[llvm] [llvm][aarch64][x86] Implement a lightweight spectre v1 mitigation, like MSVC /Qspectre (PR #116450)

[Chandler Carruth via llvm-commits]

chandlerc wrote:

FWIW, I do not recommend the mitigation approach you cited from MSVC given the concerns raised in the security community about its efficacy: https://www.paulkocher.com/doc/MicrosoftCompilerSpectreMitigation.html

It's worth noting that this technique and the patterns used were developed when Spectre was very new, and there is a large body of research since that I think has expanded the security community's understanding of the full scope of these issues.

For example, there is also a systematic review of the different categories and structures of Spectre-style attacks and the defenses for them here: https://www.usenix.org/system/files/sec19-canella.pdf

If LLVM is going to take on complexity to support compiler-based Spectre mitigations, I would encourage it to be documented based on the taxonomy in that systematic review. It would also be good to know if any of the researchers from this space have evaluated this new mixture of technique? I think it would be important to have some research or other supporting evidence for the effectiveness of mitigations that we continue to carry in-tree.

https://github.com/llvm/llvm-project/pull/116450