Optimize stpcpy's size and speed

Author: colinleroy

libsrc/common/stpcpy.c

@@ -0,0 +1,22 @@
+;
+; Colin Leroy-Mira, 4 Sept. 2024
+;
+; char* stpcpy (char* dest, const char* src);
+;
+
+        .export         _stpcpy
+        .import         _strcpy
+
+        .importzp       tmp1, ptr2
+
+_stpcpy:
+        jsr     _strcpy
+
+        ldx     ptr2+1        ; Load dest pointer's last high byte
+        tya                   ; Get the last offset strcpy wrote to
+
+        clc
+        adc     ptr2          ; Add to low byte value
+        bcc     :+
+        inx
+:       rts                   ; Return pointer to dest's terminator

libsrc/common/stpcpy.s

@@ -25,6 +25,9 @@ L1:     lda     (ptr1),y
         inc     ptr2+1
         bne     L1
 
-L9:     lda     ptr2            ; X still contains high byte
-        rts
+L9:     lda     ptr2            ; X still contains dest's original high byte
 
+        ; On exit, we want AX to be dest (as this is what strcpy returns).
+        ; We also want (ptr2),y to still point to dest's terminator, as this
+        ; is used by stpcpy().
+        rts

libsrc/common/strcpy.s

@@ -8,10 +8,12 @@
 #define STR_SHORT   "Hello, World!"
 #define STR_LONG    "This is a longer test string for stpcpy."
 
+char dest[512];
+char multi_page[300];
+
 int
 main ()
 {
-    char dest[50];
     const char *src_empty;
     const char *src_short;
     const char *src_long;
@@ -38,7 +40,14 @@ main ()
     assert(end == &dest[sizeof (STR_LONG) - 1]);
     printf ("Test 3 passed.\n");
 
+    memset(multi_page, 'a', sizeof(multi_page)-1);
+    multi_page[sizeof(multi_page)-1] = '\0';
+    end = stpcpy (dest, multi_page);
+    assert(!strcmp (dest, multi_page));
+    assert(!*end);
+    assert(end == &dest[sizeof (multi_page) - 1]);
+    printf ("Test 4 passed.\n");
+
     printf ("All tests passed.\n");
     return EXIT_SUCCESS;
 }
-