Add option to enable spectre mitigation in MSVC

[donny-dont]

Adds an option ENABLE_MSVC_SPECTRE_MITIGATION which will add the /Qspectre flag on MSVC.

[donny-dont]

This is to align the build with the one in vcpkg https://github.com/microsoft/vcpkg/blob/e109e2de620e50fa521f52748c47857ad2fbe350/ports/libressl/0002-suppress-msvc-warnings.patch#L9-L12

If its a no that's fine will keep the patch locally there and close this. Just throwing it out there since vcpkg is building it in this manner.

[botovq]

This makes sense to me. I'll leave this open for a bit to see if anyone using windows builds objects. @vszakats, any opinion on this?

[vszakats]

This can be done by passing -DCMAKE_C_FLAGS=/Qspectre to CMake.

A dedicated, nearly 1 to 1 MSVC-specific CMake option doesn't simplify this IMO. Perhaps it would, if it enabled it for other compilers. But, there are several spectre mitigations (e.g. in gcc), making it somewhat difficult to figure out a single combination that makes everyone happy. Dropping "MSVC" from the option name/desc would be a nice touch either way.

[donny-dont]

This can be done by passing -DCMAKE_C_FLAGS=/Qspectre to CMake.

A dedicated, nearly 1 to 1 MSVC-specific CMake option doesn't simplify this IMO. Perhaps it would, if it enabled it for other compilers. But, there are several spectre mitigations (e.g. in gcc), making it somewhat difficult to figure out a single combination that makes everyone happy. Dropping "MSVC" from the option name/desc would be a nice touch either way.

Sounds good. Any particular options you want for this on clang/gcc? Researching it some on my end as well.

[vszakats]

These are the ones I'm aware of, it's a 2021 list, there may be more options since:

• llvm/clang: -mretpoline (since 6.0.0, perhaps also in 5.0.2)
• gcc: -mindirect-branch -mfunction-return -mindirect-branch-register

IIRC they caused a significant performance drop, so ended up not using them.

Also for MSVC there is: /Qspectre-load, /Qspectre-load-cf, /Qspectre-jmp.

[donny-dont]

This can be done by passing -DCMAKE_C_FLAGS=/Qspectre to CMake.

After talking to the vcpkg folks they would also prefer this way of doing things. Since there are so many options and trade offs I think thats the path so I will close this.

Thanks again for your time on this!