core:: SIMD primitives not inlined with stable Rust (1.51.0) with target-cpu=native

[hkratz]

I don't know why, but for some reason the SIMD core primitives are not being inlined if I build simd-json with stable Rust. It works fine with nightly (2021-04-23) though.

Stable:

$ cargo clean; RUSTFLAGS="-C target-cpu=native" cargo asm --rust simd_json::Deserializer::from_slice_with_buffers|grep core::
     call    core::core_arch::x86::avx::_mm256_setzero_si256
     call    core::core_arch::x86::avx::_mm256_setzero_si256
     call    core::core_arch::x86::avx::_mm256_setzero_si256
     call    core::core_arch::x86::avx::_mm256_loadu_si256
     [... 126 more lines elides ...]

Nightly:

$ cargo +nightly clean; RUSTFLAGS="-C target-cpu=native" cargo +nightly asm --rust simd_json::Deserializer::from_slice_w
ith_buffers|grep core::|wc -l
0

This leads to much worse performance on stable.

Benchmarks

Stable:

apache_builds/simd_json::to_tape
                        time:   [372.11 us 373.84 us 376.01 us]
                        thrpt:  [334.02 MiB/s 335.95 MiB/s 337.52 MiB/s]

Nightly:

[...]
apache_builds/simd_json::to_tape
                        time:   [67.727 us 68.317 us 68.884 us]
                        thrpt:  [1.7805 GiB/s 1.7953 GiB/s 1.8109 GiB/s]
[...]

[hkratz]

Ahh, that is a known issue which was fixed during the beta but not backported to 1.51.0. Urgh.

Not sure if we want to do something in the docs about this given that this is what is recommended in the README. Leaving open for triage.

[Licenser]

This is a really great find! Perhaps we should change the REAME to suggest using target-feature=+avx,+avx2,+sse4.2 instead? I can make this change but since you found it I don't want to steal the contribution :)

[hkratz]

This is a really great find! Perhaps we should change the REAME to suggest using target-feature=+avx,+avx2,+sse4.2 instead? I can make this change but since you found it I don't want to steal the contribution :)

Rust 1.52 is scheduled to be released in two days and I have verified that it is fixed in that version. So I think we can get away with not changing the docs and just closing this after a while as this bug really only affects Rust 1.51.

[hkratz]

Confirmed fixed in 1.52 and not the fault of simd-json anyway.

[Licenser]

Thank you!