From f4ff0d493c87b9fd301a222bc73bdc3288a47d1f Mon Sep 17 00:00:00 2001
From: Paris Oplopoios <parisoplop@gmail.com>
Date: Tue, 3 Sep 2024 20:41:22 +0300
Subject: [PATCH] Set CF correctly on BLSI patch (#722)

* Set CF correctly on BLSI patch

* Remove redundant CLC
---
 src/core/cpu_patches.cpp | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/core/cpu_patches.cpp b/src/core/cpu_patches.cpp
index 151d34986..07253b46f 100644
--- a/src/core/cpu_patches.cpp
+++ b/src/core/cpu_patches.cpp
@@ -307,9 +307,24 @@ static void GenerateBLSI(const ZydisDecodedOperand* operands, Xbyak::CodeGenerat
 
     SaveRegisters(c, {scratch});
 
+    // BLSI sets CF to zero if source is zero, otherwise it sets CF to one.
+    Xbyak::Label set_carry, clear_carry, end;
+
     c.mov(scratch, *src);
-    c.neg(scratch);
+    c.neg(scratch); // NEG, like BLSI, clears CF if the source is zero and sets it otherwise
+    c.jc(set_carry);
+    c.jmp(clear_carry);
+
+    c.L(set_carry);
     c.and_(scratch, *src);
+    c.stc(); // setting/clearing carry needs to happen after the AND because that clears CF
+    c.jmp(end);
+
+    c.L(clear_carry);
+    c.and_(scratch, *src);
+    // We don't need to clear carry here since AND does that for us
+
+    c.L(end);
     c.mov(dst, scratch);
 
     RestoreRegisters(c, {scratch});