Reland of [ignition/turbo] Perform liveness analysis on the bytecodes

src/interpreter/bytecode-generator.cc

Index: src/interpreter/bytecode-generator.cc
diff --git a/src/interpreter/bytecode-generator.cc b/src/interpreter/bytecode-generator.cc
index 1200cacd0707baeb4b9dbaf7ffbb079fc94479ad..c325a8a0c807ddd545aca6b92e8a13b41cbc3196 100644
--- a/src/interpreter/bytecode-generator.cc
+++ b/src/interpreter/bytecode-generator.cc
@@ -722,22 +722,26 @@ void BytecodeGenerator::VisitIterationHeader(IterationStatement* stmt,
                                              LoopBuilder* loop_builder) {
   // Recall that stmt->yield_count() is always zero inside ordinary
   // (i.e. non-generator) functions.
+  if (stmt->yield_count() == 0) {
+    loop_builder->LoopHeader();
+  } else {
+    // Collect all labels for generator resume points within the loop (if any)
+    // so
+    // that they can be bound to the loop header below. Also create fresh labels
+    // for these resume points, to be used inside the loop.
+    ZoneVector<BytecodeLabel> resume_points_in_loop(zone());
+    size_t first_yield = stmt->first_yield_id();
+    DCHECK_LE(first_yield + stmt->yield_count(),
+              generator_resume_points_.size());
+    for (size_t id = first_yield; id < first_yield + stmt->yield_count();
+         id++) {
+      auto& label = generator_resume_points_[id];
+      resume_points_in_loop.push_back(label);
+      generator_resume_points_[id] = BytecodeLabel();
+    }
+
+    loop_builder->LoopHeader(&resume_points_in_loop);

[rmcilroy, 2016/11/28 10:00:31]

Why did you need to change this code here?

[Leszek Swirski, 2016/11/28 11:07:22]

Oh, this was a drive-by fix -- I had changed LoopBuilder::LoopHeader to allow
nullptr so that I could use it in tests, and I noticed that we could
fast-path/clarify the non-generator case here and avoid initialising a
ZoneVector and risk allocating anything on the zone (afaik in most
implementations vectors are constructed without an initial allocation, but it
doesn't hurt to be careful).

 
-  // Collect all labels for generator resume points within the loop (if any) so
-  // that they can be bound to the loop header below. Also create fresh labels
-  // for these resume points, to be used inside the loop.
-  ZoneVector<BytecodeLabel> resume_points_in_loop(zone());
-  size_t first_yield = stmt->first_yield_id();
-  DCHECK_LE(first_yield + stmt->yield_count(), generator_resume_points_.size());
-  for (size_t id = first_yield; id < first_yield + stmt->yield_count(); id++) {
-    auto& label = generator_resume_points_[id];
-    resume_points_in_loop.push_back(label);
-    generator_resume_points_[id] = BytecodeLabel();
-  }
-
-  loop_builder->LoopHeader(&resume_points_in_loop);
-
-  if (stmt->yield_count() > 0) {
     // If we are not resuming, fall through to loop body.
     // If we are resuming, perform state dispatch.
     BytecodeLabel not_resuming;