1 //===- InductionVars.cpp - Induction Variable Cannonicalization code --------=//
3 // This file implements induction variable cannonicalization of loops.
5 // Specifically, after this executes, the following is true:
6 // - There is a single induction variable for each loop (at least loops that
7 // used to contain at least one induction variable)
8 // * This induction variable starts at 0 and steps by 1 per iteration
9 // * This induction variable is represented by the first PHI node in the
10 // Header block, allowing it to be found easily.
11 // - All other preexisting induction variables are adjusted to operate in
12 // terms of this primary induction variable
13 // - Induction variables with a step size of 0 have been eliminated.
15 // This code assumes the following is true to perform its full job:
16 // - The CFG has been simplified to not have multiple entrances into an
17 // interval header. Interval headers should only have two predecessors,
18 // one from inside of the loop and one from outside of the loop.
20 //===----------------------------------------------------------------------===//
22 #include "llvm/Transforms/Scalar/InductionVars.h"
23 #include "llvm/Constants.h"
24 #include "llvm/iPHINode.h"
25 #include "llvm/Type.h"
26 #include "llvm/Support/CFG.h"
27 #include "llvm/Analysis/IntervalPartition.h"
28 #include "Support/STLExtras.h"
33 // isLoopInvariant - Return true if the specified value/basic block source is
34 // an interval invariant computation.
36 static bool isLoopInvariant(Interval *Int, Value *V) {
37 assert(isa<Constant>(V) || isa<Instruction>(V) || isa<Argument>(V));
39 if (!isa<Instruction>(V))
40 return true; // Constants and arguments are always loop invariant
42 BasicBlock *ValueBlock = cast<Instruction>(V)->getParent();
43 assert(ValueBlock && "Instruction not embedded in basic block!");
45 // For now, only consider values from outside of the interval, regardless of
46 // whether the expression could be lifted out of the loop by some LICM.
48 // TODO: invoke LICM library if we find out it would be useful.
50 return !Int->contains(ValueBlock);
54 // isLinearInductionVariableH - Return isLIV if the expression V is a linear
55 // expression defined in terms of loop invariant computations, and a single
56 // instance of the PHI node PN. Return isLIC if the expression V is a loop
57 // invariant computation. Return isNLIV if the expression is a negated linear
58 // induction variable. Return isOther if it is neither.
60 // Currently allowed operators are: ADD, SUB, NEG
61 // TODO: This should allow casts!
63 enum LIVType { isLIV, isLIC, isNLIV, isOther };
65 // neg - Negate the sign of a LIV expression.
66 inline LIVType neg(LIVType T) {
67 assert(T == isLIV || T == isNLIV && "Negate Only works on LIV expressions");
68 return T == isLIV ? isNLIV : isLIV;
71 static LIVType isLinearInductionVariableH(Interval *Int, Value *V,
73 if (V == PN) { return isLIV; } // PHI node references are (0+PHI)
74 if (isLoopInvariant(Int, V)) return isLIC;
76 // loop variant computations must be instructions!
77 Instruction *I = cast<Instruction>(V);
78 switch (I->getOpcode()) { // Handle each instruction seperately
79 case Instruction::Add:
80 case Instruction::Sub: {
81 Value *SubV1 = cast<BinaryOperator>(I)->getOperand(0);
82 Value *SubV2 = cast<BinaryOperator>(I)->getOperand(1);
83 LIVType SubLIVType1 = isLinearInductionVariableH(Int, SubV1, PN);
84 if (SubLIVType1 == isOther) return isOther; // Early bailout
85 LIVType SubLIVType2 = isLinearInductionVariableH(Int, SubV2, PN);
87 switch (SubLIVType2) {
88 case isOther: return isOther; // Unknown subexpression type
89 case isLIC: return SubLIVType1; // Constant offset, return type #1
92 // So now we know that we have a linear induction variable on the RHS of
93 // the ADD or SUB instruction. SubLIVType1 cannot be isOther, so it is
94 // either a Loop Invariant computation, or a LIV type.
95 if (SubLIVType1 == isLIC) {
96 // Loop invariant computation, we know this is a LIV then.
97 return (I->getOpcode() == Instruction::Add) ?
98 SubLIVType2 : neg(SubLIVType2);
101 // If the LHS is also a LIV Expression, we cannot add two LIVs together
102 if (I->getOpcode() == Instruction::Add) return isOther;
104 // We can only subtract two LIVs if they are the same type, which yields
105 // a LIC, because the LIVs cancel each other out.
106 return (SubLIVType1 == SubLIVType2) ? isLIC : isOther;
111 default: // Any other instruction is not a LINEAR induction var
116 // isLinearInductionVariable - Return true if the specified expression is a
117 // "linear induction variable", which is an expression involving a single
118 // instance of the PHI node and a loop invariant value that is added or
119 // subtracted to the PHI node. This is calculated by walking the SSA graph
121 static inline bool isLinearInductionVariable(Interval *Int, Value *V,
123 return isLinearInductionVariableH(Int, V, PN) == isLIV;
127 // isSimpleInductionVar - Return true iff the cannonical induction variable PN
128 // has an initializer of the constant value 0, and has a step size of constant
130 static inline bool isSimpleInductionVar(PHINode *PN) {
131 assert(PN->getNumIncomingValues() == 2 && "Must have cannonical PHI node!");
132 Value *Initializer = PN->getIncomingValue(0);
133 if (!isa<Constant>(Initializer)) return false;
135 if (Initializer->getType()->isSigned()) { // Signed constant value...
136 if (((ConstantSInt*)Initializer)->getValue() != 0) return false;
137 } else if (Initializer->getType()->isUnsigned()) { // Unsigned constant value
138 if (((ConstantUInt*)Initializer)->getValue() != 0) return false;
140 return false; // Not signed or unsigned? Must be FP type or something
143 Value *StepExpr = PN->getIncomingValue(1);
144 if (!isa<Instruction>(StepExpr) ||
145 cast<Instruction>(StepExpr)->getOpcode() != Instruction::Add)
148 BinaryOperator *I = cast<BinaryOperator>(StepExpr);
149 assert(isa<PHINode>(I->getOperand(0)) &&
150 "PHI node should be first operand of ADD instruction!");
152 // Get the right hand side of the ADD node. See if it is a constant 1.
153 Value *StepSize = I->getOperand(1);
154 if (!isa<Constant>(StepSize)) return false;
156 if (StepSize->getType()->isSigned()) { // Signed constant value...
157 if (((ConstantSInt*)StepSize)->getValue() != 1) return false;
158 } else if (StepSize->getType()->isUnsigned()) { // Unsigned constant value
159 if (((ConstantUInt*)StepSize)->getValue() != 1) return false;
161 return false; // Not signed or unsigned? Must be FP type or something
164 // At this point, we know the initializer is a constant value 0 and the step
165 // size is a constant value 1. This is our simple induction variable!
169 // InjectSimpleInductionVariable - Insert a cannonical induction variable into
170 // the interval header Header. This assumes that the flow graph is in
171 // simplified form (so we know that the header block has exactly 2 predecessors)
173 // TODO: This should inherit the largest type that is being used by the already
174 // present induction variables (instead of always using uint)
176 static PHINode *InjectSimpleInductionVariable(Interval *Int) {
177 std::string PHIName, AddName;
179 BasicBlock *Header = Int->getHeaderNode();
180 Function *M = Header->getParent();
182 if (M->hasSymbolTable()) {
183 // Only name the induction variable if the function isn't stripped.
185 AddName = "ind_var_next";
188 // Create the neccesary instructions...
189 PHINode *PN = new PHINode(Type::UIntTy, PHIName);
190 Constant *One = ConstantUInt::get(Type::UIntTy, 1);
191 Constant *Zero = ConstantUInt::get(Type::UIntTy, 0);
192 BinaryOperator *AddNode = BinaryOperator::create(Instruction::Add,
195 // Figure out which predecessors I have to play with... there should be
196 // exactly two... one of which is a loop predecessor, and one of which is not.
