Have flatecut's huffman.decode load more bits
name old time/op new time/op delta
pkg:github.com/google/wuffs/lib/flatecut goos:linux goarch:amd64
Cut-56 1.03ms ± 0% 0.92ms ± 0% -10.44% (p=0.008 n=5+5)
pkg:github.com/google/wuffs/lib/zlibcut goos:linux goarch:amd64
Cut-56 2.44ms ± 0% 2.34ms ± 1% -4.19% (p=0.008 n=5+5)
diff --git a/lib/flatecut/flatecut.go b/lib/flatecut/flatecut.go
index b0b7391..b574222 100644
--- a/lib/flatecut/flatecut.go
+++ b/lib/flatecut/flatecut.go
@@ -84,6 +84,11 @@
mostNegativeInt32 = -0x80000000
)
+func loadU32LE(b []byte) uint32 {
+ _ = b[3] // bounds check hint to compiler; see golang.org/issue/14808
+ return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
+}
+
type bitstream struct {
// bytes[index] is the next byte to load into the 'bits' field.
bytes []byte
@@ -157,6 +162,13 @@
func (h *huffman) decode(b *bitstream) int32 {
if b.nBits >= 8 {
// No-op.
+ } else if b.index < (len(b.bytes) - 4) {
+ // This is "Variant 4" of
+ // https://fgiesen.wordpress.com/2018/02/20/reading-bits-in-far-too-many-ways-part-2/
+ u := loadU32LE(b.bytes[b.index:])
+ b.bits |= u << b.nBits
+ b.index += int((31 - b.nBits) >> 3)
+ b.nBits |= 24
} else if b.index < len(b.bytes) {
b.bits |= uint32(b.bytes[b.index]) << b.nBits
b.nBits += 8
