From f36973d9961a3dd583dd4896a78a55895199aeb2 Mon Sep 17 00:00:00 2001
From: Jean Boussier <jean.boussier@gmail.com>
Date: Sun, 9 Feb 2025 19:10:57 +0100
Subject: [PATCH] Optimize Cartridge::Mbc1

Closes: https://github.com/sacckey/rubyboy/pull/23

Having to call a proc for every memory access isn't ideal, that
quite a bit of overhead.

If instead we use a `case` statement with only immediate values
and a few bitwise operations we can let YJIT generate very efficient
code.

two runs on main with ruby 3.4.1 / YJIT:

```
Ruby: 3.4.1
YJIT: true
1: 4.051815 sec
2: 3.989457 sec
3: 3.963555 sec
FPS: 374.849216902501

Ruby: 3.4.1
YJIT: true
1: 3.872855 sec
2: 3.943231 sec
3: 4.024268 sec
FPS: 380.05620440064547
```

On this branch:

```
Ruby: 3.4.1
YJIT: true
1: 3.625805 sec
2: 3.615032 sec
3: 3.572852 sec
FPS: 416.1392102177157

Ruby: 3.4.1
YJIT: true
1: 3.543953 sec
2: 3.520738 sec
3: 3.506004 sec
FPS: 425.70521616601366
```
---
 lib/rubyboy/cartridge/mbc1.rb | 86 ++++++++++++++---------------------
 1 file changed, 34 insertions(+), 52 deletions(-)

diff --git a/lib/rubyboy/cartridge/mbc1.rb b/lib/rubyboy/cartridge/mbc1.rb
index 833eeee..1f1a474 100644
--- a/lib/rubyboy/cartridge/mbc1.rb
+++ b/lib/rubyboy/cartridge/mbc1.rb
@@ -10,65 +10,47 @@ def initialize(rom, ram)
         @ram_bank = 0
         @ram_enable = false
         @ram_banking_mode = false
-
-        @read_methods = Array.new(0xc000)
-        @write_methods = Array.new(0xc000)
-
-        set_methods
       end
 
-      def set_methods
-        0xc000.times do |addr|
-          @read_methods[addr] =
-            case addr
-            when 0x0000..0x3fff then -> { @rom.data[addr] }
-            when 0x4000..0x7fff then -> { @rom.data[addr + (@rom_bank - 1) * 0x4000] }
-            when 0xa000..0xbfff
-              lambda do
-                if @ram_enable
-                  if @ram_banking_mode
-                    @ram.eram[addr - 0xa000 + @ram_bank * 0x800]
-                  else
-                    @ram.eram[addr - 0xa000]
-                  end
-                else
-                  0xff
-                end
-              end
-            end
-        end
-
-        0xc000.times do |addr|
-          @write_methods[addr] =
-            case addr
-            when 0x0000..0x1fff then ->(value) { @ram_enable = value & 0x0f == 0x0a }
-            when 0x2000..0x3fff
-              lambda do |value|
-                @rom_bank = value & 0x1f
-                @rom_bank = 1 if @rom_bank == 0
-              end
-            when 0x4000..0x5fff then ->(value) { @ram_bank = value & 0x03 }
-            when 0x6000..0x7fff then ->(value) { @ram_banking_mode = value & 0x01 == 0x01 }
-            when 0xa000..0xbfff
-              lambda do |value|
-                if @ram_enable
-                  if @ram_banking_mode
-                    @ram.eram[addr - 0xa000 + @ram_bank * 0x800] = value
-                  else
-                    @ram.eram[addr - 0xa000] = value
-                  end
-                end
-              end
+      def read_byte(addr)
+        case (addr >> 12)
+        when 0x0, 0x1, 0x2, 0x3
+          @rom.data[addr]
+        when 0x4, 0x5, 0x6, 0x7
+          @rom.data[addr + ((@rom_bank - 1) << 14)]
+        when 0xa, 0xb
+          if @ram_enable
+            if @ram_banking_mode
+              @ram.eram[addr - 0xa000 + (@ram_bank << 11)]
+            else
+              @ram.eram[addr - 0xa000]
             end
+          else
+            0xff
+          end
         end
       end
 
-      def read_byte(addr)
-        @read_methods[addr].call
-      end
-
       def write_byte(addr, value)
-        @write_methods[addr].call(value)
+        case addr >> 12
+        when 0x0, 0x1
+          @ram_enable = value & 0x0f == 0x0a
+        when 0x2, 0x3
+          @rom_bank = value & 0x1f
+          @rom_bank = 1 if @rom_bank == 0
+        when 0x4, 0x5
+          @ram_bank = value & 0x03
+        when 0x6, 0x7
+          @ram_banking_mode = value & 0x01 == 0x01
+        when 0xa, 0xb
+          if @ram_enable
+            if @ram_banking_mode
+              @ram.eram[addr - 0xa000 + (@ram_bank << 11)] = value
+            else
+              @ram.eram[addr - 0xa000] = value
+            end
+          end
+        end
       end
     end
   end