CPU-feature dependent code in builtins

[Florian Loitsch]

On RISC-V we need to store vector registers in the builtin Generate_WasmCompileLazy (and others). However, not all CPUs have vector support. It's dynamically detected, and then used accordingly.

Given that the builtins are precompiled, we can't use `CpuFeatures::IsSupported` as that wouldn't take the actual hardware into account.

We would either remove these functions from precompilation, or have a dynamic branch looking at the CPU feature in the generated assembly.

Did other architectures run into similar issues?

Any suggestions on how to handle this?

Thanks.

[dmerc...@google.com]

Hi,

> have a dynamic branch looking at the CPU feature in the generated assembly.

I would suggest to do this.

You can look for instance at `IncsspqIfSupported` in builtins-x64.cc to see how you can retrieve CpuFeatures at runtime from builtins. And another example is the `supports_wasm_simd_128_address` external reference, used on a few architectures.

You should be able to do a similar thing with the CodeStubAssembler if needed.

Cheers,

Darius

[Florian Loitsch]

I will try that.

Thanks for the fast response.

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