experiment

src/runtime/gc_blocks.go

@@ -32,7 +32,6 @@ package runtime
 
 import (
 	"internal/task"
-	"math/bits"
 	"runtime/interrupt"
 	"unsafe"
 )
@@ -125,29 +124,24 @@ func (b gcBlock) address() uintptr {
 	return addr
 }
 
-// findHead returns the head (first block) of an object, assuming the block
-// points to an allocated object. It returns the same block if this block
-// already points to the head.
-func (b gcBlock) findHead() gcBlock {
-	stateBytePtr := (*uint8)(unsafe.Add(metadataStart, b/blocksPerStateByte))
+//const blockStateByteAllTails = uint8(blockStateTail<<(stateBits*3) | blockStateTail<<(stateBits*2) | blockStateTail<<(stateBits*1) | blockStateTail<<(stateBits*0))
 
-	// XOR the stateByte with byte containing all tails to turn tail bits to 0 and
-	// mask out the bits that are not part of the object
-	otherObjectBlocks := int(blocksPerStateByte - (b%blocksPerStateByte + 1))
-	stateByte := ((*stateBytePtr) ^ blockStateByteAllTails) & (uint8(1<<(8-(otherObjectBlocks*stateBits))) - 1)
+func (b gcBlock) findHead() gcBlock {
+	for {
+		rem := b % blocksPerStateByte
+		stateByte := *(*uint8)(unsafe.Add(metadataStart, b/blocksPerStateByte))
+		if stateByte == blockStateByteAllTails {
+			b -= rem + 1
+			continue
+		}
 
-	// loop until state byte is not all tails
-	for stateByte == 0 {
-		stateBytePtr = (*uint8)(unsafe.Add(unsafe.Pointer(stateBytePtr), -1))
-		stateByte = (*stateBytePtr) ^ blockStateByteAllTails
-		b -= blocksPerStateByte
+		state := blockState(stateByte>>(rem*stateBits)) & blockStateMask
+		if state != blockStateTail {
+			break
+		}
+		b--
 	}
 
-	// in the first state byte which is not all tails, count the number of leading bits that are 0 and
-	// divide it by stateBits to get the number of tail blocks. Subtract otherObjectBlocks to exclude
-	// blocks that are not part of the object
-	b -= gcBlock((bits.LeadingZeros8(stateByte) / stateBits) - otherObjectBlocks)
-
 	if gcAsserts {
 		if b.state() != blockStateHead && b.state() != blockStateMark {
 			runtimePanic("gc: found tail without head")