From: Ivan Maidanski <ivmai@mail.ru>
Date: Mon, 26 Mar 2018 18:30:32 +0000 (+0300)
Subject: Reduce probability of collision in threads hashtable for 64-bit targets
X-Git-Tag: v8.0.0~272
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=553ed5607385e85f30aefe8d60b737b5c232439c;p=gc

Reduce probability of collision in threads hashtable for 64-bit targets

* include/private/pthread_support.h [GC_PTHREADS && !GC_WIN32_THREADS
&& CPP_WORDSZ==64] (THREAD_TABLE_INDEX): Xor also 4th byte of id value.
---

diff --git a/include/private/pthread_support.h b/include/private/pthread_support.h
index 8df6bc21..ae5c2e9e 100644
--- a/include/private/pthread_support.h
+++ b/include/private/pthread_support.h
@@ -140,10 +140,17 @@ typedef struct GC_Thread_Rep {
 # define THREAD_TABLE_SZ 256    /* Power of 2 (for speed). */
 #endif
 
-#define THREAD_TABLE_INDEX(id) \
+#if CPP_WORDSZ == 64
+# define THREAD_TABLE_INDEX(id) \
+    (int)(((((NUMERIC_THREAD_ID(id) >> 8) ^ NUMERIC_THREAD_ID(id)) >> 16) \
+          ^ ((NUMERIC_THREAD_ID(id) >> 8) ^ NUMERIC_THREAD_ID(id))) \
+         % THREAD_TABLE_SZ)
+#else
+# define THREAD_TABLE_INDEX(id) \
                 (int)(((NUMERIC_THREAD_ID(id) >> 16) \
                        ^ (NUMERIC_THREAD_ID(id) >> 8) \
                        ^ NUMERIC_THREAD_ID(id)) % THREAD_TABLE_SZ)
+#endif
 
 GC_EXTERN volatile GC_thread GC_threads[THREAD_TABLE_SZ];