandSegmentSizes = array<i32: 0, 1, 1, 0>,\n'
       '            resultSegmentSizes = array<i32: 0, 0>}> ({\n'
       '        ^bb0(%channel: !dataflow.channel<i32>, %target: memref<4xi32>):\n'
       '          %message = dataflow.channel.receive %channel : !dataflow.channel<i32>\n'
       '          %zero = arith.constant 0 : index\n'
       '          memref.store %message, %target[%zero] : memref<4xi32>\n'
       '          "loom.spatial_yield"()\n'
       '              <{operandSegmentSizes = array<i32: 0, 0>}> : () -> ()\n'
       '      }) {graph_name = "graph_' idx '", source_maps = [affine_map<() -> ()>]} :\n'
       '          (!dataflow.channel<i32>, memref<4xi32>) -> ()\n';