JitCommon: Signed 32-bit division magic constants

Add a function to calculate the magic constants required to optimize
signed 32-bit division.

Since this optimization is not exclusive to any particular architecture,
JitCommon seemed like a good place to put this.

Source/Core/Core/CMakeLists.txt

@@ -430,6 +430,8 @@ add_library(core
   PowerPC/Interpreter/Interpreter_Tables.cpp
   PowerPC/Interpreter/Interpreter.cpp
   PowerPC/Interpreter/Interpreter.h
+	PowerPC/JitCommon/DivUtils.cpp
+	PowerPC/JitCommon/DivUtils.h
   PowerPC/JitCommon/JitAsmCommon.cpp
   PowerPC/JitCommon/JitAsmCommon.h
   PowerPC/JitCommon/JitBase.cpp

Source/Core/Core/PowerPC/JitCommon/DivUtils.cpp

@@ -0,0 +1,57 @@
+// Copyright 2021 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#include <cstdlib>
+
+#include "Core/PowerPC/JitCommon/DivUtils.h"
+
+namespace JitCommon
+{
+Magic SignedDivisionConstants(s32 d)
+{
+  const u32 two31 = 2147483648;
+
+  const u32 ad = std::abs(d);
+  const u32 t = two31 - (d < 0);
+  const u32 anc = t - 1 - t % ad;
+  u32 q1 = two31 / anc;
+  u32 r1 = two31 - q1 * anc;
+  u32 q2 = two31 / ad;
+  u32 r2 = two31 - q2 * ad;
+
+  s32 p = 31;
+  u32 delta;
+
+  do
+  {
+    p++;
+
+    q1 *= 2;
+    r1 *= 2;
+    if (r1 >= anc)
+    {
+      q1++;
+      r1 -= anc;
+    }
+
+    q2 *= 2;
+    r2 *= 2;
+    if (r2 >= ad)
+    {
+      q2++;
+      r2 -= ad;
+    }
+    delta = ad - r2;
+  } while (q1 < delta || (q1 == delta && r1 == 0));
+
+  Magic mag;
+  mag.multiplier = q2 + 1;
+  if (d < 0)
+    mag.multiplier = -mag.multiplier;
+  mag.shift = p - 32;
+
+  return mag;
+}
+
+}  // namespace JitCommon

Source/Core/Core/PowerPC/JitCommon/DivUtils.h

@@ -0,0 +1,22 @@
+// Copyright 2021 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include "Common/CommonTypes.h"
+
+namespace JitCommon
+{
+struct Magic
+{
+  s32 multiplier;
+  u8 shift;
+};
+
+// Calculate the constants required to optimize a signed 32-bit integer division.
+// Taken from The PowerPC Compiler Writer's Guide and LLVM.
+// Divisor must not be -1, 0, and 1.
+Magic SignedDivisionConstants(s32 divisor);
+
+}  // namespace JitCommon

Source/Core/DolphinLib.vcxproj

@@ -27,6 +27,12 @@
       <Project>{41279555-f94f-4ebc-99de-af863c10c5c4}</Project>
     </ProjectReference>
   </ItemGroup>
+  <ItemGroup>
+    <ClInclude Include="Core\PowerPC\JitCommon\DivUtils.h" />
+  </ItemGroup>
+  <ItemGroup>
+    <ClCompile Include="Core\PowerPC\JitCommon\DivUtils.cpp" />
+  </ItemGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
   <ImportGroup Label="ExtensionTargets" />
 </Project>

Source/UnitTests/DivUtilsTest.cpp

@@ -0,0 +1,33 @@
+// Copyright 2021 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#include <gtest/gtest.h>
+
+#include "Core/PowerPC/JitCommon/DivUtils.h"
+
+using namespace JitCommon;
+
+TEST(DivUtils, Signed)
+{
+  Magic m3 = SignedDivisionConstants(3);
+  Magic m5 = SignedDivisionConstants(5);
+  Magic m7 = SignedDivisionConstants(7);
+  Magic minus3 = SignedDivisionConstants(-3);
+  Magic minus5 = SignedDivisionConstants(-5);
+  Magic minus7 = SignedDivisionConstants(-7);
+
+  EXPECT_EQ(0x55555556, m3.multiplier);
+  EXPECT_EQ(0, m3.shift);
+  EXPECT_EQ(0x66666667, m5.multiplier);
+  EXPECT_EQ(1, m5.shift);
+  EXPECT_EQ(-0x6DB6DB6D, m7.multiplier);
+  EXPECT_EQ(2, m7.shift);
+
+  EXPECT_EQ(-0x55555556, minus3.multiplier);
+  EXPECT_EQ(0, minus3.shift);
+  EXPECT_EQ(-0x66666667, minus5.multiplier);
+  EXPECT_EQ(1, minus5.shift);
+  EXPECT_EQ(0x6DB6DB6D, minus7.multiplier);
+  EXPECT_EQ(2, minus7.shift);
+}

Source/UnitTests/UnitTests.vcxproj

@@ -68,6 +68,7 @@
     <ClCompile Include="Core\IOS\FS\FileSystemTest.cpp" />
     <ClCompile Include="Core\MMIOTest.cpp" />
     <ClCompile Include="Core\PageFaultTest.cpp" />
+    <ClCompile Include="DivUtilsTest.cpp" />
     <ClCompile Include="FileUtil.cpp" />
     <ClCompile Include="VideoCommon\VertexLoaderTest.cpp" />
     <ClCompile Include="StubHost.cpp" />