llvm dev - Jan 2011 - [LLVMdev] How to change the type of an Instruction?

Hello guys,

I wonder how I can change the type of an integer variable. For instance,
given the instruction "%3 = add i32 %1, %2" I would like to alter the
instruction to "%3 = add i16 %1, %2". Is there any way to do this?

Best wishes,

Douglas
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On 21 January 2011 12:56, Douglas do Couto Teixeira <
douglasdocouto at gmail.com> wrote:
> Hello guys,
>
> I wonder how I can change the type of an integer variable. For instance,
> given the instruction "%3 = add i32 %1, %2" I would like to alter
the
> instruction to "%3 = add i16 %1, %2". Is there any way to do
this?
>
No. Instead you create a new Instruction, in this case with
BinaryOperator::CreateAdd, then OldInst->replaceAllUsesWith(NewInst) to
update all the users, then OldInst->eraseFromParent() since it's now dead
code.

Also, all values have types immutably assigned at creation, so you'll need
to insert casts (trunc instructions in your case) to cast %1 and %2 from i32
to i16 for the smaller add.

Nick
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Hi,

Nick, thanks for the reply.
I still have a problem: I only need to "clone" an Instruction,
changing its
type. That is, I would like to keep all characteristics of the old
Instruction and create a new one only with a different type. I am trying
create a new Instruction thus:

%3 = add nsw i32 %1, %2 ; <i16> [#uses=2]  //Old Instruction

Value* Op0 = I->getOperand(0);
Value* Op1 = I->getOperand(1);
Value* V0 = new Value(Type::getInt16Ty(Op0->getContext()),
Op0->getValueID());
Value* V1 = new Value(Type::getInt16Ty(Op1->getContext()),
Op1->getValueID());
Instruction* newInst = BinaryOperator::CreateNSWAdd(V0, V1, "test");
errs() << "NewInst:\n" << *newInst <<
"\n";


But I get something like this:

%test = add nsw i16 <badref>, <badref> ; <i16> [#uses=0]


What I am doing wrong?


Best,

Douglas

On Fri, Jan 21, 2011 at 8:25 PM, Nick Lewycky <nlewycky at google.com>
wrote:
> On 21 January 2011 12:56, Douglas do Couto Teixeira <
> douglasdocouto at gmail.com> wrote:
>
>> Hello guys,
>>
>> I wonder how I can change the type of an integer variable. For
instance,
>> given the instruction "%3 = add i32 %1, %2" I would like to
alter the
>> instruction to "%3 = add i16 %1, %2". Is there any way to do
this?
>>
>
> No. Instead you create a new Instruction, in this case with
> BinaryOperator::CreateAdd, then OldInst->replaceAllUsesWith(NewInst) to
> update all the users, then OldInst->eraseFromParent() since it's now
dead
> code.
>
> Also, all values have types immutably assigned at creation, so you'll
need
> to insert casts (trunc instructions in your case) to cast %1 and %2 from
i32
> to i16 for the smaller add.
>
> Nick
>
>
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