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    SubjectRe: [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e
    On Wed, Sep 16, 2020 at 11:22:02AM -0700, Nick Desaulniers wrote:
    > I looked into this a bit, and IIRC, the issue was that compiler
    > generated functions aren't very good about keeping track of whether
    > they should or should not emit framepointer setup/teardown
    > prolog/epilogs. In LLVM's IR, -fno-omit-frame-pointer gets attached
    > to every function as a function level attribute.
    > https://godbolt.org/z/fcn9c6 ("frame-pointer"="all").
    >
    > There were some recent LLVM patches for BTI (arm64) that made some BTI
    > related command line flags module level attributes, which I thought
    > was interesting; I was wondering last night if -fno-omit-frame-pointer
    > and maybe even the level of stack protector should be? I guess LTO
    > would complicate things; not sure it would be good to merge modules
    > with different attributes; I'm not sure how that's handled today in
    > LLVM.
    >
    > Basically, when the compiler is synthesizing a new function
    > definition, it should check whether a frame pointer should be emitted
    > or not. We could do that today by maybe scanning all other function
    > definitions for the presence of "frame-pointer"="all" fn attr,
    > breaking early if we find one, and emitting the frame pointer setup in
    > that case. Though I guess it's "frame-pointer"="none" otherwise, so
    > maybe checking any other fn def would be fine; I don't see any C fn
    > attr's that allow you to keep frame pointers or not. What's tricky is
    > that the front end flag was resolved much earlier than where this code
    > gets generated, so it would need to look for traces that the flag ever
    > existed, which sounds brittle on paper to me.

    For code generated by the kernel at runtime, our current (x86) policy is
    "always use frame pointers for non-leaf functions".

    A lot of this compiler talk is over my head, but if *non-leaf* generated
    functions are rare enough then it might be worth considering to just
    always use frame pointers for them.

    --
    Josh

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