Add some much-needed comments around refbackrt::invoke.

This code is really tricky, and was not commented.

lib/RefBackend/Runtime/Runtime.cpp

@@ -178,22 +178,46 @@ void refbackrt::invoke(ModuleDescriptor *moduleDescriptor,
   assert(descriptor && "unknown function name");
   assert(inputs.size() < kMaxArity && "number of inputs exceeds kMaxArity");
   assert(outputs.size() < kMaxArity && "number of outputs exceeds kMaxArity");
+
+  // We haven't committed to using "vector" in this runtime code, so use
+  // a fixed-sized array.
   std::array<UnrankedMemref, kMaxArity> inputUnrankedMemrefs;
   std::array<UnrankedMemref, kMaxArity> outputUnrankedMemrefs;
   std::array<void *, kMaxArity * 2> packedInputs;
   std::array<void *, kMaxArity> packedOutputs;
+
+  // Deepcopy the refbackrt::Tensor's into UnrankedMemref's.
+  // TODO: Avoid the deep copy. It makes the later lifetime management code
+  // more complex though (and maybe impossible given the current abstractions).
   for (int i = 0, e = inputs.size(); i < e; i++) {
     inputUnrankedMemrefs[i] =
         convertRefbackrtTensorToUnrankedMemref(inputs[i].get());
   }
+  // Create a type-erased list of "packed inputs" to pass to the
+  // LLVM/C ABI wrapper function. Each packedInput pointer corresponds to
+  // one LLVM/C ABI argument to the underlying function.
+  //
+  // The ABI lowering on StandardToLLVM conversion side will
+  // "explode" the unranked memref descriptors on the underlying function
+  // into separate arguments for the rank and pointer-to-descriptor.
   for (int i = 0, e = inputs.size(); i < e; i++) {
     packedInputs[2 * i] = ToVoidPtr(&inputUnrankedMemrefs[i].rank);
     packedInputs[2 * i + 1] = ToVoidPtr(&inputUnrankedMemrefs[i].descriptor);
   }
+  // Create a type-erased list of "packed output" to pass to the
+  // LLVM/C ABI wrapper function.
+  //
+  // Due to how StandardToLLVM lowering works, each packedOutput pointer
+  // corresponds to a single UnrankedMemref (not "exploded").
   for (int i = 0, e = outputs.size(); i < e; i++) {
     packedOutputs[i] = ToVoidPtr(&outputUnrankedMemrefs[i]);
   }
+
+  // Actually invoke the function!
   descriptor->functionPtr(packedInputs.data(), packedOutputs.data());
+
+  // Copy out the result data into refbackrt::Tensor's.
+  // TODO: Avoid needing to make a deep copy.
   for (int i = 0, e = outputs.size(); i < e; i++) {
     // TODO: Have compiler emit the element type in the metadata.
     auto elementType = ElementType::F32;
@@ -202,6 +226,15 @@ void refbackrt::invoke(ModuleDescriptor *moduleDescriptor,
         elementType);
     outputs[i] = Ref<Tensor>(tensor);
   }
+
+  // Now, we just need to free all the UnrankedMemref's that we created.
+  // This is complicated by the fact that multiple input/output UnrankedMemref's
+  // can end up with the same backing buffer (`allocatedPtr`), and we need
+  // to avoid double-freeing.
+  // Output buffers might alias any other input or output buffer.
+  // Input buffers are guaranteed to not alias each other.
+
+  // Free the output buffers.
   for (int i = 0, e = outputs.size(); i < e; i++) {
     void *allocatedPtr = outputUnrankedMemrefs[i].descriptor->allocatedPtr;
     // Multiple returned memrefs can point into the same underlying
@@ -215,6 +248,8 @@ void refbackrt::invoke(ModuleDescriptor *moduleDescriptor,
     if (!bufferNeedsFreeing)
       std::free(allocatedPtr);
   }
+
+  // Free the input buffers.
   for (int i = 0, e = inputs.size(); i < e; i++) {
     void *allocatedPtr = inputUnrankedMemrefs[i].descriptor->allocatedPtr;
     bool bufferNeedsFreeing = true;
@@ -234,12 +269,14 @@ void refbackrt::invoke(ModuleDescriptor *moduleDescriptor,
       std::free(allocatedPtr);
   }
 
+  // Free the output descriptors.
   for (int i = 0, e = outputs.size(); i < e; i++) {
     // The LLVM lowering guarantees that each returned unranked memref
     // descriptor is separately malloc'ed, so no need to do anything special
     // like we had to do for the allocatedPtr's.
     std::free(outputUnrankedMemrefs[i].descriptor);
   }
+  // Free the input descriptors.
   for (int i = 0, e = inputs.size(); i < e; i++) {
     std::free(inputUnrankedMemrefs[i].descriptor);
   }