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SubjectRe: [RFC PATCH] perf report: add sort by file lines
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On Thu, 2011-03-31 at 16:45 +0800, Lin Ming wrote:
> I am considering if it is possible to do "instruction unwind" to get a
> map from (temporarily used) register to a specific member of a data
> structure pointed by a pointer.
>
> 4004a0: movq -8(%rbp), %rax /* load foo arg from stack
> */
> 4004a4: movq 24(%rax), %rax /* load foo->bar */
> 4004a8: movq -16(%rbp), %rdx /* load tmp arg from stack
> */
> 4004ac: movl 32(%rdx), %edx /* load tmp->blah */
> 4004af: movl %edx, 20(%rax) /* store bar->fubar */
>
> foo: -8(%rbp)
> tmp: -16(%rbp)
>
> Assume we are now at ip 4004af, from the instruction decoder, we know
> it's a store operation, and we want to find out what %rax is.
>
> 1. unwind to 4004ac
> Ignore this, because it does not touch %rax
>
> 2. unwind to 4004a8
> Ignore this, because it does not touch %rax
>
> 3. unwind to 4004a4
> 20(%rax) => 20(24(%rax)), continue to unwind because we still
> have no idea what %rax is
>
> 4. unwind to 4004a0
> 20(24(%rax)) => 20(24(-8(%rbp))), stop unwind, because we now know
> -8(%rbp) is foo.
>
> So the original 20(%rax) is replace as 20(24(-8(%rbp))), and it means
> foo->bar->fubar
>
> Does this make sense?

Yes and no, the problem is that you cannot unwind an x86 instruction
stream. Therefore its easier to start at the beginning of a function
where DWARF should be able to tell you everything you need and then do a
single fwd scan to propagate the information until you reach the point
of interest.


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