[LLVMdev] Operating on contents of virtual registers

[Ashay Rane]

Hello,

I was facing some difficulty in implementing a transform and I was wondering
if I could get some help please.

The transform needs to operate on the operands of certain instructions. For
example, given an instruction, say "%10 = load i32* %9, align 4", I have to
record the value of %9 and process it. Of course, this is only possible at
runtime and so I am instrumenting the code such that a particular function
is invoked just before the instruction of interest is executed.

The problem that I am facing is in getting the value of the operands. As I
understand, the operands could either be program variables (e.g. in "%6 =
load i32* %old, align 4") or one of the virtual registers (as in the first
load instruction). For both cases, is it possible to extract the
value/address of the operand (%9 or %old)? If yes, what should be the best
function type to use so that I can pass this value as an argument to my
function? For now, I am only concerned with load and store instructions and
so (I suppose) I have to deal with pointers only.

An easy way that I can think of is to directly insert the LLVM IR (e.g. call
void @my_function(%old)) but because I am using
Module::getOrInsertFunction(), I have to have a function type. So
alternatively, is there a way to insert direct LLVM instructions (without
going through the type hierarchy)?

Thanks,
Ashay


---

Ashay Rane
Research Associate
The University of Texas at Austin
http://www.public.asu.edu/~asrane/
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