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    SubjectRe: [PATCH] arm: Add unwinding annotations for 64bit division functions
    On Tue, Sep 20, 2011 at 02:27:01PM +0100, Nicolas Pitre wrote:
    > On Mon, 19 Sep 2011, Laura Abbott wrote:
    > > On Mon, September 19, 2011 4:22 pm, Nicolas Pitre wrote:
    > > > On Mon, 19 Sep 2011, Laura Abbott wrote:
    > > >> @@ -189,7 +191,12 @@ ENTRY(__do_div64)
    > > >> moveq yh, xh
    > > >> moveq xh, #0
    > > >> moveq pc, lr
    > > >> +UNWIND(.fnend)
    > > >>
    > > >> +UNWIND(.fnstart)
    > > >> +UNWIND(.pad #4)
    > > >> +UNWIND(.save {lr})
    > > >> +Ldiv0_64:
    > > >
    > > > Why this phony fnend+fnstart here?
    > >
    > > If a division by 0 occurs, we need to be able to access the saved LR on
    > > the stack which is setup right before calling the __div0 function. This
    > > can't go at the top of __do_div64 because if we try to do a backtrace from
    > > within __do_div64 the annotation won't be correct as the LR was never
    > > saved on the stack.
    >
    > I suppose the debug infrastructure always assume the same stack frame
    > for the whole function, hence you can't put that .pad and .save right
    > before the call to __div0 without the .fnstart and have it effective
    > only there?

    An element in the unwinding table contains a pointer to the .fnstart
    location and a set of instruction bytecodes (like .save) to the next
    .fnend statement. If more 4 bytecodes (to fit in an int), it contains an
    index to another table with more bytecodes. The elements in the
    unwinding table are sorted by the .fnstart address.

    So I think in the above case if we don't have the .fnend/.fnstart for
    Ldiv0_64 we would get all the instructions in the __do_div64 unwinding
    information. I don't think the unwinding bytecode is that smart to allow
    other tricks like checking some conditions.

    Unless anyone has a better idea to specify this:

    Acked-by: Catalin Marinas <catalin.marinas@arm.com>



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