Bug 885514: Don't emit retsub if all paths fall through r=jandem

This means that we don't have to implement retsub in Warp to handle simple cases.

Differential Revision: https://phabricator.services.mozilla.com/D142037

js/src/frontend/BytecodeControlStructures.cpp

@@ -105,6 +105,6 @@ bool LoopControl::emitLoopEnd(BytecodeEmitter* bce, JSOp op,
 }
 
 TryFinallyControl::TryFinallyControl(BytecodeEmitter* bce, StatementKind kind)
-    : NestableControl(bce, kind), emittingSubroutine_(false) {
+    : NestableControl(bce, kind) {
   MOZ_ASSERT(is<TryFinallyControl>());
 }

js/src/frontend/BytecodeControlStructures.h

@@ -140,7 +140,8 @@ inline bool NestableControl::is<LoopControl>() const {
 }
 
 class TryFinallyControl : public NestableControl {
-  bool emittingSubroutine_;
+  bool emittingSubroutine_ = false;
+  bool hasNonLocalJumps_ = false;
 
  public:
   // Offset of the last jump to this `finally`.
@@ -155,6 +156,9 @@ class TryFinallyControl : public NestableControl {
   void setEmittingSubroutine() { emittingSubroutine_ = true; }
 
   bool emittingSubroutine() const { return emittingSubroutine_; }
+
+  void setHasNonLocalJumps() { hasNonLocalJumps_ = true; }
+  bool hasNonLocalJumps() const { return hasNonLocalJumps_; }
 };
 template <>
 inline bool NestableControl::is<TryFinallyControl>() const {

js/src/frontend/BytecodeEmitter.cpp

@@ -795,6 +795,7 @@ bool NonLocalExitControl::prepareForNonLocalJump(NestableControl* target) {
             //      [stack] ...
             return false;
           }
+          finallyControl.setHasNonLocalJumps();
         }
         break;
       }

js/src/frontend/TryEmitter.cpp

@@ -261,8 +261,17 @@ bool TryEmitter::emitFinallyEnd() {
     return false;
   }
 
-  if (!bce_->emit1(JSOp::Retsub)) {
-    return false;
+  if (controlInfo_ && controlInfo_->hasNonLocalJumps()) {
+    if (!bce_->emit1(JSOp::Retsub)) {
+      return false;
+    }
+  } else {
+    // If there are no non-local jumps, then the only possible jump target
+    // is the code immediately following this finally block. Instead of
+    // emitting a retsub, we can simply pop the resume index and fall through.
+    if (!bce_->emit1(JSOp::Pop)) {
+      return false;
+    }
   }
 
   if (!ifThrowing.emitEnd()) {