How to handle missing SIMD support with a future rustup component

[bjorn3]

Cg_clif is missing support for a lot of SIMD intrinsics. To reduce the impact of this on user code cg_clif currently replaces the cpuid call used by is_x86_feature_detected! to return only the bare minimum supported features. This works just fine when the standard library is compiled by cg_clif, but once cg_clif gets shipped as rustup component it will have to work with a cg_llvm compiled standard library which doesn't have this workaround. As it is unrealistic to implement all those SIMD intrinsics in the immediate future, I will need to find a way to make is_x86_feature_detected! return false where necessary when using cg_clif even if the standard library is compiled with cg_llvm.

[bjorn3]

https://rust-lang.zulipchat.com/#narrow/stream/208962-t-libs.2Fstdarch/topic/How.20to.20handle.20missing.20SIMD.20support.20with.20cg_clif.3F

[bjorn3]

https://github.com/bjorn3/rustc_codegen_cranelift/pull/1378 implemented enough vendor intrinsics to compile cg_clif's test suite with a standard library compiled using LLVM.

[bjorn3]

As of https://github.com/bjorn3/rustc_codegen_cranelift/pull/1397 is_x86_feature_detected!() now returns all actual cpu features when inline asm support is enabled. I haven't got any reports of programs crashing with this, so I assume all actually important vendor intrinsics are now supported.