JitArm64_Integer: cmp - Subtract shifted 12-bit constant

You can encode a shifted 12-bit immediate in a SUB instruction on ARM64.
Constants in this range do not need to be sign extended, so we can
exploit this to avoid materializing the immediate.

This approach saves an instruction if it does not need to be
materialized in a register afterwards. Otherwise, we just materialize
it later and the total number of instructions stays the same.

Before:
0x52a00099   mov    w25, #0x40000             ; =262144
0x93407f7a   sxtw   x26, w27
0xcb39c35a   sub    x26, x26, w25, sxtw

After:
0x93407f7a   sxtw   x26, w27
0xd141035a   sub    x26, x26, #0x40, lsl #12  ; =0x40000

1 files changed, 5 insertions, 0 deletions

diff --git a/Source/Core/Core/PowerPC/JitArm64/JitArm64_Integer.cpp b/Source/Core/Core/PowerPC/JitArm64/JitArm64_Integer.cpp
index 370ad619c5..3a56e58b84 100644
--- a/Source/Core/Core/PowerPC/JitArm64/JitArm64_Integer.cpp
+++ b/Source/Core/Core/PowerPC/JitArm64/JitArm64_Integer.cpp
@@ -658,6 +658,11 @@ void JitArm64::cmp(UGeckoInstruction inst)
     if (const u32 imm = gpr.GetImm(b); imm != 0)
       SUB(CR, CR, imm);
   }
+  else if (gpr.IsImm(b) && (gpr.GetImm(b) & 0xFFF000) == gpr.GetImm(b))
+  {
+    SXTW(CR, gpr.R(a));
+    SUB(CR, CR, gpr.GetImm(b) >> 12, true);
+  }
   else
   {
     ARM64Reg RA = gpr.R(a);