runtime: use integer math for hashmap overLoadFactor

Change-Id: I92cf39a05e738a03d956779d7a1ab1ef8074b2ab
Reviewed-on: https://go-review.googlesource.com/54655
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/hashmap.go

@@ -64,8 +64,10 @@ const (
 	bucketCntBits = 3
 	bucketCnt     = 1 << bucketCntBits
 
-	// Maximum average load of a bucket that triggers growth.
-	loadFactor = 6.5
+	// Maximum average load of a bucket that triggers growth is 6.5.
+	// Represent as loadFactorNum/loadFactDen, to allow integer math.
+	loadFactorNum = 13
+	loadFactorDen = 2
 
 	// Maximum key or value size to keep inline (instead of mallocing per element).
 	// Must fit in a uint8.
@@ -984,8 +986,7 @@ func hashGrow(t *maptype, h *hmap) {
 
 // overLoadFactor reports whether count items placed in 1<<B buckets is over loadFactor.
 func overLoadFactor(count int64, B uint8) bool {
-	// TODO: rewrite to use integer math and comparison?
-	return count >= bucketCnt && float32(count) >= loadFactor*float32((uint64(1)<<B))
+	return count >= bucketCnt && uint64(count) >= loadFactorNum*((uint64(1)<<B)/loadFactorDen)
 }
 
 // tooManyOverflowBuckets reports whether noverflow buckets is too many for a map with 1<<B buckets.

src/runtime/stubs.go

@@ -91,7 +91,7 @@ func reflect_memmove(to, from unsafe.Pointer, n uintptr) {
 }
 
 // exported value for testing
-var hashLoad = loadFactor
+var hashLoad = float32(loadFactorNum) / float32(loadFactorDen)
 
 //go:nosplit
 func fastrand() uint32 {