Added a compilation order heuristic for compiling C before Nc process…

…es (lava-nc#275)

* DiGraphBase: collapse a cycle of length 1 correctly.

Signed-off-by: Risbud, Sumedh <[email protected]>

* Added 'C-first-Nc-second compilation heuristic for heterogeneous ProcGroups containing C and Nc Processes

Signed-off-by: Risbud, Sumedh <[email protected]>

* Bugfix to C-first-Nc-second heuristic

Signed-off-by: Risbud, Sumedh <[email protected]>

src/lava/magma/compiler/compiler.py

@@ -299,13 +299,42 @@ def _create_subcompilers(
             initialized with the Processes they will compile.
         """
         subcompilers = []
+        c_idx = []
+        nc_idx = []
         # Go through all required subcompiler classes...
-        for subcompiler_class, procs in compiler_type_to_procs.items():
+        for idx, (subcompiler_class, procs) in \
+                enumerate(compiler_type_to_procs.items()):
             # ...create the subcompiler instance...
             compiler = subcompiler_class(procs, self._compile_config)
             # ...and add it to the list.
             subcompilers.append(compiler)
-
+            # Remember the index for C and Nc subcompilers:
+            if isinstance(compiler, CProcCompiler):
+                c_idx.append(idx)
+            if isinstance(compiler, NcProcCompiler):
+                nc_idx.append(idx)
+
+        # Implement the heuristic "C-first Nc-second" whenever C and Nc
+        # compilers are in the list. The subcompiler.compile is called in
+        # their order of appearance in this list.
+        # ToDo: In the implementation below, we assume that there is only one
+        #  instance of each subcompiler (Py/C/Nc) per ProcGroup. This should
+        #  be true as long as the `compiler_type_to_procs` mapping comes from
+        #  `self._map_subcompiler_type_to_procs`.
+        # 1. Confirm that there is only one instance of C and Nc subcompilers
+        if len(c_idx) > 1 or len(nc_idx) > 1:
+            raise AssertionError("More than one instance of C or Nc "
+                                 "subcompiler detected.")
+        # 2. If the index of C subcompiler is larger (appears later in the
+        # list), then swap it with Nc subcompiler.
+        if len(c_idx) > 0 and len(nc_idx) > 0:
+            if c_idx[0] > nc_idx[0]:
+                subcompilers[c_idx[0]], subcompilers[nc_idx[0]] = subcompilers[
+                    nc_idx[0]], subcompilers[c_idx[0]]
+        # We have ensured that C subcompiler appears before Nc subcompiler in
+        # the list we return. As compile() is called serially on each
+        # subcompiler, C Processes will be compiled before Nc Processes
+        # within a ProcGroup.
         return subcompilers
 
     @staticmethod