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/ConstantVals.h"
24 #include "llvm/Analysis/IntervalPartition.h"
25 #include "llvm/Assembly/Writer.h"
26 #include "llvm/SymbolTable.h"
27 #include "llvm/iPHINode.h"
28 #include "llvm/Method.h"
29 #include "Support/STLExtras.h"
34 // isLoopInvariant - Return true if the specified value/basic block source is
35 // an interval invariant computation.
37 static bool isLoopInvariant(cfg::Interval *Int, Value *V) {
38 assert(isa<Constant>(V) || isa<Instruction>(V) || isa<MethodArgument>(V));
40 if (!isa<Instruction>(V))
41 return true; // Constants and arguments are always loop invariant
43 BasicBlock *ValueBlock = cast<Instruction>(V)->getParent();
44 assert(ValueBlock && "Instruction not embedded in basic block!");
46 // For now, only consider values from outside of the interval, regardless of
47 // whether the expression could be lifted out of the loop by some LICM.
49 // TODO: invoke LICM library if we find out it would be useful.
51 return !Int->contains(ValueBlock);
55 // isLinearInductionVariableH - Return isLIV if the expression V is a linear
56 // expression defined in terms of loop invariant computations, and a single
57 // instance of the PHI node PN. Return isLIC if the expression V is a loop
58 // invariant computation. Return isNLIV if the expression is a negated linear
59 // induction variable. Return isOther if it is neither.
61 // Currently allowed operators are: ADD, SUB, NEG
62 // TODO: This should allow casts!
64 enum LIVType { isLIV, isLIC, isNLIV, isOther };
66 // neg - Negate the sign of a LIV expression.
67 inline LIVType neg(LIVType T) {
68 assert(T == isLIV || T == isNLIV && "Negate Only works on LIV expressions");
69 return T == isLIV ? isNLIV : isLIV;
72 static LIVType isLinearInductionVariableH(cfg::Interval *Int, Value *V,
74 if (V == PN) { return isLIV; } // PHI node references are (0+PHI)
75 if (isLoopInvariant(Int, V)) return isLIC;
77 // loop variant computations must be instructions!
78 Instruction *I = cast<Instruction>(V);
79 switch (I->getOpcode()) { // Handle each instruction seperately
80 case Instruction::Add:
81 case Instruction::Sub: {
82 Value *SubV1 = cast<BinaryOperator>(I)->getOperand(0);
83 Value *SubV2 = cast<BinaryOperator>(I)->getOperand(1);
84 LIVType SubLIVType1 = isLinearInductionVariableH(Int, SubV1, PN);
85 if (SubLIVType1 == isOther) return isOther; // Early bailout
86 LIVType SubLIVType2 = isLinearInductionVariableH(Int, SubV2, PN);
88 switch (SubLIVType2) {
89 case isOther: return isOther; // Unknown subexpression type
90 case isLIC: return SubLIVType1; // Constant offset, return type #1
93 // So now we know that we have a linear induction variable on the RHS of
94 // the ADD or SUB instruction. SubLIVType1 cannot be isOther, so it is
95 // either a Loop Invariant computation, or a LIV type.
96 if (SubLIVType1 == isLIC) {
97 // Loop invariant computation, we know this is a LIV then.
98 return (I->getOpcode() == Instruction::Add) ?
99 SubLIVType2 : neg(SubLIVType2);
102 // If the LHS is also a LIV Expression, we cannot add two LIVs together
103 if (I->getOpcode() == Instruction::Add) return isOther;
105 // We can only subtract two LIVs if they are the same type, which yields
106 // a LIC, because the LIVs cancel each other out.