198 pred_iterator PI = pred_begin(Header);
199 assert(PI != pred_end(Header) && "Header node should have 2 preds!");
200 BasicBlock *Pred1 = *PI; ++PI;
201 assert(PI != pred_end(Header) && "Header node should have 2 preds!");
202 BasicBlock *Pred2 = *PI;
203 assert(++PI == pred_end(Header) && "Header node should have 2 preds!");
205 // Make Pred1 be the loop entrance predecessor, Pred2 be the Loop predecessor
206 if (Int->contains(Pred1)) std::swap(Pred1, Pred2);
208 assert(!Int->contains(Pred1) && "Pred1 should be loop entrance!");
209 assert( Int->contains(Pred2) && "Pred2 should be looping edge!");
211 // Link the instructions into the PHI node...
212 PN->addIncoming(Zero, Pred1); // The initializer is first argument
213 PN->addIncoming(AddNode, Pred2); // The step size is second PHI argument
215 // Insert the PHI node into the Header of the loop. It shall be the first
216 // instruction, because the "Simple" Induction Variable must be first in the
219 BasicBlock::InstListType &IL = Header->getInstList();
222 // Insert the Add instruction as the first (non-phi) instruction in the
223 // header node's basic block.
224 BasicBlock::iterator I = IL.begin();
225 while (isa<PHINode>(*I)) ++I;
226 IL.insert(I, AddNode);
230 // ProcessInterval - This function is invoked once for each interval in the
231 // IntervalPartition of the program. It looks for auxilliary induction
232 // variables in loops. If it finds one, it:
233 // * Cannonicalizes the induction variable. This consists of:
234 // A. Making the first element of the PHI node be the loop invariant
235 // computation, and the second element be the linear induction portion.
236 // B. Changing the first element of the linear induction portion of the PHI
237 // node to be of the form ADD(PHI, <loop invariant expr>).
238 // * Add the induction variable PHI to a list of induction variables found.
240 // After this, a list of cannonical induction variables is known. This list
241 // is searched to see if there is an induction variable that counts from
242 // constant 0 with a step size of constant 1. If there is not one, one is
243 // injected into the loop. Thus a "simple" induction variable is always known
245 // One a simple induction variable is known, all other induction variables are
246 // modified to refer to the "simple" induction variable.
248 static bool ProcessInterval(Interval *Int) {
249 if (!Int->isLoop()) return false; // Not a loop? Ignore it!
251 std::vector<PHINode *> InductionVars;
253 BasicBlock *Header = Int->getHeaderNode();
254 // Loop over all of the PHI nodes in the interval header...
255 for (BasicBlock::iterator I = Header->begin(), E = Header->end();
256 I != E && isa<PHINode>(*I); ++I) {
257 PHINode *PN = cast<PHINode>(*I);
258 if (PN->getNumIncomingValues() != 2) { // These should be eliminated by now.
259 cerr << "Found interval header with more than 2 predecessors! Ignoring\n";
260 return false; // Todo, make an assertion.
263 // For this to be an induction variable, one of the arguments must be a
264 // loop invariant expression, and the other must be an expression involving
265 // the PHI node, along with possible additions and subtractions of loop
268 BasicBlock *BB1 = PN->getIncomingBlock(0);
269 Value *V1 = PN->getIncomingValue(0);
270 BasicBlock *BB2 = PN->getIncomingBlock(1);
271 Value *V2 = PN->getIncomingValue(1);
273 // Figure out which computation is loop invariant...
274 if (!isLoopInvariant(Int, V1)) {
275 // V1 is *not* loop invariant. Check to see if V2 is:
276 if (isLoopInvariant(Int, V2)) {
277 // They *are* loop invariant. Exchange BB1/BB2 and V1/V2 so that
278 // V1 is always the loop invariant computation.
279 std::swap(V1, V2); std::swap(BB1, BB2);
281 // Neither value is loop invariant. Must not be an induction variable.
