From: Hongli Lai <hongli@...99.net>
Date: 2008-03-16T01:15:10+09:00
Subject: Re: Copy-on-write friendly garbage collector

This is a multi-part message in MIME format.
--------------020001080201060600020504
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

Hongli Lai wrote:
> Great work Daniel. I don't measure the same amount of speedup that you 
> claim in your email, but there is definitely a small speedup.
> 
> I've added some further further optimizations. 
> find_position_in_bitfield() now uses bit operators instead of division 
> and modulo operators. This should speed things up a little more.
> 
> The attached patch is created against Ruby 1.8, but it shows what I've 
> exactly changed.

I've been such a fool. The patch I sent uses platform-specific macros to 
figure out the base 2 log of sizeof(int)*8.

Here is a new patch, which uses straight C code to figure out that 
value. There is no performance overhead because the code is easily 
optimizable by the compiler, and is a lot more portable.

--------------020001080201060600020504
Content-Type: text/x-patch;
 name="ruby-portable-bitfield-operations.diff"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline;
 filename="ruby-portable-bitfield-operations.diff"

diff --git a/marktable.c b/marktable.c
index 84f88fe..5890835 100644
--- a/marktable.c
+++ b/marktable.c
@@ -51,20 +51,30 @@ find_position_in_bitfield(struct heaps_slot *hs, RVALUE *object,
 	 * We use bit operators to calculate the position in the bit field, whenever possible.
 	 * This only works if sizeof(int) is a multiple of 2, but I don't know of any platform
 	 * on which that is not true.
-	 *
-	 * The INT_BITS_LOG macro's value must be equal to the base 2 logarithm of sizeof(int).
 	 */
-	#ifdef __i386__
-		#define INT_BITS_LOG 5
-	#endif
-	
-	#ifdef INT_BITS_LOG
-		*bitfield_index = index >> INT_BITS_LOG;
-		*bitfield_offset = index & ((1 << INT_BITS_LOG) - 1);
-	#else
+	if (sizeof(int) == 4 || sizeof(int) == 8 || sizeof(int) == 16) {
+		int int_bits_log; /* Must be equal to the base 2 logarithm of sizeof(int) * 8 */
+		
+		switch (sizeof(int)) {
+		case 4:
+			int_bits_log = 5;
+			break;
+		case 8:
+			int_bits_log = 6;
+			break;
+		case 16:
+			int_bits_log = 7;
+			break;
+		default:
+			int_bits_log = 0; /* Shut up compiler warning. */
+			abort();
+		}
+		*bitfield_index = index >> int_bits_log;
+		*bitfield_offset = index & ((sizeof(int) * 8) - 1);
+	} else {
 		*bitfield_index = index / (sizeof(int) * 8);
 		*bitfield_offset = index % (sizeof(int) * 8);
-	#endif
+	}
 }
 
 

--------------020001080201060600020504--