[turbofan] trim effect chain nodes when they have no side-effect

Bug: 
Change-Id: Ic1b6dc6fcd8bfc4f0c3dbb101a38106aa3596a12
Reviewed-on: https://chromium-review.googlesource.com/863886
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50588}

src/compiler/graph-trimmer.cc

@@ -5,6 +5,7 @@
 #include "src/compiler/graph-trimmer.h"
 
 #include "src/compiler/graph.h"
+#include "src/compiler/node-properties.h"
 
 namespace v8 {
 namespace internal {
@@ -18,6 +19,18 @@ GraphTrimmer::GraphTrimmer(Zone* zone, Graph* graph)
 
 GraphTrimmer::~GraphTrimmer() {}
 
+namespace {
+bool IsSideEffectFree(const Operator* op) {
+  switch (op->opcode()) {
+    case IrOpcode::kLoad:
+    case IrOpcode::kLoadElement:
+    case IrOpcode::kLoadField:
+      return true;
+    default:
+      return false;
+  }
+}
+}  // namespace
 
 void GraphTrimmer::TrimGraph() {
   // Mark end node as live.
@@ -25,8 +38,44 @@ void GraphTrimmer::TrimGraph() {
   // Compute transitive closure of live nodes.
   for (size_t i = 0; i < live_.size(); ++i) {
     Node* const live = live_[i];
-    for (Node* const input : live->inputs()) MarkAsLive(input);
+    for (Edge edge : live->input_edges()) {
+      Node* input = edge.to();
+      if (NodeProperties::IsEffectEdge(edge)) {
+        while (IsSideEffectFree(input->op())) {
+          // Side-effect free effect chain nodes can be removed from the effect
+          // chain when only their effect output is used. So we ignore the
+          // effect chain use when determining liveness.
+          DCHECK_EQ(1, input->op()->EffectInputCount());
+          input = NodeProperties::GetEffectInput(input);
+        }
+      }
+      MarkAsLive(input);
+    }
   }
+
+  // Remove unused side-effect-free nodes from the effect chain.
+  // These are exactly the nodes we skipped before and that don't have value
+  // uses. This has to be done before we remove dead->live edges because we need
+  // to read the effect predecessor from the dead effect chain node.
+  for (Node* const live : live_) {
+    DCHECK(IsLive(live));
+    for (Edge edge : live->input_edges()) {
+      Node* input = edge.to();
+      if (NodeProperties::IsEffectEdge(edge)) {
+        Node* new_input = input;
+        while (!IsLive(new_input)) {
+          DCHECK(IsSideEffectFree(new_input->op()));
+          if (FLAG_trace_turbo_trimming) {
+            OFStream os(stdout);
+            os << "DeadEffectChainNode: " << *new_input << std::endl;
+          }
+          new_input = NodeProperties::GetEffectInput(new_input);
+        }
+        if (new_input != input) edge.UpdateTo(new_input);
+      }
+    }
+  }
+
   // Remove dead->live edges.
   for (Node* const live : live_) {
     DCHECK(IsLive(live));

src/compiler/graph-trimmer.h

@@ -15,7 +15,6 @@ namespace compiler {
 // Forward declarations.
 class Graph;
 
-
 // Trims dead nodes from the node graph.
 class V8_EXPORT_PRIVATE GraphTrimmer final {
  public: