From 1e037108a13bc876af9adf09d133dcc3274aa277 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C3=89tienne=20Barri=C3=A9?= <etienne.barrie@gmail.com>
Date: Mon, 4 Nov 2024 11:41:04 +0100
Subject: [PATCH] Precompute hash only once when interning string literals

When a fake string is interned, use the capa field to store the string
hash. This lets us compute it once for hash lookup and embedding the
hash in the interned string.

Co-authored-by: Jean Boussier <byroot@ruby-lang.org>
---
 string.c | 32 ++++++++++++++++++++++++++------
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/string.c b/string.c
index 11a2ec1b7126e2..83c3cd7abaf586 100644
--- a/string.c
+++ b/string.c
@@ -352,9 +352,24 @@ static int fstring_cmp(VALUE a, VALUE b);
 
 static VALUE register_fstring(VALUE str, bool copy, bool precompute_hash);
 
+#if SIZEOF_LONG == SIZEOF_VOIDP
+static st_index_t
+fstring_hash(VALUE str)
+{
+    if (FL_TEST_RAW(str, STR_FAKESTR)) {
+        // register_fstring precomputes the hash and stores it in capa for fake strings
+        return (st_index_t)RSTRING(str)->as.heap.aux.capa;
+    }
+    else {
+        return rb_str_hash(str);
+    }
+}
+#else
+#define fstring_hash rb_str_hash
+#endif
 const struct st_hash_type rb_fstring_hash_type = {
     fstring_cmp,
-    rb_str_hash,
+    fstring_hash,
 };
 
 #define BARE_STRING_P(str) (!FL_ANY_RAW(str, FL_EXIVAR) && RBASIC_CLASS(str) == rb_cString)
@@ -371,7 +386,7 @@ str_do_hash(VALUE str)
 }
 
 static VALUE
-str_precompute_hash(VALUE str)
+str_store_precomputed_hash(VALUE str, st_index_t hash)
 {
     RUBY_ASSERT(!FL_TEST_RAW(str, STR_PRECOMPUTED_HASH));
     RUBY_ASSERT(STR_EMBED_P(str));
@@ -382,7 +397,6 @@ str_precompute_hash(VALUE str)
     RUBY_ASSERT(free_bytes >= sizeof(st_index_t));
 #endif
 
-    st_index_t hash = str_do_hash(str);
     memcpy(RSTRING_END(str) + TERM_LEN(str), &hash, sizeof(hash));
 
     FL_SET(str, STR_PRECOMPUTED_HASH);
@@ -399,7 +413,6 @@ struct fstr_update_arg {
 static int
 fstr_update_callback(st_data_t *key, st_data_t *value, st_data_t data, int existing)
 {
-
     struct fstr_update_arg *arg = (struct fstr_update_arg *)data;
     VALUE str = (VALUE)*key;
 
@@ -429,7 +442,7 @@ fstr_update_callback(st_data_t *key, st_data_t *value, st_data_t data, int exist
                     STR_SET_LEN(new_str, RSTRING_LEN(str));
                     TERM_FILL(RSTRING_END(new_str), TERM_LEN(str));
                     rb_enc_copy(new_str, str);
-                    str_precompute_hash(new_str);
+                    str_store_precomputed_hash(new_str, fstring_hash(str));
                 }
                 else {
                     new_str = str_new(rb_cString, RSTRING(str)->as.heap.ptr, RSTRING(str)->len);
@@ -509,6 +522,14 @@ register_fstring(VALUE str, bool copy, bool precompute_hash)
         .precompute_hash = precompute_hash
     };
 
+#if SIZEOF_VOIDP == SIZEOF_LONG
+    if (FL_TEST_RAW(str, STR_FAKESTR)) {
+        // if the string hasn't been interned, we'll need the hash twice, so we
+        // compute it once and store it in capa
+        RSTRING(str)->as.heap.aux.capa = (long)str_do_hash(str);
+    }
+#endif
+
     RB_VM_LOCK_ENTER();
     {
         st_table *frozen_strings = rb_vm_fstring_table();
@@ -531,7 +552,6 @@ static VALUE
 setup_fake_str(struct RString *fake_str, const char *name, long len, int encidx)
 {
     fake_str->basic.flags = T_STRING|RSTRING_NOEMBED|STR_NOFREE|STR_FAKESTR;
-    /* SHARED to be allocated by the callback */
 
     if (!name) {
         RUBY_ASSERT_ALWAYS(len == 0);