llvm dev - Mar 2011 - [LLVMdev] Operating on contents of virtual registers

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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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

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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