feat(//core/conversion): Adds the ability to evaluate loops

Note: This is ungaurded currently, loops either must be able to be
evaluated at compile time or the module is not supported. Upcomming work
on RNNs will add support for more types of loops

Signed-off-by: Naren Dasan <naren@narendasan.com>
Signed-off-by: Naren Dasan <narens@nvidia.com>

Author: narendasan

core/conversion/conversion.cpp

@@ -190,6 +190,57 @@ void AddParamsToCtxValueMap(ConversionCtx* ctx, GraphParams& params) {
     }
 }
 
+void MapIValues(ConversionCtx* ctx, c10::ArrayRef<const torch::jit::Value*> in_list, c10::ArrayRef<const torch::jit::Value*> out_list, int64_t in_offset, int64_t out_offset) {
+    std::vector<std::pair<const torch::jit::Value*, const torch::jit::Value*>> input_output_pairs;
+    std::transform(in_list.begin() + in_offset, in_list.end(), out_list.begin() + out_offset,
+        std::back_inserter(input_output_pairs),
+        [](auto in, auto out){
+            return std::make_pair(in, out);
+        });
+
+    for (auto p : input_output_pairs) {
+        auto input = ctx->evaluated_value_map[p.first];
+        ctx->evaluated_value_map[p.second] = torch::jit::IValue(input);
+    }
+}
+
+// TODO: With functionalization pass we may be able to make this into a regular evaluator later
+void EvaluateLoopBlock(ConversionCtx* ctx, const torch::jit::Node* n) {
+    auto max_trip_count = ctx->evaluated_value_map[n->input(0)];
+    auto start_cond = ctx->evaluated_value_map[n->input(1)];
+    ctx->evaluated_value_map[n->blocks()[0]->inputs()[0]] = torch::jit::IValue(0);
+    auto trip_count = ctx->evaluated_value_map[n->blocks()[0]->inputs()[0]];
+
+    MapIValues(ctx, n->inputs(), n->outputs(), 2, 0);
+
+    LOG_DEBUG("(Loop Evaluation) Evaluating loop " << *n);
+    LOG_DEBUG("(Loop Evaluation) Max Trip Count: " << max_trip_count.toInt());
+    LOG_DEBUG("(Loop Evaluation) Start Condition: " << start_cond.toBool());
+    LOG_DEBUG("(Loop Evaluation) Current Trip Count: " << trip_count.toInt());
+
+    while (start_cond.toBool() && trip_count.toInt() < max_trip_count.toInt()) {
+        MapIValues(ctx, n->outputs(), n->blocks()[0]->inputs(), 0, 1);
+        for (auto bn : n->blocks()[0]->nodes()) {
+            auto eval = EvaluateNode(ctx, bn);
+            if (eval) {
+                if (!eval.value().isTensor()) {
+                    LOG_DEBUG(ctx->logger, "(Loop Evaluation) Found the value to be: " << eval.value());
+                } else {
+                    LOG_DEBUG(ctx->logger, "(Loop Evaluation) Found the value to be a tensor (shape " << eval.value().toTensor().sizes() << ')');
+                }
+                ctx->AssociateValueAndIValue(bn->output(0), eval.value());
+            }
+        }
+
+        MapIValues(ctx, n->blocks()[0]->outputs(), n->outputs(), 1, 0);
+        start_cond = ctx->evaluated_value_map[n->blocks()[0]->outputs()[0]];
+        auto new_trip_count = torch::jit::IValue(trip_count.toInt() + 1);
+        trip_count.swap(new_trip_count);
+        LOG_DEBUG("(Loop Evaluation) Condition: " << start_cond.toBool());
+        LOG_DEBUG("(Loop Evaluation) Current Trip Count: " << trip_count.toInt());
+    }
+}
+
 void ConvertBlockToNetDef(ConversionCtx* ctx, const torch::jit::Block* b, ConversionInfo build_info, GraphParams& static_params) {
      LOG_INFO(ctx->logger, "Converting Block");
 
@@ -202,7 +253,19 @@ void ConvertBlockToNetDef(ConversionCtx* ctx, const torch::jit::Block* b, Conver
     for (const auto n : nodes) {
         bool to_eval = evaluators::shouldEvalAtConversionTime(n);
         bool blacklisted = isNodeConversionBlacklisted(n);
-        if (!to_eval && !blacklisted) {
+        if (n->kind() == torch::jit::prim::Loop) {
+            EvaluateLoopBlock(ctx, n);
+        } else if (to_eval) {
+            auto eval = EvaluateNode(ctx, n);
+            if (eval) {
+                if (!eval.value().isTensor()) {
+                    LOG_DEBUG(ctx->logger, "Found the value to be: " << eval.value());
+                } else {
+                    LOG_DEBUG(ctx->logger, "Found the value to be a tensor (shape " << eval.value().toTensor().sizes() << ')');
+                }
+                ctx->AssociateValueAndIValue(n->output(0), eval.value());
+            }
+        } else if (!blacklisted) {
             // Should error out if something fails
             AddLayer(ctx, n);
         } else {
@@ -237,22 +300,29 @@ std::string ConvertBlockToEngine(const torch::jit::Block* b, ConversionInfo buil
     return engine;
 }
 
-bool VerifyConverterSupportForBlock(const torch::jit::Block* b) {
-    bool supported = true;
+std::set<std::string> GetUnsupportedOpsInBlock(const torch::jit::Block* b ) {
     std::set<std::string> unsupported_ops;
     for (const auto n : b->nodes()) {
-        if (!OpSupported(n)) {
+        if (!OpSupported(n) && n->kind() != torch::jit::prim::Loop) {
             auto schema = n->maybeSchema();
             TRTORCH_CHECK(schema, "Unable to get schema for Node " << util::node_info(n) \
                                     << " (conversion.VerifyCoverterSupportForBlock");
             std::stringstream ss;
             ss << *schema;
             unsupported_ops.insert(ss.str());
-            supported = false;
+        }
+        for (const auto sub_b : n->blocks()) {
+            auto sub_b_unsupported_ops = GetUnsupportedOpsInBlock(sub_b);
+            unsupported_ops.insert(sub_b_unsupported_ops.begin(), sub_b_unsupported_ops.end());
         }
     }
+    return unsupported_ops;
+}
+
+bool VerifyConverterSupportForBlock(const torch::jit::Block* b) {
+    auto unsupported_ops = GetUnsupportedOpsInBlock(b);
 
-    if (!supported) {
+    if (unsupported_ops.size() != 0) {
         std::stringstream unsupported_msg;
          unsupported_msg << "Method requested cannot be compiled by TRTorch.\nUnsupported operators listed below:" << std::endl;
         for (auto s : unsupported_ops) {
@@ -261,8 +331,10 @@ bool VerifyConverterSupportForBlock(const torch::jit::Block* b) {
         unsupported_msg << "You can either implement converters for these ops in your application or request implementation" << std::endl;
         unsupported_msg <<  "https://www.github.com/nvidia/TRTorch/issues" << std::endl;
         LOG_ERROR(unsupported_msg.str());
+        return false;
+    } else {
+        return true;
     }
-    return supported;
 }
 
 } // namespace conversion