[llvm-bugs] [Bug 40964] New: clang-cl doesn't support /Qspectre

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40964

            Bug ID: 40964
           Summary: clang-cl doesn't support /Qspectre
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: nicolasweber at gmx.de
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

cl.exe has spectre type 1 mitigation under /Qspectre, and apparently cmake now
(sometimes?) sets this, leading to stuff like
https://twitter.com/never_released/status/1101439626357686272

https://devblogs.microsoft.com/cppblog/spectre-mitigations-in-msvc/ describes
what the flag does.

We could maybe map it to -mspeculative-load-hardening

There's also a declspec for disabling spectre mitigations on a per-function
level: https://docs.microsoft.com/en-us/cpp/cpp/spectre?view=vs-2017
https://reviews.llvm.org/D51157 suggests that we already have something like
this on the IR level, so adding a declspec might not be too difficult.

Finally, there's a warning (C5045) which flags where the mitigation would be
inserted:
https://devblogs.microsoft.com/cppblog/spectre-diagnostic-in-visual-studio-2017-version-15-7-preview-4/
This part might be difficult.

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