JitArm64: MultiplyImmediate - Handle -(2^n)

ARM64's flexible shifting of input registers also allows us to calculate
a negative power of two in one instruction; shift the input of a NEG
instruction.

Before:
0x128001f7   mov    w23, #-0x10
0x1b1a7efa   mul    w26, w23, w26
0x93407f58   sxtw   x24, w26

After:
0x4b1a13fa   neg    w26, w26, lsl #4
0x93407f58   sxtw   x24, w26

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 6cd902d0c9..e3dac61ed0 100644
--- a/Source/Core/Core/PowerPC/JitArm64/JitArm64_Integer.cpp
+++ b/Source/Core/Core/PowerPC/JitArm64/JitArm64_Integer.cpp
@@ -917,6 +917,16 @@ bool JitArm64::MultiplyImmediate(u32 imm, int a, int d, bool rc)
     if (rc)
       ComputeRC0(gpr.R(d));
   }
+  else if (MathUtil::IsPow2(~imm + 1))
+  {
+    // Multiplication by a negative power of two (-(2^n)).
+    const int shift = IntLog2(~imm + 1);
+
+    gpr.BindToRegister(d, d == a);
+    NEG(gpr.R(d), 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.