Clang insertion of fence instructions to mitigate Spectre variant 1?

As far as I’ve been able to learn, the only way to avoid security vulnerabilities due to Spectre variant 1 (CVE-2017-5753, “bounds check bypass”) is to insert fences to control the relevant speculative reads. I’m interested in doing this because I work on a numerical modelling library that is used in many applications, which are used to handle valuable information. There’s been at least one piece of malware that specifically targeted one of those applications, so I work at a moderate level of paranoia.

I’ve found information about __builtin_load_no_speculate, but inserting those by hand into ten million lines of branchy C code that’s under active development is not an attractive prospect.

MSVC has recently gained a /QSpectre option that tries to do this for you ( It only handles a very limited range of cases at present (, but Microsoft are working on improving that. Red Hat tell me that there is work underway to add something similar to GCC, although it’s probably a year away.

While such a capability can’t be completely fool-proof, I can well believe that it’s possible to do as good a job as bored humans, and it will be much cheaper.

Are there any plans to add something equivalent to Clang?


There’s a patch on Phabricator to implement a detector pass, in a more static-analysis mode; it seems to be doing better than the Microsoft option but still isn’t catching everything.


Thanks: glad progress is being made.

I know Chandler is working on a different, hopefully less expensive, mitigation based on load masking. He has a document that he is polishing that should be sent for RFC soon.

I’ve posted an RFC about this mitigation approach now:

My patch also happens to implement fence insertion, but the costs seem prohibitive.