107 return (SubLIVType1 == SubLIVType2) ? isLIC : isOther;
112 default: // Any other instruction is not a LINEAR induction var
117 // isLinearInductionVariable - Return true if the specified expression is a
118 // "linear induction variable", which is an expression involving a single
119 // instance of the PHI node and a loop invariant value that is added or
120 // subtracted to the PHI node. This is calculated by walking the SSA graph
122 static inline bool isLinearInductionVariable(cfg::Interval *Int, Value *V,
124 return isLinearInductionVariableH(Int, V, PN) == isLIV;
128 // isSimpleInductionVar - Return true iff the cannonical induction variable PN
129 // has an initializer of the constant value 0, and has a step size of constant
131 static inline bool isSimpleInductionVar(PHINode *PN) {
132 assert(PN->getNumIncomingValues() == 2 && "Must have cannonical PHI node!");
133 Value *Initializer = PN->getIncomingValue(0);
134 if (!isa<Constant>(Initializer)) return false;
136 if (Initializer->getType()->isSigned()) { // Signed constant value...
137 if (((ConstantSInt*)Initializer)->getValue() != 0) return false;
138 } else if (Initializer->getType()->isUnsigned()) { // Unsigned constant value
139 if (((ConstantUInt*)Initializer)->getValue() != 0) return false;
141 return false; // Not signed or unsigned? Must be FP type or something
144 Value *StepExpr = PN->getIncomingValue(1);
145 if (!isa<Instruction>(StepExpr) ||
146 cast<Instruction>(StepExpr)->getOpcode() != Instruction::Add)
149 BinaryOperator *I = cast<BinaryOperator>(StepExpr);
150 assert(isa<PHINode>(I->getOperand(0)) &&
151 "PHI node should be first operand of ADD instruction!");
153 // Get the right hand side of the ADD node. See if it is a constant 1.
154 Value *StepSize = I->getOperand(1);
155 if (!isa<Constant>(StepSize)) return false;
157 if (StepSize->getType()->isSigned()) { // Signed constant value...
158 if (((ConstantSInt*)StepSize)->getValue() != 1) return false;
159 } else if (StepSize->getType()->isUnsigned()) { // Unsigned constant value
160 if (((ConstantUInt*)StepSize)->getValue() != 1) return false;
162 return false; // Not signed or unsigned? Must be FP type or something
165 // At this point, we know the initializer is a constant value 0 and the step
166 // size is a constant value 1. This is our simple induction variable!
170 // InjectSimpleInductionVariable - Insert a cannonical induction variable into
171 // the interval header Header. This assumes that the flow graph is in
172 // simplified form (so we know that the header block has exactly 2 predecessors)
174 // TODO: This should inherit the largest type that is being used by the already
175 // present induction variables (instead of always using uint)
177 static PHINode *InjectSimpleInductionVariable(cfg::Interval *Int) {
178 std::string PHIName, AddName;
180 BasicBlock *Header = Int->getHeaderNode();
181 Method *M = Header->getParent();
183 if (M->hasSymbolTable()) {
184 // Only name the induction variable if the method isn't stripped.
185 PHIName = M->getSymbolTable()->getUniqueName(Type::UIntTy, "ind_var");
186 AddName = M->getSymbolTable()->getUniqueName(Type::UIntTy, "ind_var_next");
189 // Create the neccesary instructions...
190 PHINode *PN = new PHINode(Type::UIntTy, PHIName);
191 Constant *One = ConstantUInt::get(Type::UIntTy, 1);
192 Constant *Zero = ConstantUInt::get(Type::UIntTy, 0);
193 BinaryOperator *AddNode = BinaryOperator::create(Instruction::Add,
196 // Figure out which predecessors I have to play with... there should be
197 // exactly two... one of which is a loop predecessor, and one of which is not.
