From cefb1a823d6d9c241adf1a90dc380af745dca96d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ciro=20Santilli=20=E5=85=AD=E5=9B=9B=E4=BA=8B=E4=BB=B6=20?=
 =?UTF-8?q?=E6=B3=95=E8=BD=AE=E5=8A=9F?= <ciro.santilli@gmail.com>
Date: Wed, 12 Jun 2019 00:00:01 +0000
Subject: [PATCH] x86 asm: move cmp from x86-assembly-cheat

---
 README.adoc                |  1 +
 userland/arch/x86_64/add.S | 14 ++++++++++++++
 userland/arch/x86_64/cmp.S | 21 +++++++++++++++++++++
 3 files changed, 36 insertions(+)
 create mode 100644 userland/arch/x86_64/cmp.S

diff --git a/README.adoc b/README.adoc
index d55d16f..785286e 100644
--- a/README.adoc
+++ b/README.adoc
@@ -12364,6 +12364,7 @@ Bibliography:
 ** link:userland/arch/x86_64/div_overflow.S[DIV overflow]
 ** link:userland/arch/x86_64/div_zero.S[DIV zero]
 ** link:userland/arch/x86_64/idiv.S[IDIV]
+* link:userland/arch/x86_64/cmp.S[CMP]
 
 === x86 SIMD
 
diff --git a/userland/arch/x86_64/add.S b/userland/arch/x86_64/add.S
index 31193b9..9e08abf 100644
--- a/userland/arch/x86_64/add.S
+++ b/userland/arch/x86_64/add.S
@@ -42,4 +42,18 @@ LKMC_PROLOGUE
      */
     addq memory_immediate, memory_register
 #endif
+
+    /* ADD and many other instructions automatically update the status flags
+     * which controls branches.
+     */
+
+        /* Equals 0. */
+        mov $1, %rax
+        add $-1, %rax
+        LKMC_ASSERT(je)
+
+        /* Not equals 0. */
+        mov $2, %rax
+        add $-1, %rax
+        LKMC_ASSERT(jne)
 LKMC_EPILOGUE
diff --git a/userland/arch/x86_64/cmp.S b/userland/arch/x86_64/cmp.S
new file mode 100644
index 0000000..8cf1b8b
--- /dev/null
+++ b/userland/arch/x86_64/cmp.S
@@ -0,0 +1,21 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#userland-assembly
+ *
+ * Compare two numbers and set the flags register.
+ *
+ * `cmp X, Y` does `X - Y` and ignores the exact result, but sets
+ * all flags that would be set on the subtraction, just like SUB does.
+ */
+
+#include <lkmc.h>
+
+LKMC_PROLOGUE
+    /* 2 == 2 */
+    mov $2, %rax
+    cmp $2, %rax
+    LKMC_ASSERT(je)
+
+    /* 2 > 1 */
+    mov $2, %rax
+    cmp $1, %rax
+    LKMC_ASSERT(ja)
+LKMC_EPILOGUE