strcat: Better logic for determining max amount of characters to copy

src/string.asm

@@ -60,30 +60,25 @@ strcpy:
 ;	rsi*	(arg) Pointer to src[]
 ;	rdx	(arg) Size of dest[]
 ;	rcx*	Counter for movsb
-;	r8*	Backup for rdi
+;	r8*	Backup for rdi (point to start of dest)
 ;	r9*	Backup for rsi
-;	r10*	Current length of dest[]
+;	r10*	Backup for rdi (point to end of dest)
 strcat:
 	mov	r8, rdi
 	mov	r9, rsi
+	call	strlen
+	sub	rdx, rax
+	dec	rdi
+	mov	r10, rdi
 	mov	rdi, rsi
 	call	strlen
-	mov	rcx, rax
-	mov	rdi, r8
-	call	strlen
-	mov	r10, rax
-	mov	rdi, r8
-	add	rcx, rax
-	cmp	rcx, rdx
-	cmova	rcx, rdx
-	cmp	rcx, rdx
+	cmp	rax, rdx
 	jb	.copy
-	dec	rcx
-	.copy:	
-	mov	rdi, r8
+	dec	rdx
+	.copy:
+	mov	rcx, rdx
 	mov	rsi, r9
-	add	rdi, r10
-	sub	rcx, r10
+	mov	rdi, r10
 	cld
 	rep	movsb
 	mov	byte [rdi], EOS