How to find "the" definition point of a Value in LLVM? - llvm

Question

LLVM uses static single assignment (SSA) form for its IR, meaning that every Value has a single definition point. So what is the easiest (and most generic) way to find "the" definition point of a Value without having to examine each use and determine how our Value is being used? In the code below, I am interested in definition points of Values used as function arguments.
protected: void getValueDefs( Function * F ) {
for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) {
ImmutableCallSite CS( cast<Value>(I) );
if ( !CS || isa<IntrinsicInst>(I) ) continue;
for ( User::const_op_iterator Ab = CS.arg_begin(), Ae = CS.arg_end(); Ab != Ae; ++Ab ) {
for ( User *U : Ab->get()->users() ) {
if (Instruction *Inst = dyn_cast<Instruction>(U)) {
/* How to do the check here?? */
}
}
}
}
}

Answer 1

That's a bit of an odd way to write it. I'd probably do something like this:
void scanFunc(Function &F) {
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
for (BasicBlock::iterator BBI = BB->begin(), BBE = BB->end(); BBI != BBE;
++BBI) {
if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
dbgs() << "Call: ";
CI->dump();
dbgs() << "\n";
ImmutableCallSite CS(CI);
for (ImmutableCallSite::arg_iterator I = CS.arg_begin(),
E = CS.arg_end();
I != E; ++I) {
if (Instruction *Inst = dyn_cast<Instruction>(*I)) {
// Do stuff
dbgs() << "\tInst: ";
Inst->dump();
dbgs() << "\n";
}
}
}
}
}
}
Basically though, if it is an instruction, it is the definition of the value. That's how the IR works. Otherwise, it might be a constant etc. If you take this code:
int a (int b) {
return b + 4;
}
int b (int c) {
return a(c) + a(c-1);
}
int d (int e, int f, int g) {
int h = a(4);
int i = b(5);
int j = b(6);
return h + i + j + e + f + g;
}
and compile it to IR and run this code on it you'll see:
Call: %call = call i32 #_Z1ai(i32 %0)
Inst: %0 = load i32* %c.addr, align 4
Call: %call1 = call i32 #_Z1ai(i32 %sub)
Inst: %sub = sub nsw i32 %1, 1
Call: %call = call i32 #_Z1ai(i32 4)
Call: %call1 = call i32 #_Z1bi(i32 5)
Call: %call2 = call i32 #_Z1bi(i32 6)