Abhinash Jain
2013-Jun-10 21:05 UTC
[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. -- 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.
Duncan Sands
2013-Jun-11 14:10 UTC
[LLVMdev] Getting the memory address of all operands on an expression
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 >
Abhinash Jain
2013-Jun-18 06:49 UTC
[LLVMdev] Getting the memory address of all operands on an expression
> in LLVM IR, the operands of most expression are registers, so don't have amemory 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). -- View this message in context: http://llvm.1065342.n5.nabble.com/Getting-the-memory-address-of-all-operands-on-an-expression-tp58435p58583.html Sent from the LLVM - Dev mailing list archive at Nabble.com.
David Chisnall
2013-Jun-19 10:13 UTC
[LLVMdev] Getting the memory address of all operands on an expression
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],r3Your 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
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