[cfe-dev] What else is Clang hiding from me? (Aka how to find ALL supported flags)

[Gaier, Bjoern via cfe-dev]

Hello everyone,

(There is a TL;DR down there)

I started to look into the Spectre Mitigation features of the MSVC compiler - we are planning to apply those to our code base finally. I was able to find out, that the MSVC compiler supports 3 flags for this:
/Qspectre -> "Specifies compiler generation of instructions to mitigate certain Spectre variant 1 security vulnerabilities."
/Qspectre-load -> "Specifies compiler generation of serializing instructions for every load instruction."
/Qspectre-load-cf -> "Specifies compiler generation of serializing instructions for every control-flow instruction that contains a load."

That was surprisingly helpful! However, we are also using the Clang-Cl compiler, so I wanted to find out which features are supported in terms of the Spectre Mitigation. This was difficult! I found at least this website:
https://llvm.org/docs/SpeculativeLoadHardening.html - Which is really interesting but... too much for me >o<
Next hint I got was the attribute "speculative_load_hardening" but there the 'frustration' started. On the site, the compiler flag "-mno-speculative-load-hardening" was mentioned - I figured that the flag "-mspeculative-load-hardening" would be a thing. The link https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mspeculative-load-hardening was of no help however, because the flag isn't mentioned there as it seems. Sooo... Next thing I did was executing "clang -help" and "clang --help-hidden" to my surprise the flag wasn't mentioned there as well...

So I went to one of our projects and added it by hand, checking if it will throw an error or not - it did not.

TL;DR:
My problem as a user is... I'm really unsure how many undocumented flags are supported by Clang and which one might be of interested for me. For example, do I know all the flags now that are related to the spectre mitigation? I know that there is also -mreptoline (or something), but I can't know about the flags I don't know about - and when not even Clang itself dumps them, then I'm really screwed.

Also, how can I be sure, that the flag is also supported by Clang-Cl? Or by Clang-Cl -Xclang?

Also a kinda confusing thing... When I go to the LLVM/Clang online documentation, then it says Clang 11, when I ask my Clang about the version it replies with Clang 9 - also on the website I can only download LLVM 9 - That makes me feel like I missed 2 Versions already - even though I think to recall that LLVM 10 is next.

Yet it seems like I'm the only one with that issue .w.

Kind greetings
Björn
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