From 1b5acebef2d447a3dbed6cf5e146fda74b81f10d Mon Sep 17 00:00:00 2001
From: normal <normal@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>
Date: Thu, 2 Oct 2014 21:17:13 +0000
Subject: [PATCH] st.c (new_size): use next_pow2 function

Reduces object size slightly on x86-64:

   text	   data	    bss	    dec	    hex	filename
2782359	  22400	  71880	2876639	 2be4df	ruby.orig
2781831	  22400	  71880	2876111	 2be2cf	ruby.pow2

And on 32-bit x86:

   text	   data	    bss	    dec	    hex	filename
2814751	  12100	  30552	2857403	 2b99bb	ruby.orig
2814051	  12100	  30552	2856703	 2b96ff	ruby.pow2

This is not a performance-critical function, but the
smaller icache footprint seems worth it.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47767 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
---
 ChangeLog |  5 +++++
 st.c      | 25 +++++++++++++++++++++----
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 9dffbed6f4..21d35117d2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Oct  3 06:06:28 2014  Eric Wong  <e@80x24.org>
+
+	* st.c (next_pow2): new function (from old bignum.c)
+	  (new_size): use next_pow2 function
+
 Fri Oct  3 05:58:58 2014  Eric Wong  <e@80x24.org>
 
 	* vm_trace.c (rb_tp_t): pack 56 => 48 bytes on 64-bit
diff --git a/st.c b/st.c
index fa3fa3f037..ac58e4d44b 100644
--- a/st.c
+++ b/st.c
@@ -140,14 +140,31 @@ remove_safe_packed_entry(st_table *table, st_index_t i, st_data_t never)
     PHASH_SET(table, i, 0);
 }
 
+static st_index_t
+next_pow2(st_index_t x)
+{
+    x |= x >> 1;
+    x |= x >> 2;
+    x |= x >> 4;
+    x |= x >> 8;
+    x |= x >> 16;
+#if SIZEOF_ST_INDEX_T == 8
+    x |= x >> 32;
+#endif
+    return x + 1;
+}
+
 static st_index_t
 new_size(st_index_t size)
 {
-    st_index_t i;
+    st_index_t n;
 
-    for (i=3; i<31; i++) {
-	if ((st_index_t)(1<<i) > size) return 1<<i;
-    }
+    if (size && (size & ~(size - 1)) == size) /* already a power-of-two? */
+	return size;
+
+    n = next_pow2(size);
+    if (n > size)
+	return n;
 #ifndef NOT_RUBY
     rb_raise(rb_eRuntimeError, "st_table too big");
 #endif