From e926f0b39360fd12489a6f111352f29dd906112c Mon Sep 17 00:00:00 2001
From: MerryMage <MerryMage@users.noreply.github.com>
Date: Fri, 1 Jan 2021 13:35:01 +0000
Subject: [PATCH] emit_x64_data_processing: Minor optimization for immediates
 in EmitSub

---
 src/backend/x64/emit_x64_data_processing.cpp | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/src/backend/x64/emit_x64_data_processing.cpp b/src/backend/x64/emit_x64_data_processing.cpp
index e7b9495e..88201cbb 100644
--- a/src/backend/x64/emit_x64_data_processing.cpp
+++ b/src/backend/x64/emit_x64_data_processing.cpp
@@ -1090,19 +1090,21 @@ static void EmitSub(BlockOfCode& code, EmitContext& ctx, IR::Inst* inst, int bit
     // TODO: Optimize CMP case.
     // Note that x64 CF is inverse of what the ARM carry flag is here.
 
+    bool invert_output_carry = true;
+
     if (args[1].IsImmediate() && args[1].GetType() == IR::Type::U32) {
         const u32 op_arg = args[1].GetImmediateU32();
         if (carry_in.IsImmediate()) {
             if (carry_in.GetImmediateU1()) {
                 code.sub(result, op_arg);
             } else {
-                code.stc();
-                code.sbb(result, op_arg);
+                code.add(result, ~op_arg);
+                invert_output_carry = false;
             }
         } else {
             code.bt(carry.cvt32(), 0);
-            code.cmc();
-            code.sbb(result, op_arg);
+            code.adc(result, ~op_arg);
+            invert_output_carry = false;
         }
     } else {
         OpArg op_arg = ctx.reg_alloc.UseOpArg(args[1]);
@@ -1122,14 +1124,20 @@ static void EmitSub(BlockOfCode& code, EmitContext& ctx, IR::Inst* inst, int bit
     }
 
     if (nzcv_inst) {
-        code.cmc();
+        if (invert_output_carry) {
+            code.cmc();
+        }
         code.lahf();
         code.seto(code.al);
         ctx.reg_alloc.DefineValue(nzcv_inst, nzcv);
         ctx.EraseInstruction(nzcv_inst);
     }
     if (carry_inst) {
-        code.setnc(carry);
+        if (invert_output_carry) {
+            code.setnc(carry);
+        } else {
+            code.setc(carry);
+        }
         ctx.reg_alloc.DefineValue(carry_inst, carry);
         ctx.EraseInstruction(carry_inst);
     }