Messages in this thread | | | Subject | Re: [RFC PATCH] perf report: add sort by file lines | From | Peter Zijlstra <> | Date | Thu, 31 Mar 2011 16:01:57 +0200 |
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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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