[LLVMdev] Operating on contents of virtual registers

[Ashay Rane]

Thanks Eli, that worked!

For the record, I had to make one change though. I had to bitcast to i32*
(instead of i8*) otherwise I was seeing errors like:
Instruction referencing instruction not embedded in a basic block!
  %retval = alloca i32
  <badref> = bitcast i32* %retval to i8*

and sometimes:
Instruction does not dominate all uses!

That makes me curious to ask, what does the choice of i32 or i8 depend on?
Is it the architecture?

Ashay


On Wed, Mar 16, 2011 at 11:40 PM, Eli Friedman <eli.friedman at gmail.com>wrote:

> On Wed, Mar 16, 2011 at 9:33 PM, Ashay Rane <ashay.rane at asu.edu> wrote:
> > 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
>
> I mean calling IRBuilder::CreateBitCast with the return value from
> getOperand().
>
> -Eli
>
> > 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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