From 589c3771e9e0d3afb70caccd080491221925d524 Mon Sep 17 00:00:00 2001
From: Luke Wilde <lukew@serenityos.org>
Date: Sun, 27 Mar 2022 19:59:25 +0100
Subject: [PATCH] LibJS: Only store MemberExpression object when loading a
 computed prop

When calling emit_load_from_reference with a MemberExpression, it is
only necessary to store the result of evaluating MemberExpression's
object when performing computed property lookup.

This allows us to skip unnecessary stores for identifier lookup.
For example, this would generate 3 unnecessary stores:
```
> Temporal.PlainDateTime.prototype.add
JS::Bytecode::Executable (REPL)
1:
[   0] GetVariable 0 (Temporal)
[  28] Store $2
[  30] GetById 1 (PlainDateTime)
[  40] Store $3
[  48] GetById 2 (prototype)
[  58] Store $4
[  60] GetById 3 (add)
```

With this, it generates:
```
> Temporal.PlainDateTime.prototype.add
JS::Bytecode::Executable (REPL)
1:
[   0] GetVariable 0 (Temporal)
[  28] GetById 1 (PlainDateTime)
[  38] GetById 2 (prototype)
[  48] GetById 3 (add)
```
---
 Userland/Libraries/LibJS/Bytecode/Generator.cpp | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/Userland/Libraries/LibJS/Bytecode/Generator.cpp b/Userland/Libraries/LibJS/Bytecode/Generator.cpp
index f6221072b79..36d6b54c413 100644
--- a/Userland/Libraries/LibJS/Bytecode/Generator.cpp
+++ b/Userland/Libraries/LibJS/Bytecode/Generator.cpp
@@ -137,10 +137,10 @@ CodeGenerationErrorOr<void> Generator::emit_load_from_reference(JS::ASTNode cons
         auto& expression = static_cast<MemberExpression const&>(node);
         TRY(expression.object().generate_bytecode(*this));
 
-        auto object_reg = allocate_register();
-        emit<Bytecode::Op::Store>(object_reg);
-
         if (expression.is_computed()) {
+            auto object_reg = allocate_register();
+            emit<Bytecode::Op::Store>(object_reg);
+
             TRY(expression.property().generate_bytecode(*this));
             emit<Bytecode::Op::GetByValue>(object_reg);
         } else if (expression.property().is_identifier()) {