binary size optimization

src/runtime/gc_blocks.go

@@ -32,7 +32,6 @@ package runtime
 
 import (
 	"internal/task"
-	"math/bits"
 	"runtime/interrupt"
 	"unsafe"
 )
@@ -80,6 +79,20 @@ const (
 // turned into variable and assigned using inline function.
 const blockStateByteAllTails = uint8(blockStateTail<<(stateBits*3) | blockStateTail<<(stateBits*2) | blockStateTail<<(stateBits*1) | blockStateTail<<(stateBits*0))
 
+const len6tab = "" +
+	"\x00\x01\x02\x02\x03\x03\x03\x03\x04\x04\x04\x04\x04\x04\x04\x04" +
+	"\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05" +
+	"\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06" +
+	"\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06"
+
+// leadingZeros6 returns the number of leading zero bits in x; the result is 6 for x == 0.
+func leadingZeros6(x uint8) int { return 6 - len6(x) }
+
+// len6 returns the minimum number of bits required to represent x; the result is 0 for x == 0.
+func len6(x uint8) int {
+	return int(len6tab[x])
+}
+
 // String returns a human-readable version of the block state, for debugging.
 func (s blockState) String() string {
 	switch s {
@@ -151,7 +164,7 @@ func (b gcBlock) findHead() gcBlock {
 	// at this point stateByte is set to the first state byte of the object that we encountered which is not all tails
 	// and all tail bits in it are turned to zero. We count number of bytes that are 0 (tail) using LeadingZeros8
 	// and divide it by stateBits to get the number of tail blocks in state bits.
-	b -= gcBlock(bits.LeadingZeros8(stateByte) / stateBits)
+	b -= gcBlock(leadingZeros6(stateByte>>2) / stateBits)
 
 	if gcAsserts {
 		if b.state() != blockStateHead && b.state() != blockStateMark {