282 // This case can happen if there is an unreachable loop in the CFG that
283 // has two tail loops in it that was not split by the cleanup phase
289 // At this point, we know that BB1/V1 are loop invariant. We don't know
290 // anything about BB2/V2. Check now to see if V2 is a linear induction
293 cerr << "Found loop invariant computation: " << V1 << "\n";
295 if (!isLinearInductionVariable(Int, V2, PN))
296 continue; // No, it is not a linear ind var, ignore the PHI node.
297 cerr << "Found linear induction variable: " << V2;
299 // TODO: Cannonicalize V2
301 // Add this PHI node to the list of induction variables found...
302 InductionVars.push_back(PN);
305 // No induction variables found?
306 if (InductionVars.empty()) return false;
308 // Search to see if there is already a "simple" induction variable.
309 std::vector<PHINode*>::iterator It =
310 find_if(InductionVars.begin(), InductionVars.end(), isSimpleInductionVar);
312 PHINode *PrimaryIndVar;
314 // A simple induction variable was not found, inject one now...
315 if (It == InductionVars.end()) {
316 PrimaryIndVar = InjectSimpleInductionVariable(Int);
318 // Move the PHI node for this induction variable to the start of the PHI
319 // list in HeaderNode... we do not need to do this for the inserted case
320 // because the inserted node will always be placed at the beginning of
324 BasicBlock::iterator i =
325 find(Header->begin(), Header->end(), PrimaryIndVar);
326 assert(i != Header->end() &&
327 "How could Primary IndVar not be in the header!?!!?");
329 if (i != Header->begin())
330 std::iter_swap(i, Header->begin());
333 // Now we know that there is a simple induction variable PrimaryIndVar.
334 // Simplify all of the other induction variables to use this induction
335 // variable as their counter, and destroy the PHI nodes that correspond to
341 cerr << "Found Interval Header with indvars (primary indvar should be first "
342 << "phi): \n" << Header << "\nPrimaryIndVar: " << PrimaryIndVar;
344 return false; // TODO: true;
348 // ProcessIntervalPartition - This function loops over the interval partition
349 // processing each interval with ProcessInterval
351 static bool ProcessIntervalPartition(IntervalPartition &IP) {
352 // This currently just prints out information about the interval structure
353 // of the function...
355 static unsigned N = 0;
356 cerr << "\n***********Interval Partition #" << (++N) << "************\n\n";
357 copy(IP.begin(), IP.end(), ostream_iterator<Interval*>(cerr, "\n"));
359 cerr << "\n*********** PERFORMING WORK ************\n\n";
361 // Loop over all of the intervals in the partition and look for induction
362 // variables in intervals that represent loops.
364 return reduce_apply(IP.begin(), IP.end(), bitwise_or<bool>(), false,
365 std::ptr_fun(ProcessInterval));
368 // DoInductionVariableCannonicalize - Simplify induction variables in loops.
369 // This function loops over an interval partition of a program, reducing it
370 // until the graph is gone.
372 bool InductionVariableCannonicalize::doIt(Function *M, IntervalPartition &IP) {
374 bool Changed = false;
377 while (!IP->isDegeneratePartition()) {
378 Changed |= ProcessIntervalPartition(*IP);
380 // Calculate the reduced version of this graph until we get to an
381 // irreducible graph or a degenerate graph...
383 IntervalPartition *NewIP = new IntervalPartition(*IP, false);
384 if (NewIP->size() == IP->size()) {
385 cerr << "IRREDUCIBLE GRAPH FOUND!!!\n";
398 bool InductionVariableCannonicalize::runOnFunction(Function *F) {
399 return doIt(F, getAnalysis<IntervalPartition>());
402 // getAnalysisUsage - This function works on the call graph of a module.
403 // It is capable of updating the call graph to reflect the new state of the
406 void InductionVariableCannonicalize::getAnalysisUsage(AnalysisUsage &AU) const {
407 AU.addRequired(IntervalPartition::ID);