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runtime: optimize findHead
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This is similar to #3899, but
smaller and hopefully just as efficient.

Thanks to @HattoriHanzo031 for starting this work, benchmarking, and for
improving the performance of the code even further.
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aykevl committed Nov 12, 2024
1 parent 0edeaf6 commit 07ba1e3
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Showing 2 changed files with 40 additions and 6 deletions.
6 changes: 3 additions & 3 deletions builder/sizes_test.go
Original file line number Diff line number Diff line change
Expand Up @@ -41,9 +41,9 @@ func TestBinarySize(t *testing.T) {
// This is a small number of very diverse targets that we want to test.
tests := []sizeTest{
// microcontrollers
{"hifive1b", "examples/echo", 4568, 280, 0, 2268},
{"microbit", "examples/serial", 2868, 388, 8, 2272},
{"wioterminal", "examples/pininterrupt", 6104, 1484, 116, 6832},
{"hifive1b", "examples/echo", 4588, 280, 0, 2268},
{"microbit", "examples/serial", 2888, 388, 8, 2272},
{"wioterminal", "examples/pininterrupt", 6124, 1484, 116, 6832},

// TODO: also check wasm. Right now this is difficult, because
// wasm binaries are run through wasm-opt and therefore the
Expand Down
40 changes: 37 additions & 3 deletions src/runtime/gc_blocks.go
Original file line number Diff line number Diff line change
Expand Up @@ -76,6 +76,13 @@ const (
blockStateMask blockState = 3 // 11
)

// The byte value of a block where every block is a 'tail' block.
const blockStateByteAllTails = 0 |
uint8(blockStateTail<<(stateBits*3)) |
uint8(blockStateTail<<(stateBits*2)) |
uint8(blockStateTail<<(stateBits*1)) |
uint8(blockStateTail<<(stateBits*0))

// String returns a human-readable version of the block state, for debugging.
func (s blockState) String() string {
switch s {
Expand Down Expand Up @@ -123,7 +130,25 @@ func (b gcBlock) address() uintptr {
// points to an allocated object. It returns the same block if this block
// already points to the head.
func (b gcBlock) findHead() gcBlock {
for b.state() == blockStateTail {
for {
// Optimization: check whether the current block state byte (which
// contains the state of multiple blocks) is composed entirely of tail
// blocks. If so, we can skip back to the last block in the previous
// state byte.
// This optimization speeds up findHead for pointers that point into a
// large allocation.
stateByte := b.stateByte()
if stateByte == blockStateByteAllTails {
b -= (b % blocksPerStateByte) + 1
continue
}

// Check whether we've found a non-tail block, which means we found the
// head.
state := b.stateFromByte(stateByte)
if state != blockStateTail {
break
}
b--
}
if gcAsserts {
Expand All @@ -146,10 +171,19 @@ func (b gcBlock) findNext() gcBlock {
return b
}

func (b gcBlock) stateByte() byte {
return *(*uint8)(unsafe.Add(metadataStart, b/blocksPerStateByte))
}

// Return the block state given a state byte. The state byte must have been
// obtained using b.stateByte(), otherwise the result is incorrect.
func (b gcBlock) stateFromByte(stateByte byte) blockState {
return blockState(stateByte>>((b%blocksPerStateByte)*stateBits)) & blockStateMask
}

// State returns the current block state.
func (b gcBlock) state() blockState {
stateBytePtr := (*uint8)(unsafe.Add(metadataStart, b/blocksPerStateByte))
return blockState(*stateBytePtr>>((b%blocksPerStateByte)*stateBits)) & blockStateMask
return b.stateFromByte(b.stateByte())
}

// setState sets the current block to the given state, which must contain more
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