199 BasicBlock::pred_iterator PI = Header->pred_begin();
200 assert(PI != Header->pred_end() && "Header node should have 2 preds!");
201 BasicBlock *Pred1 = *PI; ++PI;
202 assert(PI != Header->pred_end() && "Header node should have 2 preds!");
203 BasicBlock *Pred2 = *PI;
204 assert(++PI == Header->pred_end() && "Header node should have 2 preds!");
206 // Make Pred1 be the loop entrance predecessor, Pred2 be the Loop predecessor
207 if (Int->contains(Pred1)) std::swap(Pred1, Pred2);
209 assert(!Int->contains(Pred1) && "Pred1 should be loop entrance!");
210 assert( Int->contains(Pred2) && "Pred2 should be looping edge!");
212 // Link the instructions into the PHI node...
213 PN->addIncoming(Zero, Pred1); // The initializer is first argument
214 PN->addIncoming(AddNode, Pred2); // The step size is second PHI argument
216 // Insert the PHI node into the Header of the loop. It shall be the first
217 // instruction, because the "Simple" Induction Variable must be first in the
220 BasicBlock::InstListType &IL = Header->getInstList();
223 // Insert the Add instruction as the first (non-phi) instruction in the
224 // header node's basic block.
225 BasicBlock::iterator I = IL.begin();
226 while (isa<PHINode>(*I)) ++I;
227 IL.insert(I, AddNode);
231 // ProcessInterval - This function is invoked once for each interval in the
232 // IntervalPartition of the program. It looks for auxilliary induction
233 // variables in loops. If it finds one, it:
234 // * Cannonicalizes the induction variable. This consists of:
235 // A. Making the first element of the PHI node be the loop invariant
236 // computation, and the second element be the linear induction portion.
237 // B. Changing the first element of the linear induction portion of the PHI
238 // node to be of the form ADD(PHI, <loop invariant expr>).
239 // * Add the induction variable PHI to a list of induction variables found.
241 // After this, a list of cannonical induction variables is known. This list
242 // is searched to see if there is an induction variable that counts from
243 // constant 0 with a step size of constant 1. If there is not one, one is
244 // injected into the loop. Thus a "simple" induction variable is always known
246 // One a simple induction variable is known, all other induction variables are
247 // modified to refer to the "simple" induction variable.
249 static bool ProcessInterval(cfg::Interval *Int) {
250 if (!Int->isLoop()) return false; // Not a loop? Ignore it!
252 std::vector<PHINode *> InductionVars;
254 BasicBlock *Header = Int->getHeaderNode();
255 // Loop over all of the PHI nodes in the interval header...
256 for (BasicBlock::iterator I = Header->begin(), E = Header->end();
257 I != E && isa<PHINode>(*I); ++I) {
258 PHINode *PN = cast<PHINode>(*I);
259 if (PN->getNumIncomingValues() != 2) { // These should be eliminated by now.
260 cerr << "Found interval header with more than 2 predecessors! Ignoring\n";
261 return false; // Todo, make an assertion.
264 // For this to be an induction variable, one of the arguments must be a
265 // loop invariant expression, and the other must be an expression involving
266 // the PHI node, along with possible additions and subtractions of loop
269 BasicBlock *BB1 = PN->getIncomingBlock(0);
270 Value *V1 = PN->getIncomingValue(0);
271 BasicBlock *BB2 = PN->getIncomingBlock(1);
272 Value *V2 = PN->getIncomingValue(1);
274 // Figure out which computation is loop invariant...
275 if (!isLoopInvariant(Int, V1)) {
276 // V1 is *not* loop invariant. Check to see if V2 is:
277 if (isLoopInvariant(Int, V2)) {
278 // They *are* loop invariant. Exchange BB1/BB2 and V1/V2 so that
279 // V1 is always the loop invariant computation.
280 std::swap(V1, V2); std::swap(BB1, BB2);
282 // Neither value is loop invariant. Must not be an induction variable.
283 // This case can happen if there is an unreachable loop in the CFG that
284 // has two tail loops in it that was not split by the cleanup phase
290 // At this point, we know that BB1/V1 are loop invariant. We don't know
291 // anything about BB2/V2. Check now to see if V2 is a linear induction
294 cerr << "Found loop invariant computation: " << V1 << "\n";
296 if (!isLinearInductionVariable(Int, V2, PN))
297 continue; // No, it is not a linear ind var, ignore the PHI node.
