From d3bca3b7fab4aefdb6e86afd3b9cdec81fb1f3d7 Mon Sep 17 00:00:00 2001
From: zeertzjq <zeertzjq@outlook.com>
Date: Sat, 14 Mar 2026 08:14:34 +0800
Subject: [PATCH] vim-patch:9.2.0152: concatenating strings is slow (#38286)

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

closes: vim/vim#19642

https://github.com/vim/vim/commit/16d421a4d95b45ebbcf47ab60173cdb1b87ce419

Co-authored-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
---
 src/nvim/eval.c          | 22 ++++++++++++++++++++++
 src/nvim/eval/executor.c |  9 +++++++--
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index ab02298d2b..e95ce22f6b 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -2267,6 +2267,23 @@ static int eval_addlist(typval_T *tv1, typval_T *tv2)
   return OK;
 }
 
+/// Append string "s2" to the string in "tv1".
+/// Returns OK if "tv1" was grown in place, FAIL otherwise.
+int grow_string_tv(typval_T *tv1, const char *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 *p = xrealloc(tv1->vval.v_string, len1 + len2 + 1);
+
+  memmove(p + len1, s2, len2 + 1);
+  tv1->vval.v_string = p;
+  return OK;
+}
+
 /// Concatenate strings "tv1" and "tv2" and store the result in "tv1".
 static int eval_concat_str(typval_T *tv1, typval_T *tv2)
 {
@@ -2281,6 +2298,11 @@ static int 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;
+  }
+
   char *p = concat_str(s1, s2);
   tv_clear(tv1);
   tv1->v_type = VAR_STRING;
diff --git a/src/nvim/eval/executor.c b/src/nvim/eval/executor.c
index 39b8df9760..d8d10992a6 100644
--- a/src/nvim/eval/executor.c
+++ b/src/nvim/eval/executor.c
@@ -129,10 +129,15 @@ static int tv_op_string(typval_T *tv1, const typval_T *tv2, const char *op)
     return FAIL;
   }
 
+  char numbuf[NUMBUFLEN];
+  const char *s2 = tv_get_string_buf(tv2, numbuf);
+  if (grow_string_tv(tv1, s2) == OK) {
+    return OK;
+  }
+
   // str .= str
   const char *tvs = tv_get_string(tv1);
-  char numbuf[NUMBUFLEN];
-  char *const s = concat_str(tvs, tv_get_string_buf(tv2, numbuf));
+  char *const s = concat_str(tvs, s2);
   tv_clear(tv1);
   tv1->v_type = VAR_STRING;
   tv1->vval.v_string = s;