x86 asm: move cmp from x86-assembly-cheat

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
 

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

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