patch 9.2.0152: concatenating strings is slow

Problem:  concatenating strings is slow
Solution: Use grow_string_tv() to grow the existing string buffer in
          place when possible (Yasuhiro Matsumoto).

closes: #19642

Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/eval.c

@@ -2639,6 +2639,28 @@ tv_op_number(typval_T *tv1, typval_T *tv2, char_u *op)
     return failed ? FAIL : OK;
 }
 
+/*
+ * Append string "s2" to the string in "tv1".
+ * Returns OK if "tv1" was grown in place, FAIL otherwise.
+ */
+    static int
+grow_string_tv(typval_T *tv1, char_u *s2)
+{
+    if (tv1->v_type != VAR_STRING || tv1->vval.v_string == NULL)
+	return FAIL;
+
+    size_t len1 = STRLEN(tv1->vval.v_string);
+    size_t len2 = STRLEN(s2);
+    char_u *p = vim_realloc(tv1->vval.v_string, len1 + len2 + 1);
+
+    if (p == NULL)
+	return FAIL;
+
+    mch_memmove(p + len1, s2, len2 + 1);
+    tv1->vval.v_string = p;
+    return OK;
+}
+
 /*
  * Handle "str1 .= str2"
  * Returns OK or FAIL.
@@ -2648,13 +2670,18 @@ tv_op_string(typval_T *tv1, typval_T *tv2, char_u *op UNUSED)
 {
     char_u	numbuf[NUMBUFLEN];
     char_u	*s;
+    char_u	*s2;
 
     if (tv2->v_type == VAR_FLOAT)
 	return FAIL;
 
     // str .= str
+    s2 = tv_get_string_buf(tv2, numbuf);
+    if (grow_string_tv(tv1, s2) == OK)
+	return OK;
+
     s = tv_get_string(tv1);
-    s = concat_str(s, tv_get_string_buf(tv2, numbuf));
+    s = concat_str(s, s2);
     clear_tv(tv1);
     tv1->v_type = VAR_STRING;
     tv1->vval.v_string = s;
@@ -4371,6 +4398,10 @@ eval_concat_str(typval_T *tv1, typval_T *tv2)
 	return FAIL;
     }
 
+    // When possible, grow the existing string in place to avoid alloc/free.
+    if (grow_string_tv(tv1, s2) == OK)
+	return OK;
+
     p = concat_str(s1, s2);
     clear_tv(tv1);
     tv1->v_type = VAR_STRING;

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    152,
 /**/
     151,
 /**/