x86 asm: move string instructions from x86-assembly-cheat

userland/arch/x86_64/rep.S

@@ -0,0 +1,73 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#x86-rep-prefix */
+
+#include <lkmc.h>
+
+.bss
+    src: .skip 16
+    dst: .skip 16
+LKMC_PROLOGUE
+
+    /* memset: REP STOSQ */
+    cld
+    lea dst(%rip), %rdi
+    /* 2 elements. */
+    mov $2, %rcx
+    /* Set every element to 42. */
+    mov $0x2A, %rax
+    rep stosq
+    /* RCX was decremented down to zero. */
+    LKMC_ASSERT_EQ(%rcx, $0)
+    /* And the memory was set. */
+    LKMC_ASSERT_EQ(dst + 0, $0x2A)
+    LKMC_ASSERT_EQ(dst + 8, $0x2A)
+
+    /* memcpy: REP MOVSQ */
+    cld
+    movq $2, src + 0
+    movq $3, src + 8
+    lea src(%rip), %rsi
+    lea dst(%rip), %rdi
+    mov $2, %rcx
+    rep movsq
+    LKMC_ASSERT_EQ(dst + 0, $2)
+    LKMC_ASSERT_EQ(dst + 8, $3)
+
+    /* memcmp: REPZ CMPSQ */
+
+        /* Setup src. */
+        movl $2, src + 0x0
+        movl $3, src + 0x4
+        movl $4, src + 0x8
+        movl $5, src + 0xA
+
+        /* Equal. */
+        movl $2, dst + 0x0
+        movl $3, dst + 0x4
+        movl $4, dst + 0x8
+        movl $5, dst + 0xA
+        cld
+        mov $src, %rsi
+        mov $dst, %rdi
+        mov $4, %rcx
+        repz cmpsl
+        mov %rcx, %r12
+        /* Last flag was equal. */
+        LKMC_ASSERT(jz)
+        /* RCX was decreased all the way to zero. */
+        LKMC_ASSERT_EQ(%r12, $0)
+
+        /* Different. */
+        movl $2, dst + 0x0
+        movl $3, dst + 0x4
+        movl $2, dst + 0x8
+        movl $5, dst + 0xA
+        mov $src, %rsi
+        mov $dst, %rdi
+        mov $4, %rcx
+        repz cmpsl
+        mov %rcx, %r12
+        LKMC_ASSERT(jnz)
+        /* We stopped half-way with 1 comparision missing. */
+        LKMC_ASSERT_EQ(%r12, $1)
+
+LKMC_EPILOGUE