From f0828bc714992a1eb3b34069d995e03c6e329038 Mon Sep 17 00:00:00 2001
From: Matthew Gaudet <mgaudet@mozilla.com>
Date: Wed, 17 May 2023 14:59:51 +0000
Subject: [PATCH] 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                 |  5 +++++
 js/src/frontend/NameOpEmitter.cpp | 23 ++++++++++++++++++++---
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/js/public/Value.h b/js/public/Value.h
index e3579da8e99e..de8db0fed860 100644
--- a/js/public/Value.h
+++ b/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) {
diff --git a/js/src/frontend/NameOpEmitter.cpp b/js/src/frontend/NameOpEmitter.cpp
index 074aaabf6d86..f36c7a2028b7 100644
--- a/js/src/frontend/NameOpEmitter.cpp
+++ b/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;