Polish up strcpy code

.gitignore

@@ -0,0 +1,3 @@
+*.o
+a.out
+build

driver/alloca_driver.S

@@ -1,8 +1,4 @@
-.intel_syntax noprefix
+.include "lib/alloca.S"
-
-.macro  alloca size
-sub rsp, \size
-.endm
 
 .text
 

driver/strcpy_driver.S

@@ -1,3 +1,5 @@
+    .include "lib/strcpy.S"
+
     .intel_syntax noprefix
 
     .globl _start
@@ -5,22 +7,6 @@
     .text
 
 
-# String-copying routine.
-# Commented-out lines would also return the starting address of the destination address.
-strcpy:
-#    push rsi
-.loop:
-    movb r10b, [rdi]
-    cmpb r10b, 0
-    je .end
-    movb [rsi], r10b
-    inc rdi
-    inc rsi
-    jmp .loop
-.end:
-#    pop rax
-    ret
-
 _start:
 
     mov rdi, offset str2

lib/alloca.S

@@ -0,0 +1,5 @@
+.intel_syntax noprefix
+
+.macro  alloca size
+sub rsp, \size
+.endm

lib/strcpy.S

@@ -0,0 +1,17 @@
+    .intel_syntax noprefix
+
+    .text
+
+# String-copying routine. Strings should be null-terminated.
+# Input: rdi = source string base, rsi = destination string base
+# No check is done for buffer overflow.
+strcpy:
+    movb r10b, [rdi]
+    cmpb r10b, 0
+    je .end
+    movb [rsi], r10b
+    inc rdi
+    inc rsi
+    jmp strcpy
+.end:
+    ret