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SubjectRe: [PATCH] arm: Add unwinding support for memset and memzero.
On 3/14/2012 4:15 AM, Dave Martin wrote:
>
> For functions like this, the unwinding requirements are different
> depending on where we are in the function. But the unwinder annotations
> don't remember exact instruction locations; only the extent of the
> whole unwind block is recorded, along with the sequence (but not
> location) of unwinder directives.
>
> As is, the unwinding may be wrong depending on which part of the function
> is executing when the fault occurs.
>

Hmmmm, I thought I could get away with only one annotation based on the
structure of memset/memzero but looking again you are right, it really
requires multiple annotations to be correct.
> It may be possible to split the function into multiple unwind blocks,
> e.g.:
>
> ENTRY(somefunc)
> UNWIND(.fnstart)
>
> UNWIND(.save {r4,lr})
> stfmd sp!, {r4,lr}
>
> /* check something */
>
> blt _the_other_way
> /* maybe carry out our job this way */
>
> ldmfd sp!, {r4,lr}
> UNWIND(.fnend)
>
> _the_other_way:
> UNWIND(.fnstart)
> UNWIND(.save {r4,lr})
> UNWIND(.save {r5-r8})
> stmfd sp!, {r5-r8} /* !! */
>
> /* carry out our job the other way */
>
> ldmfd sp!, {r5-r8}
> ldmfd sp!, {r4,pc} /* !! */
> UNWIND(.fnend)
>
>
> This is still not exactly right (it's hard to be exactly right,
> since the unwind tables are not meant for handling asynchronous
> unwinding), but unwinding should be correct for the main bits of code
> where most time is spent and/or faults are most likely (the "carry out
> our job" comments).
>

Would a compiler be able to generate code such as this and still
generate correct completely unwinding annotations? Or if the compiler
knows unwinding is necessary, is the only option to generate code in
'unwindable blocks'? (alternatively, no compiler is smart/stupid enough
to generate this code?)

> You'd have to experiment to see whether the backtracer does something
> sensible with unwind tables like this. It might need tweaking to
> find the correct function symbol if a fault occurs in the second
> unwind block for example ... that perhaps it will already do the
> right thing.
>

Yes, I'll look into this. memcpy is missing annotations as well but that
code is significantly more convoluted.

> Cheers
> ---Dave
>

Thanks,
Laura

--
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.


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