Bug 1833484 - Optimize bytecode emission for Infinity, NaN and undefined where possible. r=arai

This is possible because once we are looking at the global we know that these values aren't
configurable and therefore have well known values.

Differential Revision: https://phabricator.services.mozilla.com/D178243

js/public/Value.h

@@ -463,6 +463,11 @@ static MOZ_ALWAYS_INLINE double GenericNaN() {
   return mozilla::BitwiseCast<double>(detail::CanonicalizedNaNBits);
 }
 
+// Return the infinity the engine uses
+static MOZ_ALWAYS_INLINE double Infinity() {
+  return mozilla::BitwiseCast<double>(detail::InfinityBits);
+}
+
 // Convert an arbitrary double to one compatible with JS::Value representation
 // by replacing any NaN value with a canonical one.
 static MOZ_ALWAYS_INLINE double CanonicalizeNaN(double d) {

js/src/frontend/NameOpEmitter.cpp

@@ -12,6 +12,7 @@
 #include "frontend/SharedContext.h"
 #include "frontend/TDZCheckCache.h"
 #include "frontend/ValueUsage.h"
+#include "js/Value.h"
 #include "vm/Opcodes.h"
 
 using namespace js;
@@ -44,9 +45,25 @@ bool NameOpEmitter::emitGet() {
           return false;
         }
       } else {
-        if (!bce_->emitAtomOp(JSOp::GetGName, name_)) {
-          //        [stack] VAL
-          return false;
+        // Some names on the global are not configurable and have fixed values
+        // which we can emit instead.
+        if (name_ == TaggedParserAtomIndex::WellKnown::undefined()) {
+          if (!bce_->emit1(JSOp::Undefined)) {
+            return false;
+          }
+        } else if (name_ == TaggedParserAtomIndex::WellKnown::NaN()) {
+          if (!bce_->emitDouble(JS::GenericNaN())) {
+            return false;
+          }
+        } else if (name_ == TaggedParserAtomIndex::WellKnown::Infinity()) {
+          if (!bce_->emitDouble(JS::Infinity())) {
+            return false;
+          }
+        } else {
+          if (!bce_->emitAtomOp(JSOp::GetGName, name_)) {
+            //        [stack] VAL
+            return false;
+          }
         }
       }
       break;