[LLVMdev] How to properly use copyValue?

[Nick Lewycky]

Ryan Taylor wrote:
> Nick,
>   First, thanks for responding. Secondly, I just want to create an exact
> copy of an already exisitng Value, this should be pretty simple to do
> but I'm unclear on how to do this?
>   For example, PHINode::getIncomingValue (unsigned) returns a "Value"
> and I want to make a copy of that Value. Is there some other way to do
> this other than copyValue() that I'm missing?

There is no copy instruction in llvm, but you can fake one up with a 
bitcast.

Nick

>
> On Sat, Feb 4, 2012 at 3:23 PM, Nick Lewycky <nicholas at mxc.ca
> <mailto:nicholas at mxc.ca>> wrote:
>
>     Ryan Taylor wrote:
>
>         Since there are no constructors for Value, how do you properly
>         insert a
>         new Value?
>
>
>     Value is a pure base. If you want to construct a new Value, then you
>     want to construct a global variable or instruction or argument or
>     something in particular.
>
>
>         If I create a pointer Value *newValue and then call
>         AA.copyValue(oldValue, newValue), this does not work, since
>         newValue is
>         not allocated.
>
>
>     AA.copyValue is used to tell the AliasAnalysis that a transformation
>     is replacing the instruction in oldValue with newValue, so AA should
>     update its internal tables. (Note that BasicAA and TBAA don't have
>     internal tables, so the function is a no-op.)
>
>     As to your original question, if you need to copy a value, some
>     types have copy methods such as Instruction::clone(). For
>     BasicBlocks, Functions and Modules, there's the Cloning interface in
>     llvm/Transforms/Utils/Cloning. h. Constants not in the GlobalValue
>     hierarchy are impossible to make copies of as they are designed to
>     be pointer-comparable and permitting copies would break that.
>
>     Inserting is a separate issue. Supposing you create a new
>     instruction using "BO = BinaryOperator::createAdd(X, Y);" you then
>     need to insert it using BO->insertBefore or BO->insertAfter.
>
>     Nick
>
>