<bit>: Use popcount for has_single_bit() if it's always available

[pps83]

provides 1.05 to 1.1 times speed up on x64 targets.

x64 results

Benchmark                               Time 
---------------------------------------------
bm_has_single_bit_if<uint8_t>        13.4 ns 
bm_has_single_bit_if_new<uint8_t>    12.7 ns 
bm_has_single_bit_if<uint16_t>       13.9 ns 
bm_has_single_bit_if_new<uint16_t>   10.8 ns 
bm_has_single_bit_if<uint32_t>       15.9 ns 
bm_has_single_bit_if_new<uint32_t>   12.6 ns 
bm_has_single_bit_if<uint64_t>       14.3 ns 
bm_has_single_bit_if_new<uint64_t>   11.0 ns 
bm_has_single_bit<uint8_t>           6.34 ns 
bm_has_single_bit_new<uint8_t>       5.88 ns 
bm_has_single_bit<uint16_t>          6.19 ns 
bm_has_single_bit_new<uint16_t>      4.52 ns 
bm_has_single_bit<uint32_t>          5.57 ns 
bm_has_single_bit_new<uint32_t>      5.99 ns 
bm_has_single_bit<uint64_t>          6.07 ns 
bm_has_single_bit_new<uint64_t>      5.41 ns 

fixes #5359

[pps83]

@AlexGuteniev, do I need to update benchmarks, or simply put benchmark numbers as part of the commit message?

[AlexGuteniev]

@AlexGuteniev, do I need to update benchmarks, or simply put benchmark numbers as part of the commit message?

I think the current benchmarks are fine, just post before/after number

[AlexGuteniev]

Ideally, we'd need ARM64 numbers as well. But I think we can do without them.

[AlexGuteniev]

I think the current benchmarks are fine, just post before/after number

You also need to pass somehow /arch:AVX or greater to make it different, perhaps by temporarily alterint CmakeFIle.txt locally, or by any other working method.

[pps83]

You also need to pass somehow /arch:AVX or greater to make it different, perhaps by temporarily alterint CmakeFIle.txt locally, or by any other working method.

shouldn't unit tests automatically run with/without these switches?

[AlexGuteniev]

shouldn't unit tests automatically run with/without these switches?

Unit tests should.
I mean benchmarks, which currently don't

[AlexGuteniev]

Also please edit the PR description to link it with the issue using one of Github keywords.

[pps83]

I'm getting conflicting results for the test: popcount on my pc is somehow slower by around 30%
it's a tiger-lake-u core i7-11390H, shouldn't have slow popcount :)

[pps83]

I'm getting conflicting results for the test: popcount on my pc is somehow slower by around 30% it's a tiger-lake-u core i7-11390H, shouldn't have slow popcount :)

no idea how is that it doesn't get inlined :)

[pps83]

wow, accidentially, i had /Only __inline (/Ob1) option. Apparently, this destroys most of std code that doesn't have inlines. Imo, all these functions should be marked inline.

[pps83]

popcount is on the right. So, it does improve, but will be 30% slower for projects using /Ob1

[AlexGuteniev]

Imo, all these functions should be marked inline.

No; I asked a while ago, see #619

[AlexGuteniev]

provides 5-10% speed up

✋ we don't do that here.

See https://github.com/microsoft/STL/wiki/Benchmarking-the-STL#calculating-speedups

So you probably mean 1.05 to 1.1 times speed up

[pps83]

So you probably mean 1.05 to 1.1 times speed up

updated. I based it on my previous commit message: #5367

(actual git history doesn't have that anyways)

[AlexGuteniev]

Mixed results for _if version, but consistent improvement of the branchless one

i5-1235U (Alder Lake)

P Cores

Benchmark	Before	After	Speedup
bm_has_single_bit_if<uint8_t>	9.78 ns	9.90 ns	0.99
bm_has_single_bit_if<uint16_t>	12.1 ns	10.1 ns	1.20
bm_has_single_bit_if<uint32_t>	11.4 ns	12.3 ns	0.93
bm_has_single_bit_if<uint64_t>	9.66 ns	12.7 ns	0.76
bm_has_single_bit<uint8_t>	4.44 ns	5.56 ns	0.80
bm_has_single_bit<uint16_t>	4.66 ns	3.59 ns	1.30
bm_has_single_bit<uint32_t>	4.66 ns	3.59 ns	1.30
bm_has_single_bit<uint64_t>	4.90 ns	3.47 ns	1.41

E Cores

Benchmark	Before	After	Speedup
bm_has_single_bit_if<uint8_t>	14.7 ns	14.0 ns	1.05
bm_has_single_bit_if<uint16_t>	16.6 ns	14.7 ns	1.13
bm_has_single_bit_if<uint32_t>	14.7 ns	14.3 ns	1.03
bm_has_single_bit_if<uint64_t>	14.7 ns	14.1 ns	1.04
bm_has_single_bit<uint8_t>	9.09 ns	8.84 ns	1.03
bm_has_single_bit<uint16_t>	9.09 ns	7.24 ns	1.26
bm_has_single_bit<uint32_t>	9.07 ns	7.25 ns	1.25
bm_has_single_bit<uint64_t>	9.08 ns	7.27 ns	1.25

[StephanTLavavej]

Thanks! 😻 Results on my 5950X, comparing main to this PR with set _CL_=/arch:AVX2 injected non-intrusively:

Benchmark	Before	After	Speedup
bm_has_single_bit_if<uint8_t>	12.9 ns	11.2 ns	1.15
bm_has_single_bit_if<uint16_t>	12.9 ns	11.6 ns	1.11
bm_has_single_bit_if<uint32_t>	13.1 ns	12.9 ns	1.02
bm_has_single_bit_if<uint64_t>	13.3 ns	13.1 ns	1.02
bm_has_single_bit<uint8_t>	5.10 ns	5.30 ns	0.96
bm_has_single_bit<uint16_t>	5.09 ns	3.62 ns	1.41
bm_has_single_bit<uint32_t>	5.10 ns	3.62 ns	1.41
bm_has_single_bit<uint64_t>	5.13 ns	3.61 ns	1.42

I'm not worried about the 0.96, that looks like noise (I reran and it was slightly better), and the 1.41 speedups are very healthy indeed.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[pps83]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

no changes planned.

[StephanTLavavej]

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