Optimize AtomicBool for target that don't support byte-sized atomics

[Amanieu]

AtomicBool is defined to have the same layout as bool, which means that we guarantee that it has a size of 1 byte. However on certain architectures such as RISC-V, LLVM will emulate byte atomics using a masked CAS loop on an aligned word.

We can take advantage of the fact that bool only ever has a value of 0 or 1 to replace swap operations with and/or operations that LLVM can lower to word-sized atomic and/or operations. This takes advantage of the fact that the incoming value to a swap or compare_exchange for AtomicBool is often a compile-time constant.

Example

pub fn swap_true(atomic: &AtomicBool) -> bool {
    atomic.swap(true, Ordering::Relaxed)
}

Old

	andi	a1, a0, -4
	slli	a0, a0, 3
	li	a2, 255
	sllw	a2, a2, a0
	li	a3, 1
	sllw	a3, a3, a0
	slli	a3, a3, 32
	srli	a3, a3, 32
.LBB1_1:
	lr.w	a4, (a1)
	mv	a5, a3
	xor	a5, a5, a4
	and	a5, a5, a2
	xor	a5, a5, a4
	sc.w	a5, a5, (a1)
	bnez	a5, .LBB1_1
	srlw	a0, a4, a0
	andi	a0, a0, 255
	snez	a0, a0
	ret

New

	andi	a1, a0, -4
	slli	a0, a0, 3
	li	a2, 1
	sllw	a2, a2, a0
	amoor.w	a1, a2, (a1)
	srlw	a0, a1, a0
	andi	a0, a0, 255
	snez	a0, a0
	ret

[rustbot]

r? @cuviper

(rustbot has picked a reviewer for you, use r? to override)

[thomcc]

This looks fine to me, you can r=me when you're happy with the set of platforms.

[Amanieu]

As far as I can tell LoongArch and RISC-V are the main ones that people care about. There might be other but I will leave those to follow-up PRs.

@bors r=thomcc

[bors]

📌 Commit 9e5077dce91db0f4761aba04567b42fa6884f789 has been approved by thomcc

It is now in the queue for this repository.

[bors]

⌛ Testing commit 9e5077dce91db0f4761aba04567b42fa6884f789 with merge 8bd1c3eef862bd2ecdc2774529142004de3febf5...

[bors]

💔 Test failed - checks-actions

[Amanieu]

@bors r=thomcc

[bors]

📌 Commit 3282132c7c428b01821d3d7454c5e015deb7a860 has been approved by thomcc

It is now in the queue for this repository.

[Amanieu]

@bors r=thomcc

[bors]

📌 Commit 3dbee5b has been approved by thomcc

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3dbee5b with merge e7d6ce3...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing e7d6ce3 to master...

[rust-timer]

Finished benchmarking commit (e7d6ce3): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-0.8%, -0.3%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.5%	[-0.8%, -0.3%]	2

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	4

Bootstrap: 652.686s -> 656.285s (0.55%)