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setPreservesCFG to be less confusing. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4255 91177308-0d34-0410-b5e6-96231b3b80d8
183 lines
6.9 KiB
C++
183 lines
6.9 KiB
C++
//===- PiNodeInsertion.cpp - Insert Pi nodes into a program ---------------===//
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//
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// PiNodeInsertion - This pass inserts single entry Phi nodes into basic blocks
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// that are preceeded by a conditional branch, where the branch gives
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// information about the operands of the condition. For example, this C code:
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// if (x == 0) { ... = x + 4;
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// becomes:
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// if (x == 0) {
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// x2 = phi(x); // Node that can hold data flow information about X
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// ... = x2 + 4;
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//
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// Since the direction of the condition branch gives information about X itself
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// (whether or not it is zero), some passes (like value numbering or ABCD) can
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// use the inserted Phi/Pi nodes as a place to attach information, in this case
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// saying that X has a value of 0 in this scope. The power of this analysis
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// information is that "in the scope" translates to "for all uses of x2".
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//
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// This special form of Phi node is refered to as a Pi node, following the
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// terminology defined in the "Array Bounds Checks on Demand" paper.
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//
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// As a really trivial example of what the Pi nodes are good for, this pass
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// replaces values compared for equality with direct constants with the constant
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// itself in the branch it's equal to the constant. In the case above, it would
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// change the body to be "... = 0 + 4;" Real value numbering can do much more.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Pass.h"
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#include "llvm/Function.h"
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#include "llvm/iTerminators.h"
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#include "llvm/iOperators.h"
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#include "llvm/iPHINode.h"
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#include "llvm/Support/CFG.h"
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#include "Support/Statistic.h"
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namespace {
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Statistic<> NumInserted("pinodes", "Number of Pi nodes inserted");
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struct PiNodeInserter : public FunctionPass {
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virtual bool runOnFunction(Function &F);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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AU.addRequired<DominatorSet>();
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}
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// insertPiNodeFor - Insert a Pi node for V in the successors of BB if our
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// conditions hold. If Rep is not null, fill in a value of 'Rep' instead of
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// creating a new Pi node itself because we know that the value is a simple
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// constant.
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//
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bool insertPiNodeFor(Value *V, BasicBlock *BB, Value *Rep = 0);
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};
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RegisterOpt<PiNodeInserter> X("pinodes", "Pi Node Insertion");
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}
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Pass *createPiNodeInsertionPass() { return new PiNodeInserter(); }
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bool PiNodeInserter::runOnFunction(Function &F) {
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bool Changed = false;
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for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
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TerminatorInst *TI = I->getTerminator();
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// FIXME: Insert PI nodes for switch statements too
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// Look for conditional branch instructions... that branch on a setcc test
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if (BranchInst *BI = dyn_cast<BranchInst>(TI))
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if (BI->isConditional())
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// TODO: we could in theory support logical operations here too...
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if (SetCondInst *SCI = dyn_cast<SetCondInst>(BI->getCondition())) {
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// Calculate replacement values if this is an obvious constant == or
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// != comparison...
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Value *TrueRep = 0, *FalseRep = 0;
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// Make sure the the constant is the second operand if there is one...
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// This fits with our cannonicalization patterns used elsewhere in the
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// compiler, without depending on instcombine running before us.
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//
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if (isa<Constant>(SCI->getOperand(0)) &&
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!isa<Constant>(SCI->getOperand(1))) {
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SCI->swapOperands();
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Changed = true;
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}
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if (isa<Constant>(SCI->getOperand(1))) {
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if (SCI->getOpcode() == Instruction::SetEQ)
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TrueRep = SCI->getOperand(1);
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else if (SCI->getOpcode() == Instruction::SetNE)
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FalseRep = SCI->getOperand(1);
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}
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BasicBlock *TB = BI->getSuccessor(0); // True block
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BasicBlock *FB = BI->getSuccessor(1); // False block
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// Insert the Pi nodes for the first operand to the comparison...
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Changed |= insertPiNodeFor(SCI->getOperand(0), TB, TrueRep);
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Changed |= insertPiNodeFor(SCI->getOperand(0), FB, FalseRep);
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// Insert the Pi nodes for the second operand to the comparison...
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Changed |= insertPiNodeFor(SCI->getOperand(1), TB);
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Changed |= insertPiNodeFor(SCI->getOperand(1), FB);
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}
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}
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return Changed;
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}
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// alreadyHasPiNodeFor - Return true if there is already a Pi node in BB for V.
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static bool alreadyHasPiNodeFor(Value *V, BasicBlock *BB) {
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for (Value::use_iterator I = V->use_begin(), E = V->use_end(); I != E; ++I)
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if (PHINode *PN = dyn_cast<PHINode>(*I))
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if (PN->getParent() == BB)
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return true;
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return false;
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}
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// insertPiNodeFor - Insert a Pi node for V in the successors of BB if our
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// conditions hold. If Rep is not null, fill in a value of 'Rep' instead of
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// creating a new Pi node itself because we know that the value is a simple
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// constant.
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//
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bool PiNodeInserter::insertPiNodeFor(Value *V, BasicBlock *Succ, Value *Rep) {
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// Do not insert Pi nodes for constants!
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if (isa<Constant>(V)) return false;
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// Check to make sure that there is not already a PI node inserted...
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if (alreadyHasPiNodeFor(V, Succ) && Rep == 0)
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return false;
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// Insert Pi nodes only into successors that the conditional branch dominates.
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// In this simple case, we know that BB dominates a successor as long there
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// are no other incoming edges to the successor.
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//
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// Check to make sure that the successor only has a single predecessor...
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pred_iterator PI = pred_begin(Succ);
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BasicBlock *Pred = *PI;
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if (++PI != pred_end(Succ)) return false; // Multiple predecessor? Bail...
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// It seems to be safe to insert the Pi node. Do so now...
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// Create the Pi node...
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Value *Pi = Rep;
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if (Rep == 0) // Insert the Pi node in the successor basic block...
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Pi = new PHINode(V->getType(), V->getName() + ".pi", Succ->begin());
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// Loop over all of the uses of V, replacing ones that the Pi node
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// dominates with references to the Pi node itself.
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//
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DominatorSet &DS = getAnalysis<DominatorSet>();
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for (unsigned i = 0; i < V->use_size(); ) {
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if (Instruction *U = dyn_cast<Instruction>(*(V->use_begin()+i)))
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if (U->getParent()->getParent() == Succ->getParent() &&
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DS.dominates(Succ, U->getParent())) {
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// This instruction is dominated by the Pi node, replace reference to V
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// with a reference to the Pi node.
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//
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U->replaceUsesOfWith(V, Pi);
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continue; // Do not skip the next use...
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}
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// This use is not dominated by the Pi node, skip it...
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++i;
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}
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// Set up the incoming value for the Pi node... do this after uses have been
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// replaced, because we don't want the Pi node to refer to itself.
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//
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if (Rep == 0)
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cast<PHINode>(Pi)->addIncoming(V, Pred);
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++NumInserted;
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return true;
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}
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