298 cerr << "Found linear induction variable: " << V2;
300 // TODO: Cannonicalize V2
302 // Add this PHI node to the list of induction variables found...
303 InductionVars.push_back(PN);
306 // No induction variables found?
307 if (InductionVars.empty()) return false;
309 // Search to see if there is already a "simple" induction variable.
310 std::vector<PHINode*>::iterator It =
311 find_if(InductionVars.begin(), InductionVars.end(), isSimpleInductionVar);
313 PHINode *PrimaryIndVar;
315 // A simple induction variable was not found, inject one now...
316 if (It == InductionVars.end()) {
317 PrimaryIndVar = InjectSimpleInductionVariable(Int);
319 // Move the PHI node for this induction variable to the start of the PHI
320 // list in HeaderNode... we do not need to do this for the inserted case
321 // because the inserted node will always be placed at the beginning of
325 BasicBlock::iterator i =
326 find(Header->begin(), Header->end(), PrimaryIndVar);
327 assert(i != Header->end() &&
328 "How could Primary IndVar not be in the header!?!!?");
330 if (i != Header->begin())
331 std::iter_swap(i, Header->begin());
334 // Now we know that there is a simple induction variable PrimaryIndVar.
335 // Simplify all of the other induction variables to use this induction
336 // variable as their counter, and destroy the PHI nodes that correspond to
342 cerr << "Found Interval Header with indvars (primary indvar should be first "
343 << "phi): \n" << Header << "\nPrimaryIndVar: " << PrimaryIndVar;
345 return false; // TODO: true;
349 // ProcessIntervalPartition - This function loops over the interval partition
350 // processing each interval with ProcessInterval
352 static bool ProcessIntervalPartition(cfg::IntervalPartition &IP) {
353 // This currently just prints out information about the interval structure
356 static unsigned N = 0;
357 cerr << "\n***********Interval Partition #" << (++N) << "************\n\n";
358 copy(IP.begin(), IP.end(), ostream_iterator<cfg::Interval*>(cerr, "\n"));
360 cerr << "\n*********** PERFORMING WORK ************\n\n";
362 // Loop over all of the intervals in the partition and look for induction
363 // variables in intervals that represent loops.
365 return reduce_apply(IP.begin(), IP.end(), bitwise_or<bool>(), false,
366 std::ptr_fun(ProcessInterval));
369 // DoInductionVariableCannonicalize - Simplify induction variables in loops.
370 // This function loops over an interval partition of a program, reducing it
371 // until the graph is gone.
373 bool InductionVariableCannonicalize::doIt(Method *M,
374 cfg::IntervalPartition &IP) {
375 bool Changed = false;
378 while (!IP->isDegeneratePartition()) {
379 Changed |= ProcessIntervalPartition(*IP);
381 // Calculate the reduced version of this graph until we get to an
382 // irreducible graph or a degenerate graph...
384 cfg::IntervalPartition *NewIP = new cfg::IntervalPartition(*IP, false);
385 if (NewIP->size() == IP->size()) {
386 cerr << "IRREDUCIBLE GRAPH FOUND!!!\n";
399 bool InductionVariableCannonicalize::runOnMethod(Method *M) {
400 return doIt(M, getAnalysis<cfg::IntervalPartition>());
403 // getAnalysisUsageInfo - This function works on the call graph of a module.
404 // It is capable of updating the call graph to reflect the new state of the
407 void InductionVariableCannonicalize::getAnalysisUsageInfo(
408 Pass::AnalysisSet &Required,
409 Pass::AnalysisSet &Destroyed,
410 Pass::AnalysisSet &Provided) {
411 Required.push_back(cfg::IntervalPartition::ID);