[LLVMdev] Identifying the instructions that uses a pointer used as a function argument

[Henrique Santos]

Actually, it's quite the opposite. This is happening because the allocas
weren't promoted to registers references. To do so, you should run the SSA
construction pass. This is done by running "opt -mem2reg" on your
bytecode file.

Good luck.

H.


On Tue, Nov 5, 2013 at 11:26 AM, Arnamoy Bhattacharyya
<arnamoy at ualberta.ca>wrote:

> Thanks for the reply.
>
> For the source code:
>
> int main()
> {
> int a = 0;
> int *p;
> p = &a;
> call_arnamoy(p);
> int *p1;
> p1 = p;
> return 1;
> }
>
> The bit code:
>
>   %retval = alloca i32, align 4
>   %a = alloca i32, align 4
> %p = alloca i32*, align 8
>  %p1 = alloca i32*, align 8
>  store i32 0, i32* %retval
> store i32 0, i32* %a, align 4
> store i32* %a, i32** %p, align 8
> %0 = load i32** %p, align 8
> %call = call i32 @arnamoy(i32* %0)
> %1 = load i32** %p, align 8
> store i32* %1, i32** %p1, align 8
> ret i32 1
>
> I need to know where the pointer "p" is used in the IR.  But when I
> try to run the use_iterator on callInst->getArgOperand(0), it return
> only one instruct that is the Call instruction itself.  But not the
> store i32* %a, i32** %p, align 8, %1 = load i32** %p, align 8 and %0 =
> load i32** %p, align 8  I need all of those.  I am guessing this is
> happening because the argument is promoted to register.  Is there a
> way of knowing the name of the variable the value of which is stored
> in the register?
>
> If it's not possible, any work-around suggestions?
>
> Thanks a lot again;
>
> On Tue, Nov 5, 2013 at 12:41 PM, Henrique Santos
> <henrique.nazare.santos at gmail.com> wrote:
> > I think I understood the problem you're trying to solve. I didn't,
> however,
> > understand the problems you are having (maybe you're not running
> mem2reg?).
> > I do have a little piece of code that I think does (more or less) what
> you
> > want.
> > Take a look at it, try understanding it, and see if it helps you out.
> > You might have to change it a bit since you're replacing uses after an
> > instruction and not a basic block.
> > Replace the "auto"s with the iterator types if you're not using C++0x.
> > Also, sorry for the lack of comments. : )
> >
> > void replaceUsesOfWithAfter(Value *V, Value *R, BasicBlock *BB) {
> >   set<Instruction*> Replace;
> >   for (auto UI = V->use_begin(), UE = V->use_end(); UI != UE; ++UI)
> >     if (Instruction *I = dyn_cast<Instruction>(*UI)) {
> >       if (I != R && DT_->dominates(BB, I->getParent()))
> >         Replace.insert(I);
> >       else if (PHINode *Phi = dyn_cast<PHINode>(*UI))
> >         for (unsigned Idx = 0; Idx < Phi->getNumIncomingValues(); ++Idx)
> >           if (Phi->getIncomingValue(Idx) == V &&
> >               DT_->dominates(BB, Phi->getIncomingBlock(Idx)))
> >             Phi->setIncomingValue(Idx, R);
> >     }
> >
> >   for (auto& I : Replace)
> >     I->replaceUsesOfWith(V, R);
> > }
> >
> > If this doesn't help, you could try showing us the actual IR you're
> running
> > the transformation on.
> >
> > Just FMI, is it okay to post code on this mailing list?
> >
> > H.
> >
> >
> >
> > On Tue, Nov 5, 2013 at 8:59 AM, Arnamoy Bhattacharyya <
> arnamoy at ualberta.ca>
> > wrote:
> >>
> >> Hello all;
> >>
> >> So here is my goal:
> >>
> >> *** If there is a Call instruction to myFunction(int * val), I need to
> >> identify all the instructions that uses val in the IR and replace the
> >> uses with a newly created pointer.  I will also be changing the
> >> argument of myFunction to the new pointer.
> >>
> >> int * val = malloc/calloc;
> >> ...
> >> myFunction(val);
> >> ....
> >> *val = 45;
> >>
> >> becomes==
> >>
> >> int * val = malloc/calloc;
> >> int * val1 = malloc/calloc;
> >> ...
> >> myFunction(val1);
> >> ....
> >> *val1 = 45;
> >>
> >> I thought traversing the use-def chain of the function argument will
> >> give me the uses of the pointer but it is giving me only one use.
> >> (the call instruction to myFunc())
> >>
> >> Is it happening because val is a pointer and the value is stored into
> >> register first?  How can I identify the uses of val in the IR?  (And
> >> if again val is assigned to a new pointer, the uses of that new
> >> pointer?  e.g int * new_val = val; all the uses of new_val)
> >>
> >> **I don't know if this is the right path to go, but if I could get the
> >> name of the pointer used as an function argument, I could match the
> >> name against other instructions that use the same named pointer.  But
> >> this is also not working because first the pointer is loaded into
> >> register and then the register is passed in the argument.
> >>
> >> Sorry if the question does not make sense.  Any pointer or explanation
> >> will be appreciated.
> >>
> >> Thanks a lot!!
> >> --
> >> Arnamoy Bhattacharyya
> >> Athabasca Hall 143
> >> Department of Computing Science - University of Alberta
> >> Edmonton, Alberta, Canada, T6G 2E8
> >> 587-710-7073
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
> >
>
>
>
> --
> Arnamoy Bhattacharyya
> Athabasca Hall 143
> Department of Computing Science - University of Alberta
> Edmonton, Alberta, Canada, T6G 2E8
> 587-710-7073
>
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