x86 asm: move POPCNT in from x86-assembly-cheat

README.adoc

@@ -12427,6 +12427,9 @@ Do a BT and then swap the value of the tested bit.
 * link:userland/arch/x86_64/setcc.S[SETcc]
 +
 Set a a byte of a register to 0 or 1 depending on the cc condition.
+* link:userland/arch/x86_64/popcnt.S[POPCNT]
++
+Count the number of 1 bits.
 * link:userland/arch/x86_64/test.S[TEST]
 +
 Like <<x86-binary-arithmetic-instructions,CMP>> but does AND instead of SUB:

userland/arch/x86_64/popcnt.S

@@ -0,0 +1,37 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#x86-bit-and-byte-instructions */
+
+#include <lkmc.h>
+
+LKMC_PROLOGUE
+    mov $0, %rbx
+    popcnt %rbx, %rax
+    LKMC_ASSERT_EQ(%rax, $0)
+
+    mov $1, %rbx
+    popcnt %rbx, %rax
+    LKMC_ASSERT_EQ(%rax, $1)
+
+    mov $2, %rbx
+    popcnt %rbx, %rax
+    LKMC_ASSERT_EQ(%rax, $1)
+
+    mov $3, %rbx
+    popcnt %rbx, %rax
+    LKMC_ASSERT_EQ(%rax, $2)
+
+    mov $4, %rbx
+    popcnt %rbx, %rax
+    LKMC_ASSERT_EQ(%rax, $1)
+
+    mov $5, %rbx
+    popcnt %rbx, %rax
+    LKMC_ASSERT_EQ(%rax, $2)
+
+    mov $6, %rbx
+    popcnt %rbx, %rax
+    LKMC_ASSERT_EQ(%rax, $2)
+
+    mov $7, %rbx
+    popcnt %rbx, %rax
+    LKMC_ASSERT_EQ(%rax, $3)
+LKMC_EPILOGUE