[LLVMdev] Operating on contents of virtual registers

[Ashay Rane]

Hi Eli,

Thanks for the reply. The problem is that getOperand() returns an
llvm::Instruction (that refers to the definition of the operand). What I am
trying to find out is how to get the value of the operand. When you refer to
bitcasting to i8*, do you mean casting the return value from getOperand()
itself?

Ashay


On Wed, Mar 16, 2011 at 11:03 PM, Eli Friedman <eli.friedman at gmail.com>wrote:

> 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
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