From b7fa2dd0d0b7328f8f713b7cf0332d1e34c20fe4 Mon Sep 17 00:00:00 2001
From: Jean Boussier <jean.boussier@gmail.com>
Date: Tue, 3 Sep 2024 09:38:18 +0200
Subject: [PATCH] rb_enc_str_asciionly_p: avoid always fetching the encoding

Profiling of `JSON.dump` shows a significant amount of time spent
in `rb_enc_str_asciionly_p`, in large part because it fetches the
encoding.

It can be made twice as fast in this scenario by first checking the
coderange and only falling back to fetching the encoding if the
coderange is unknown.

Additionally we can skip fetching the encoding for the common
popular encodings.
---
 string.c | 32 +++++++++++++++++++++++---------
 1 file changed, 23 insertions(+), 9 deletions(-)

diff --git a/string.c b/string.c
index 025382c0f4..0dd02f8253 100644
--- a/string.c
+++ b/string.c
@@ -137,10 +137,10 @@ VALUE rb_cSymbol;
 } while (0)
 
 static inline bool
-str_enc_fastpath(VALUE str)
+str_encindex_fastpath(int encindex)
 {
     // The overwhelming majority of strings are in one of these 3 encodings.
-    switch (ENCODING_GET_INLINED(str)) {
+    switch (encindex) {
       case ENCINDEX_ASCII_8BIT:
       case ENCINDEX_UTF_8:
       case ENCINDEX_US_ASCII:
@@ -150,6 +150,12 @@ str_enc_fastpath(VALUE str)
     }
 }
 
+static inline bool
+str_enc_fastpath(VALUE str)
+{
+    return str_encindex_fastpath(ENCODING_GET_INLINED(str));
+}
+
 #define TERM_LEN(str) (str_enc_fastpath(str) ? 1 : rb_enc_mbminlen(rb_enc_from_index(ENCODING_GET(str))))
 #define TERM_FILL(ptr, termlen) do {\
     char *const term_fill_ptr = (ptr);\
@@ -862,16 +868,24 @@ rb_enc_str_coderange(VALUE str)
     return cr;
 }
 
+static inline bool
+rb_enc_str_asciicompat(VALUE str)
+{
+    int encindex = ENCODING_GET_INLINED(str);
+    return str_encindex_fastpath(encindex) || rb_enc_asciicompat(rb_enc_get_from_index(encindex));
+}
+
 int
 rb_enc_str_asciionly_p(VALUE str)
 {
-    rb_encoding *enc = STR_ENC_GET(str);
-
-    if (!rb_enc_asciicompat(enc))
-        return FALSE;
-    else if (is_ascii_string(str))
-        return TRUE;
-    return FALSE;
+    switch(ENC_CODERANGE(str)) {
+        case ENC_CODERANGE_UNKNOWN:
+            return rb_enc_str_asciicompat(str) && is_ascii_string(str);
+        case ENC_CODERANGE_7BIT:
+            return true;
+        default:
+            return false;
+    }
 }
 
 static inline void