runtime: optimize heapBitsBulkBarrier a tiny amount

This may be mostly noise but:

name                   old mean              new mean              delta
BinaryTree17            6.03s × (0.98,1.02)   5.98s × (0.97,1.03)    ~    (p=0.306)
Fannkuch11              4.42s × (0.99,1.01)   4.34s × (0.99,1.02)  -1.83% (p=0.000)
FmtFprintfEmpty        84.7ns × (0.99,1.01)  84.4ns × (1.00,1.00)    ~    (p=0.138)
FmtFprintfString        289ns × (0.98,1.02)   289ns × (1.00,1.01)    ~    (p=0.509)
FmtFprintfInt           280ns × (0.97,1.03)   272ns × (0.98,1.03)  -2.64% (p=0.003)
FmtFprintfIntInt        484ns × (0.98,1.02)   482ns × (0.98,1.03)    ~    (p=0.606)
FmtFprintfPrefixedInt   397ns × (0.98,1.03)   393ns × (0.99,1.02)    ~    (p=0.064)
FmtFprintfFloat         573ns × (0.99,1.01)   569ns × (0.99,1.01)  -0.69% (p=0.023)
FmtManyArgs            1.89µs × (0.99,1.02)  1.91µs × (0.98,1.02)    ~    (p=0.219)
GobDecode              15.4ms × (0.99,1.02)  15.1ms × (0.99,1.01)  -2.05% (p=0.000)
GobEncode              12.0ms × (0.97,1.04)  11.9ms × (0.97,1.03)    ~    (p=0.458)
Gzip                    652ms × (0.99,1.01)   653ms × (0.99,1.01)    ~    (p=0.743)
Gunzip                  144ms × (0.99,1.01)   143ms × (0.99,1.01)    ~    (p=0.134)
HTTPClientServer       91.6µs × (0.99,1.01)  91.8µs × (0.99,1.03)    ~    (p=0.678)
JSONEncode             31.9ms × (1.00,1.00)  32.0ms × (0.99,1.01)    ~    (p=0.334)
JSONDecode              110ms × (0.99,1.01)   110ms × (0.99,1.01)    ~    (p=0.315)
Mandelbrot200          6.04ms × (0.99,1.01)  6.04ms × (1.00,1.01)    ~    (p=0.596)
GoParse                6.72ms × (0.98,1.03)  6.74ms × (0.99,1.03)    ~    (p=0.577)
RegexpMatchEasy0_32     161ns × (0.99,1.01)   160ns × (1.00,1.00)  -0.83% (p=0.002)
RegexpMatchEasy0_1K     542ns × (0.99,1.02)   541ns × (0.99,1.01)    ~    (p=0.396)
RegexpMatchEasy1_32     140ns × (0.98,1.01)   137ns × (1.00,1.00)  -2.12% (p=0.000)
RegexpMatchEasy1_1K     892ns × (0.99,1.01)   891ns × (1.00,1.01)    ~    (p=0.631)
RegexpMatchMedium_32    255ns × (0.99,1.01)   253ns × (0.99,1.01)  -0.76% (p=0.008)
RegexpMatchMedium_1K   73.1µs × (1.00,1.01)  72.9µs × (1.00,1.00)    ~    (p=0.229)
RegexpMatchHard_32     3.86µs × (1.00,1.01)  3.85µs × (1.00,1.00)    ~    (p=0.341)
RegexpMatchHard_1K      117µs × (1.00,1.01)   117µs × (0.99,1.00)    ~    (p=0.955)
Revcomp                 954ms × (0.97,1.03)   955ms × (0.98,1.02)    ~    (p=0.894)
Template                133ms × (0.97,1.05)   129ms × (0.99,1.02)  -2.50% (p=0.014)
TimeParse               629ns × (0.99,1.01)   626ns × (0.99,1.01)    ~    (p=0.106)
TimeFormat              663ns × (0.99,1.01)   660ns × (0.99,1.02)    ~    (p=0.231)

Change-Id: I580e03ed01b0629cb5eae4c4637618f20127f924
Reviewed-on: https://go-review.googlesource.com/9994Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mbitmap.go

@@ -365,11 +365,13 @@ func heapBitsBulkBarrier(p, size uintptr) {
 		return
 	}
 
+	h := heapBitsForAddr(p)
 	for i := uintptr(0); i < size; i += ptrSize {
-		if heapBitsForAddr(p + i).isPointer() {
+		if h.isPointer() {
 			x := (*uintptr)(unsafe.Pointer(p + i))
 			writebarrierptr_nostore(x, *x)
 		}
+		h = h.next()
 	}
 }