go.tools/container/intsets: increase block size to 256 bits.

This consistently yields better performance with go/pointer.

Also: return int not word from ntz().

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/97570044

container/intsets/sparse.go

@@ -21,6 +21,10 @@ package intsets
 // TODO(adonovan): implement Dense, a dense bit vector with a similar API.
 // The space usage would be proportional to Max(), not Len(), and the
 // implementation would be based upon big.Int.
+//
+// TODO(adonovan): experiment with making the root block indirect (nil
+// iff IsEmpty).  This would reduce the memory usage when empty and
+// might simplify the aliasing invariants.
 
 import (
 	"bytes"
@@ -50,7 +54,7 @@ type word uintptr
 const (
 	_m            = ^word(0)
 	bitsPerWord   = 8 << (_m>>8&1 + _m>>16&1 + _m>>32&1)
-	bitsPerBlock  = 128
+	bitsPerBlock  = 256 // optimal value for go/pointer solver performance
 	wordsPerBlock = bitsPerBlock / bitsPerWord
 )
 
@@ -161,9 +165,9 @@ func (b *block) min(take bool) int {
 		if w != 0 {
 			tz := ntz(w)
 			if take {
-				b.bits[i] = w &^ (1 << tz)
+				b.bits[i] = w &^ (1 << uint(tz))
 			}
-			return b.offset + int(i*bitsPerWord) + int(tz)
+			return b.offset + int(i*bitsPerWord) + tz
 		}
 	}
 	panic("BUG: empty block")

container/intsets/util.go

@@ -44,11 +44,11 @@ func nlz(x word) int {
 
 // ntz returns the number of trailing zeros of x.
 // From Hacker's Delight, fig 5.13.
-func ntz(x word) word {
+func ntz(x word) int {
 	if x == 0 {
 		return bitsPerWord
 	}
-	var n word = 1
+	n := 1
 	if bitsPerWord == 64 {
 		if (x & 0xffffffff) == 0 {
 			n = n + 32
@@ -71,5 +71,5 @@ func ntz(x word) word {
 		n = n + 2
 		x = x >> 2
 	}
-	return n - x&1
+	return n - int(x&1)
 }