llvm dev - Jun 2013 - [LLVMdev] Getting the memory address of all operands on an expression

How to get memory address of all operands which constitutes an expression ?

eg. a=b+c; (want to know the memory address of b and c)...... Since I want
this at run time, So at assembly level this expression will become something
like as follows:-

Load r1, M[b]

Load r2, M[c]

r3=r1+r2

store M[a],r3

Now what i want to do is that, at every store instruction, I should get the
memory address of all the operands which have constituted that expression.
i.e. when am about to store at M[a], so with the help of r3, I should get
the memory address of b & c.

Have already gone through some basics of "use-def" thing but being
novice to
LLVM, am not been able to write the pass for it. Hence need help on writing
the pass for it.




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Hi Abhinash,

On 10/06/13 23:05, Abhinash Jain wrote:
> How to get memory address of all operands which constitutes an expression ?
in LLVM IR, the operands of most expression are registers, so don't have a
memory address.  In short, you can't always succeed.  However in cases where
the operands do come from memory in a fairly direct way, you can find out what
the memory is using the GetUnderlyingObject or GetUnderlyingObjects methods.

Ciao, Duncan.
>
> eg. a=b+c; (want to know the memory address of b and c)...... Since I want
> this at run time, So at assembly level this expression will become
something
> like as follows:-
>
> Load r1, M[b]
>
> Load r2, M[c]
>
> r3=r1+r2
>
> store M[a],r3
>
> Now what i want to do is that, at every store instruction, I should get the
> memory address of all the operands which have constituted that expression.
> i.e. when am about to store at M[a], so with the help of r3, I should get
> the memory address of b & c.
>
> Have already gone through some basics of "use-def" thing but
being novice to
> LLVM, am not been able to write the pass for it. Hence need help on writing
> the pass for it.
>
>
>
>
> --
> View this message in context:
http://llvm.1065342.n5.nabble.com/Getting-the-memory-address-of-all-operands-on-an-expression-tp58435.html
> Sent from the LLVM - Dev mailing list archive at Nabble.com.
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>

> in LLVM IR, the operands of most expression are registers, so don't
have a
memory address.

Yes I agree with your this statement,
But before becoming part of the expressions, the registers will actually
fetch some value from memory, through Load operations.
as shown in example "r3=r1+r2" will be the expression, where registers
such
as r1 and r2 contains (fetch) the values from the memory address b and c
resp. .
I want to know the way through which i can get this memory address of b and
c (in hexadecimal format).





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On 10 Jun 2013, at 22:05, Abhinash Jain <omnia at mailinator.com> wrote:
> So at assembly level this expression will become something
> like as follows:-
> 
> Load r1, M[b]
> 
> Load r2, M[c]
> 
> r3=r1+r2
> 
> store M[a],r3
Your question is predicated on the assumption that this is true, when in fact it
is not guaranteed.  Values in LLVM IR registers may be on the stack.  Or they
may be in registers.  Or they not exist at all at the end of the optimisation
pipeline, because even simple things like constant folding and common
subexpression elimination may end up making them redundant.  If a value only
ever exists in a register, then taking its address has no meaning.

If you want to guarantee that it is in memory, then you should manipulate it as
a pointer.  It must be created with a malloc() call, an alloca instruction, or a
global value. You can then do an inttoptr on the pointer.

If it is passed as a scalar function parameter, however, then even this
doesn't guarantee that you'll get a sensible value.  Arguments are often
passed in registers and so even if you spill it to the stack and then take the
address of the stack slot (alloca, store, ptrtoint, call printf, load), then you
will get a number, but it will not be meaningful.

It sounds like you are trying to do taint tracking, in which case you should
look at some of the related work in this area.  I know of at least two other
projects that have implemented taint tracking in LLVM.

David