sync/atomic: add OR/AND operators for unsigned types

[aktau]

Update, Aug 16 2023: Current proposal at #61395 (comment).

---

Original related proposal: #31748

Use case: we have types with methods that set a value. These methods manipulate a bitset indicating that the value was set, which is used (e.g.) for data serialization. Users of this API know to use a lock to manage concurrently reading/writing the same field, but they are allowed to concurrently write to different fields. Given that the bitsets are logically shared between different fields, we must manipulate them atomically. Currently that takes the form of a CAS loop:

type MyStruct struct{
  x int32
  y int32
  // ...

  present uint32 // Contains "present" bits for 32 fields.
}

func (m *MyStruct) SetX(v int32) {
  m.x = v
  setPresent(&m.present[0], 1)
}

func (m *MyStruct) SetY(v int32) {
  m.y = v
  setPresent(&m.present[0], 2)
}

func setPresent(part *uint32, num uint32) {
	for {
		old := atomic.LoadUint32(part)
		swapped := atomic.CompareAndSwapUint32(part, old, old|(1<<(num%32)))
		if swapped {
			return
		}
		// Yield and then try the swap again.
		runtime.Gosched()
	}
}

// similar for clearPresent, but with AND.

But, on x86-64, there are atomic versions of AND/OR that do this in one go, as mentioned in #31748. Using this would not only make the setters faster, it would likely also allow inlining them: setPresent is too complex to inline.

cc @aclements @ianlancetaylor @randall77

[rsc]

But, on x86-64, there are atomic versions of AND/OR that do this in one go, as mentioned in #31748. Using this would not only make the setters faster, it would likely also allow inlining them: setPresent is too complex to inline.

setPresent should not be calling runtime.Gosched. There's nothing another goroutine is going to do to help move things along (unlike, say, waiting for a mutex to be unlocked).

If you remove the Gosched, it should be inlinable. At least, -m says this is inlinable:

func orcas(x *uint32, bit uint32) {
	for {
		old := atomic.Load(x)
		if atomic.Cas(x, old, old|bit) {
			break
		}
	}
}

I wrote this in runtime/atomic_test.go:

package runtime_test

import (
	"runtime/internal/atomic"
	"testing"
)

func BenchmarkCas(b *testing.B) {
	var x uint32
	for i := 0; i < b.N; i++ {
		for {
			old := atomic.Load(&x)
			if atomic.Cas(&x, old, old|1) {
				break
			}
		}
	}
}

func BenchmarkOr(b *testing.B) {
	var x uint32
	for i := 0; i < b.N; i++ {
		atomic.Or(&x, 1)
	}
}

The result is:

BenchmarkCas-16                  	154634635	         7.429 ns/op
BenchmarkOr-16                   	270709120	         4.462 ns/op

So this would be an x86-only optimization that wins less than 2X. I'm not sure this really makes sense given how unusual it typically is. The main argument I can see is inlinability, but it seems to be inlinable already if written efficiently.

[aktau]

I was wondering about runtime.Gosched() as well, I couldn't figure out why it would help. Thanks for highlighting that it is indeed unnecessary.

So this would be an x86-only optimization that wins less than 2X. I'm not sure this really makes sense given how unusual it typically is. The main argument I can see is inlinability, but it seems to be inlinable already if written efficiently.

A 2x win may still be significant, but I'll know more when I add a better benchmarking setup. But also I found LDSET/LDSETA/LDSETAL/LDSETL for ARM, quoting:

Atomic bit set on word or doubleword in memory atomically loads a 32-bit word or 64-bit doubleword from memory, performs a bitwise OR with the value held in a register on it, and stores the result back to memory. The value initially loaded from memory is returned in the destination register.

I think the "atomically" may be referring to the entire operation. To be honest the mnemonic doesn't make it sound like this is a bitwise or. Perhaps an ARM expert could chime in.

[rsc]

Thanks for the link to LDSETAL. That looks like it might be good enough, although we would have to think carefully about whether the acquire-release semantics it guarantees matches the sequentially consistent semantics we want for sync/atomic when limited to the OR operation. Perhaps it does but perhaps not.

[aclements]

But also I found LDSET/LDSETA/LDSETAL/LDSETL for ARM

That's correct. ARMV8.1 added instructions for atomic And (also, Min/Max, and, for some reason, Xor). LDCLR, LDCLRA, LDCLRAL, LDCLRL is the equivalent for atomic And.

[aclements]

A 2x win may still be significant

I'll note that there was also no contention in @rsc's benchmark. Typically, CAS loops collapse far worse under contention than direct atomic operations.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[zephyrtronium]

This might be a stretch, but another option could be to rewrite the simplest form of the CAS loop to the corresponding atomic operation on platforms that support it, similar to how "intrinsics" for encoding/binary work. That way old code using the thing that works now benefits as well. New API would still be convenient, but I think atomic flag sets that want it are rare, and if it's intrinsified then it can live in any package. I am given to understand this would be harder than existing rewrite rules, though, because it spans more than a single basic block. It may also be hard to formulate proofs that the rewrite preserves semantics.

[aclements]

This might be a stretch, but another option could be to rewrite the simplest form of the CAS loop to the corresponding atomic operation on platforms that support it

@randall77 actually prototyped exactly this. I'm not necessarily opposed to having these rewrite rules, but I don't think they're a substitute for just giving people the API that they need. Often, when writing low-level code like this, the developer wants to say what they mean and have a good sense of what's actually going on, so having to express this operation through a rather opaque rewrite rule seems too subtle.