JitArm64: MultiplyImmediate - Handle -(2^n) + 1

Let's take advantage of ARM64's input register shifting one last time,
shall we?

Before:
0x1280005b   mov    w27, #-0x3
0x1b1b7f18   mul    w24, w24, w27

After:
0x4b180b18   sub    w24, w24, w24, lsl #2

1 files changed, 10 insertions, 0 deletions

diff --git a/Source/Core/Core/PowerPC/JitArm64/JitArm64_Integer.cpp b/Source/Core/Core/PowerPC/JitArm64/JitArm64_Integer.cpp
index e3dac61ed0..4810e875b3 100644
--- a/Source/Core/Core/PowerPC/JitArm64/JitArm64_Integer.cpp
+++ b/Source/Core/Core/PowerPC/JitArm64/JitArm64_Integer.cpp
@@ -927,6 +927,16 @@ bool JitArm64::MultiplyImmediate(u32 imm, int a, int d, bool rc)
     if (rc)
       ComputeRC0(gpr.R(d));
   }
+  else if (MathUtil::IsPow2(~imm + 2))
+  {
+    // Multiplication by a negative power of two plus one (-(2^n) + 1).
+    const int shift = IntLog2(~imm + 2);
+
+    gpr.BindToRegister(d, d == a);
+    SUB(gpr.R(d), gpr.R(a), gpr.R(a), ArithOption(gpr.R(a), ShiftType::LSL, shift));
+    if (rc)
+      ComputeRC0(gpr.R(d));
+  }
   else
   {
     // Immediate did not match any known special cases.