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

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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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

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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