From d656176f068a93b15cd093fafa193755d169de31 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ludovic=20Court=C3=A8s?= <ludo@gnu.org>
Date: Sat, 5 Feb 2022 23:27:48 +0100
Subject: [PATCH] 'string->symbol' avoids copying the underlying string.

* libguile/strings.c (scm_i_make_symbol): Reuse NAME's stringbuf unless
it's mutable or NAME starts at a non-zero offset.
---
 libguile/strings.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/libguile/strings.c b/libguile/strings.c
index a5b4b0386..a48f82b4a 100644
--- a/libguile/strings.c
+++ b/libguile/strings.c
@@ -763,10 +763,14 @@ SCM
 scm_i_make_symbol (SCM name, scm_t_bits flags, unsigned long hash)
 {
   SCM buf, symbol;
-  size_t length = STRING_LENGTH (name);
+  size_t start, length = STRING_LENGTH (name);
 
-  name = scm_i_substring_copy (name, 0, length);
-  buf = STRING_STRINGBUF (name);
+  get_str_buf_start (&name, &buf, &start);
+  if (SCM_UNLIKELY (STRINGBUF_MUTABLE (buf) || start != 0))
+    {
+      name = scm_i_substring_copy (name, 0, length);
+      buf = STRING_STRINGBUF (name);
+    }
 
   symbol = scm_words (scm_tc7_symbol | flags, 3);
   SCM_SET_CELL_WORD_1 (symbol, SCM_UNPACK (buf));