[LLVMdev] Operating on contents of virtual registers

[Eli Friedman]

On Wed, Mar 16, 2011 at 6:00 PM, Ashay Rane <ashay.rane at asu.edu> wrote:
> 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)?

Those are both the same case; names for instructions only exist for
the sake of readability.

> 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.

If you only need pointers, just use i8* as the type of the argument,
and bitcast the value to i8*.

> 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)?

Your approach of inserting a call is fine; making your own instruction
or intrinsic wouldn't make things any simpler.  There aren't any
shortcuts here.

-Eli