internal/fuzz: use a lookup table for SnapshotCoverage

Previously, the implementation used bit manipulation to approximate
counters to the nearest power of two.

Given the 0-255 range of byte, we can precompute values at
initialization and use a lookup table, reducing runtime computation.

Benchmarks show an 18% performance gain on AMD64 and 5% on ARM64.

* net/netip/FuzzParse (n=10, t=60s, state reset per run)
  * AMD64 (Intel Alder Lake i5-12600k):

    17,349,217 -> 20,487,756 execs/s

  * ARM64 (M3 Pro):

    19,606,471 -> 20,657,041 execs/s

* compress/gzip/FuzzReader (n=10, t=60s, mature corpus)
  * AMD64 (Intel Alder Lake i5-12600k):

    5,655,956 -> 6,707,035 execs/s

Change-Id: If11f7fe866f54c7cd2c5a48e251c027b67980df7
Reviewed-on: https://go-review.googlesource.com/c/go/+/627378
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>

src/internal/fuzz/coverage.go

@@ -23,11 +23,7 @@ func ResetCoverage() {
 func SnapshotCoverage() {
 	cov := coverage()
 	for i, b := range cov {
-		b |= b >> 1
-		b |= b >> 2
-		b |= b >> 4
-		b -= b >> 1
-		coverageSnapshot[i] = b
+		coverageSnapshot[i] = pow2Table[b]
 	}
 }
 
@@ -102,4 +98,18 @@ var (
 	// the 8-bit coverage counters reside in memory. They're known to cmd/link,
 	// which specially assigns their addresses for this purpose.
 	_counters, _ecounters [0]byte
+
+	// lookup table for faster power of two rounding
+	pow2Table [256]byte
 )
+
+func init() {
+	for i := range pow2Table {
+		b := byte(i)
+		b |= b >> 1
+		b |= b >> 2
+		b |= b >> 4
+		b -= b >> 1
+		pow2Table[i] = b
+	}
+}