[compiler] Propagate liveness across suspends

LeszekSwirski · Commit Bot · commit 41b80eeffdb6 · 2018-01-22T13:15:21.000Z
Suspend points (inside generators and async functions) have slightly funky semantics when it comes to liveness, as they save and restore a chunk of the register file as-is. In particular, this means that granular liveness information is lost, as it is assumed that all registers in that chunk of the register file are live in a suspend. Rather than marking that entire chunk of register as live/dead in suspend/restore, we can instead pattern-match the set of bytecodes in a suspend point, and propagate liveness across them. This tightens liveness estimates, and could be used to optimize which values TurboFan actually saves when suspending. Bug: chromium:798137 Change-Id: I5840cdbfc2c6edb1d3a48cf025f52615b629cdfc Reviewed-on: https://chromium-review.googlesource.com/848895 Commit-Queue: Leszek Swirski <leszeks@chromium.org> Reviewed-by: Jaroslav Sevcik <jarin@chromium.org> Cr-Commit-Position: refs/heads/master@{#50757}
diff --git a/src/compiler/bytecode-analysis.cc b/src/compiler/bytecode-analysis.cc
@@ -80,6 +80,21 @@ void UpdateInLiveness(Bytecode bytecode, BytecodeLivenessState& in_liveness,
   int num_operands = Bytecodes::NumberOfOperands(bytecode);
   const OperandType* operand_types = Bytecodes::GetOperandTypes(bytecode);
 
+  // Special case Suspend and Resume to just pass through liveness.
+  if (bytecode == Bytecode::kSuspendGenerator) {
+    // The generator object has to be live.
+    in_liveness.MarkRegisterLive(accessor.GetRegisterOperand(0).index());
+    // Suspend additionally reads and returns the accumulator
+    DCHECK(Bytecodes::ReadsAccumulator(bytecode));
+    in_liveness.MarkAccumulatorDead();
+    return;
+  }
+  if (bytecode == Bytecode::kResumeGenerator) {
+    // The generator object has to be live.
+    in_liveness.MarkRegisterLive(accessor.GetRegisterOperand(0).index());
+    return;
+  }
+
   if (Bytecodes::WritesAccumulator(bytecode)) {
     in_liveness.MarkAccumulatorDead();
   }
@@ -175,6 +190,13 @@ void UpdateOutLiveness(Bytecode bytecode, BytecodeLivenessState& out_liveness,
   int current_offset = accessor.current_offset();
   const Handle<BytecodeArray>& bytecode_array = accessor.bytecode_array();
 
+  // Special case Suspend and Resume to just pass through liveness.
+  if (bytecode == Bytecode::kSuspendGenerator ||
+      bytecode == Bytecode::kResumeGenerator) {
+    out_liveness.Union(*next_bytecode_in_liveness);
+    return;
+  }
+
   // Update from jump target (if any). Skip loops, we update these manually in
   // the liveness iterations.
   if (Bytecodes::IsForwardJump(bytecode)) {
@@ -316,7 +338,6 @@ void BytecodeAnalysis::Analyze(BailoutId osr_bailout_id) {
                         iterator, liveness_map_);
       liveness.in->CopyFrom(*liveness.out);
       UpdateInLiveness(bytecode, *liveness.in, iterator);
-
       next_bytecode_in_liveness = liveness.in;
     }
   }
