[LLVMdev] Regarding ARM CodeGen

[Gordon Henriksen]

On 2008-07-15, at 11:35, kapil anand wrote:

> I have one more query regarding the LLVM representations. In LLVM  
> Infrastructure, "label" is defined as Primitive type. So, is there  
> any way of using a variable of Type Label in some arithmetic  
> operation or in "bitcast" instruction.
>
> Is there any other way to use label in arithmetic operations. I am  
> trying this to represent Program counter in LLVM, since in ARM, we  
> can have various instructions which operate on PC , eg.

Hi Kapil,

If this isn't a FAQ, it should be. In LLVM, 'label' values (in C++,  
BasicBlock*'s) can only be used as arguments to flow control  
instructions (br, switch, invoke, and phi, to be specific). The reason  
is that the SSA representation cannot be formed in the presence of  
dynamic flow control—consider how the code generator might implement  
PHI if sources and destinations are not decidable.

What are you attempting to accomplish by reading the PC? The usual  
goal in using an 'address of label' is to implement a jump table by  
hand. In this case, your compiler should assign integers to labels and  
then, when it wants to emit a dynamic 'goto', it should branch into a  
switch statement, which the LLVM optimizer will transform into a jump  
table. If you use mem2reg, LLVM will take care of the phi nodes for you.

-- Gordon