Hi Bill and Mike, thanks a lot for these pointers! This is great example code.
I'll give it a try today. I'm not sure if I can use the method of splitting blocks. I presume this introduces additional branches. Their overhead would probably be as large as the overhead from the instructions that I try to eliminate. I will look at your code, experiment a bit, and see what happens. Best, Jonas On Wed, Jan 6, 2016 at 9:39 PM Mike Lam <lam...@jmu.edu> wrote: > Jonas, > > I'm Mike Lam, the original author of CRAFT. If you're interested in > using Dyninst to replace certain instructions, I'd encourage you to > look over the buildReplacement function in fpinst.cpp: > > http://sourceforge.net/p/crafthpc/code/ci/master/tree/src/fpinst.cpp > > That function calls buildDefaultReplacementCode (which is different > depending on what kind of FPAnalysis gets passed in), to get a Dyninst > snippet object. However, you shouldn't need to worry about that if > you're replacing with a nop because I think you can just use a > BPatch_nullExpr. > > The rest of the code splits the original basic block twice to remove > the given instruction, creates a new block for the snippet, and then > rearranges all necessary edges to insert the new block in the place of > the old instruction. > > If you have questions I can attempt to answer them although I haven't > touched this code in a while. Also, the code hasn't been updated from > 8.1.2 yet so there may be a couple of minor API changes that you'll > need to work around. > > -- mike > > > On Wed, Jan 6, 2016 at 11:57 AM, Bill Williams <b...@cs.wisc.edu> wrote: > > On 01/06/2016 07:38 AM, Jonas Wagner wrote: > > > > Hello, > > > > I’d like to dynamically modify a binary and change branches to NOPs and > > back. I wondered whether DynInst (in particular PatchAPI) would be a > > suitable tool for this. What do you think? > > > > Here’s what I’d like to do in more detail: I have a program that contains > > branches like the one below. > > > > ... some code ... > > 21d0: cmpb $0x0, branch_flag > > ↓ jne 2221 > > ... some more code ... > > > > These branches are usually not taken because the branch_flag is set to > zero. > > Nevertheless, there is some cost for loading the flag and the non-taken > > branch. I would like to get rid of this cost as much as possible, by > > replacing that sequence of instructions with NOPs. > > > > In a later stage, I’d also like to undo some of these replacements, or > even > > change the conditional branch into an unconditional branch. > > > > PatchAPI is indeed a right tool for the job here out of the Dyninst > toolkit, > > and the CRAFT tool is our best example of how to perform this type of > > instruction replacement (http://sourceforge.net/projects/crafthpc/) > > > > The general idea is that you split the block containing the instructions > you > > want to replace into those instructions and the remainder, replace the > block > > with just the undesirable instructions with your desired code, and stitch > > things back together. > > > > There are probably better ways to do this specific sort of manipulation, > > given that you can guarantee that you're either modifying whole > instructions > > or opcode bytes one-for-one. It might be more efficient and natural to > look > > at the ProcControl breakpoint mechanism and look at building something > based > > on that infrastructure combined with ParseAPI's knowledge of instruction > > locations. > > > > Hope this is helpful, and let us know how it works out. > > > > --bw > > > > Would PatchAPI be suitable for these kind of tasks? Do you have any hints > > for how I could use it, or what would be a better alternative? > > > > Thanks a lot! > > Jonas > > > > > > > > _______________________________________________ > > Dyninst-api mailing list > > Dyninst-api@cs.wisc.edu > > https://lists.cs.wisc.edu/mailman/listinfo/dyninst-api > > > > >
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