Phi translation

I just accidently sent a partially complete email, so this contains the rest (sorry!)
I’m working on translating llvm’s optimized intermediate code to another compiler’s intermediate code, and I’m working on the PHI instruction.

Here’s an example phi instruction to help explain what I’m trying to do: %inc25 = phi i32 [ 1, %entry ], [ %inc, %for.body ]

What I would want to do here is allocate some memory memory (i’m trying to use %phi1 = alloca i32, allign 4 ). Then, I want to go to the %entry block and say “store i32 1, i32* %phi1”, and in the %for.body I want to add “store i32 1, i32* %phi1”.

So first, I’m iterating through my function and basicblock:
int phi = 0;
Function::iterator func_bblock_itr = func->begin();
for(; func_bblock_itr != func->end(); ++func_bblock_itr)
BasicBlock::iterator bblock_inst_itr = func_bblock_itr->begin();
for(; bblock_inst_itr != func_bblock_itr->end(); ++bblock_inst_itr)
Then, I look for PHI instructions:
if(bblock_inst_itr->getOpcode() == Instruction::PHI) {
If I find one, I allocate space:
string name = “phi”;
Twine tname= Twine(name);
AllocaInst alloca = AllocaInst(bblock_inst_itr->getOperand(0)->getType(),tname,func_bblock_itr->begin());
and then i go through the operands one by one to find where to put store instructions:
for(unsigned int i = 0; i < bblock_inst_itr->getNumOperands();i++) {
Value *v = bblock_inst_itr->getOperand(i);
Here’s where I don’t know what to do: I want to create a store instruction in that place:
StoreInst(v, alloca?, func_bblock_itr?);

Is this a decent approach to getting the instructions inserted? and what do I put to get the pointer in the store instruction and the basicblock, the current code gives me a bunch of errors. Any help would be appreciated, thanks!

Hi Ryan,
I may be thinking loud here ,You run the reg2mem pass on the optimized IR using opt tool,that will remove the phi dependency in the CFG,.


So, this will generally work, but you actually don't need to write
this code at all.
If you look in lib/Transforms/Utils/DemoteRegToStack.cpp, there is a
function called "DemotePHIToStack" that does exactly what you want to
do below.