1 //===- PiNodeInsertion.cpp - Insert Pi nodes into a program ---------------===//
3 // PiNodeInsertion - This pass inserts single entry Phi nodes into basic blocks
4 // that are preceeded by a conditional branch, where the branch gives
5 // information about the operands of the condition. For example, this C code:
6 // if (x == 0) { ... = x + 4;
9 // x2 = phi(x); // Node that can hold data flow information about X
12 // Since the direction of the condition branch gives information about X itself
13 // (whether or not it is zero), some passes (like value numbering or ABCD) can
14 // use the inserted Phi/Pi nodes as a place to attach information, in this case
15 // saying that X has a value of 0 in this scope. The power of this analysis
16 // information is that "in the scope" translates to "for all uses of x2".
18 // This special form of Phi node is refered to as a Pi node, following the
19 // terminology defined in the "Array Bounds Checks on Demand" paper.
21 // As a really trivial example of what the Pi nodes are good for, this pass
22 // replaces values compared for equality with direct constants with the constant
23 // itself in the branch it's equal to the constant. In the case above, it would
24 // change the body to be "... = 0 + 4;" Real value numbering can do much more.
26 //===----------------------------------------------------------------------===//
28 #include "llvm/Transforms/Scalar.h"
29 #include "llvm/Analysis/Dominators.h"
30 #include "llvm/Pass.h"
31 #include "llvm/Function.h"
32 #include "llvm/BasicBlock.h"
33 #include "llvm/iTerminators.h"
34 #include "llvm/iOperators.h"
35 #include "llvm/iPHINode.h"
36 #include "llvm/Support/CFG.h"
37 #include "Support/StatisticReporter.h"
39 static Statistic<> NumInserted("pinodes\t\t- Number of Pi nodes inserted");
42 struct PiNodeInserter : public FunctionPass {
43 virtual bool runOnFunction(Function &F);
45 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
47 AU.addRequired<DominatorSet>();
50 // insertPiNodeFor - Insert a Pi node for V in the successors of BB if our
51 // conditions hold. If Rep is not null, fill in a value of 'Rep' instead of
52 // creating a new Pi node itself because we know that the value is a simple
55 bool insertPiNodeFor(Value *V, BasicBlock *BB, Value *Rep = 0);
58 RegisterOpt<PiNodeInserter> X("pinodes", "Pi Node Insertion");
61 Pass *createPiNodeInsertionPass() { return new PiNodeInserter(); }
64 bool PiNodeInserter::runOnFunction(Function &F) {
66 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
67 TerminatorInst *TI = I->getTerminator();
69 // FIXME: Insert PI nodes for switch statements too
71 // Look for conditional branch instructions... that branch on a setcc test
72 if (BranchInst *BI = dyn_cast<BranchInst>(TI))
73 if (BI->isConditional())
74 // TODO: we could in theory support logical operations here too...
75 if (SetCondInst *SCI = dyn_cast<SetCondInst>(BI->getCondition())) {
76 // Calculate replacement values if this is an obvious constant == or
78 Value *TrueRep = 0, *FalseRep = 0;
80 // Make sure the the constant is the second operand if there is one...
81 // This fits with our cannonicalization patterns used elsewhere in the
82 // compiler, without depending on instcombine running before us.
84 if (isa<Constant>(SCI->getOperand(0)) &&
85 !isa<Constant>(SCI->getOperand(1))) {
90 if (isa<Constant>(SCI->getOperand(1))) {
91 if (SCI->getOpcode() == Instruction::SetEQ)
92 TrueRep = SCI->getOperand(1);
93 else if (SCI->getOpcode() == Instruction::SetNE)
94 FalseRep = SCI->getOperand(1);
97 BasicBlock *TB = BI->getSuccessor(0); // True block
98 BasicBlock *FB = BI->getSuccessor(1); // False block
100 // Insert the Pi nodes for the first operand to the comparison...
101 Changed |= insertPiNodeFor(SCI->getOperand(0), TB, TrueRep);
102 Changed |= insertPiNodeFor(SCI->getOperand(0), FB, FalseRep);
104 // Insert the Pi nodes for the second operand to the comparison...
105 Changed |= insertPiNodeFor(SCI->getOperand(1), TB);
106 Changed |= insertPiNodeFor(SCI->getOperand(1), FB);
114 // alreadyHasPiNodeFor - Return true if there is already a Pi node in BB for V.
115 static bool alreadyHasPiNodeFor(Value *V, BasicBlock *BB) {
116 for (Value::use_iterator I = V->use_begin(), E = V->use_end(); I != E; ++I)
117 if (PHINode *PN = dyn_cast<PHINode>(*I))
118 if (PN->getParent() == BB)
124 // insertPiNodeFor - Insert a Pi node for V in the successors of BB if our
125 // conditions hold. If Rep is not null, fill in a value of 'Rep' instead of
126 // creating a new Pi node itself because we know that the value is a simple
129 bool PiNodeInserter::insertPiNodeFor(Value *V, BasicBlock *Succ, Value *Rep) {
130 // Do not insert Pi nodes for constants!
131 if (isa<Constant>(V)) return false;
133 // Check to make sure that there is not already a PI node inserted...
134 if (alreadyHasPiNodeFor(V, Succ) && Rep == 0)
137 // Insert Pi nodes only into successors that the conditional branch dominates.
138 // In this simple case, we know that BB dominates a successor as long there
139 // are no other incoming edges to the successor.
142 // Check to make sure that the successor only has a single predecessor...
143 pred_iterator PI = pred_begin(Succ);
144 BasicBlock *Pred = *PI;
145 if (++PI != pred_end(Succ)) return false; // Multiple predecessor? Bail...
147 // It seems to be safe to insert the Pi node. Do so now...
149 // Create the Pi node...
152 PHINode *Phi = new PHINode(V->getType(), V->getName() + ".pi");
154 // Insert the Pi node in the successor basic block...
155 Succ->getInstList().push_front(Phi);
159 // Loop over all of the uses of V, replacing ones that the Pi node
160 // dominates with references to the Pi node itself.
162 DominatorSet &DS = getAnalysis<DominatorSet>();
163 for (unsigned i = 0; i < V->use_size(); ) {
164 if (Instruction *U = dyn_cast<Instruction>(*(V->use_begin()+i)))
165 if (U->getParent()->getParent() == Succ->getParent() &&
166 DS.dominates(Succ, U->getParent())) {
167 // This instruction is dominated by the Pi node, replace reference to V
168 // with a reference to the Pi node.
170 U->replaceUsesOfWith(V, Pi);
171 continue; // Do not skip the next use...
174 // This use is not dominated by the Pi node, skip it...
178 // Set up the incoming value for the Pi node... do this after uses have been
179 // replaced, because we don't want the Pi node to refer to itself.
182 cast<PHINode>(Pi)->addIncoming(V, Pred);