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Add the ability to compute exit values for complex loop using unanalyzable

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operations.  This allows us to compile this testcase:

int main() {
        int h = 1;
         do h = 3 * h + 1; while (h <= 256);
        printf("%d\n", h);
        return 0;
}

into this:

int %main() {
entry:
        call void %__main( )
        %tmp.6 = call int (sbyte*, ...)* %printf( sbyte* getelementptr ([4 x sbyte]*  %.str_1, long 0, long 0), int 364 )        ; <int> [#uses=0]
        ret int 0
}

This testcase was taken directly from 256.bzip2, believe it or not.

This code is not as general as I would like.  Next up is to refactor it
a bit to handle more cases.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13019 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2004-04-17 22:58:41 +00:00
parent 20aa098ba6
commit 3221ad0db7
1 changed files with 211 additions and 74 deletions
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285
lib/Analysis/ScalarEvolution.cpp
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@ -1136,6 +1136,13 @@ namespace {
/// function as they are computed.
std::map<const Loop*, SCEVHandle> IterationCounts;
/// ConstantEvolutionLoopExitValue - This map contains entries for all of
/// the PHI instructions that we attempt to compute constant evolutions for.
/// This allows us to avoid potentially expensive recomputation of these
/// properties. An instruction maps to null if we are unable to compute its
/// exit value.
std::map<PHINode*, Constant*> ConstantEvolutionLoopExitValue;
public:
ScalarEvolutionsImpl(Function &f, LoopInfo &li)
: F(f), LI(li), UnknownValue(new SCEVCouldNotCompute()) {}
@ -1197,6 +1204,13 @@ namespace {
/// specified value for nonzero will execute. If not computable, return
/// UnknownValue
SCEVHandle HowFarToNonZero(SCEV *V, const Loop *L);
/// getConstantEvolutionLoopExitValue - If we know that the specified Phi is
/// in the header of its containing loop, we know the loop executes a
/// constant number of times, and the PHI node is just a recurrence
/// involving constants, fold it.
Constant *getConstantEvolutionLoopExitValue(PHINode *PN, uint64_t Its,
const Loop *L);
};
}
@ -1209,6 +1223,8 @@ namespace {
/// that no dangling references are left around.
void ScalarEvolutionsImpl::deleteInstructionFromRecords(Instruction *I) {
Scalars.erase(I);
if (PHINode *PN = dyn_cast<PHINode>(I))
ConstantEvolutionLoopExitValue.erase(PN);
}
@ -1552,77 +1568,23 @@ SCEVHandle ScalarEvolutionsImpl::ComputeIterationCount(const Loop *L) {
ExitBr->getSuccessor(0) == ExitBlock);
}
/// getConstantEvolvingPHI - Given an LLVM value and a loop, return a PHI node
/// in the loop that V is derived from. We allow arbitrary operations along the
/// way, but the operands of an operation must either be constants or a value
/// derived from a constant PHI. If this expression does not fit with these
/// constraints, return null.
static PHINode *getConstantEvolvingPHI(Value *V, const Loop *L) {
// If this is not an instruction, or if this is an instruction outside of the
// loop, it can't be derived from a loop PHI.
Instruction *I = dyn_cast<Instruction>(V);
if (I == 0 || !L->contains(I->getParent())) return 0;
if (PHINode *PN = dyn_cast<PHINode>(I))
if (L->getHeader() == I->getParent())
return PN;
else
// We don't currently keep track of the control flow needed to evaluate
// PHIs, so we cannot handle PHIs inside of loops.
return 0;
// If this is a call, and we have no hope of constant folding, bail early.
if (CallInst *CI = dyn_cast<CallInst>(I)) {
if (!CI->getCalledFunction() ||
!canConstantFoldCallTo(CI->getCalledFunction()))
return 0;
} else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
return 0;
/// CanConstantFold - Return true if we can constant fold an instruction of the
/// specified type, assuming that all operands were constants.
static bool CanConstantFold(const Instruction *I) {
if (isa<BinaryOperator>(I) || isa<ShiftInst>(I) ||
isa<SelectInst>(I) || isa<CastInst>(I) || isa<GetElementPtrInst>(I))
return true;
// Otherwise, we can evaluate this instruction if all of its operands but one
// are constant, and if the remaining one is derived from a constant evolving
// PHI.
unsigned Op = 0, e = I->getNumOperands();
while (Op != e && (isa<Constant>(I->getOperand(Op)) ||
isa<GlobalValue>(I->getOperand(Op))))
++Op; // Skip over all constant operands
if (Op == e) return 0; // No non-constants? Should be folded!
// Found the first non-constant operand.
unsigned NonConstantOp = Op;
// Okay, all of the rest must be constants now.
for (++Op; Op != e; ++Op)
if (!(isa<Constant>(I->getOperand(Op)) ||
isa<GlobalValue>(I->getOperand(Op))))
return 0; // Too many non-constant operands!
// This is a expression evolving from a constant PHI if the non-constant
// portion is!
return getConstantEvolvingPHI(I->getOperand(NonConstantOp), L);
if (const CallInst *CI = dyn_cast<CallInst>(I))
if (const Function *F = CI->getCalledFunction())
return canConstantFoldCallTo((Function*)F); // FIXME: elim cast
return false;
}
/// EvaluateExpression - Given an expression that passes the
/// getConstantEvolvingPHI predicate, evaluate its value assuming the PHI node
/// in the loop has the value PHIVal. If we can't fold this expression for some
/// reason, return null.
static Constant *EvaluateExpression(Value *V, Constant *PHIVal) {
if (isa<PHINode>(V)) return PHIVal;
if (Constant *C = dyn_cast<Constant>(V)) return C;
if (GlobalValue *GV = dyn_cast<GlobalValue>(V))
return ConstantPointerRef::get(GV);
Instruction *I = cast<Instruction>(V);
std::vector<Constant*> Operands;
Operands.resize(I->getNumOperands());
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
Operands[i] = EvaluateExpression(I->getOperand(i), PHIVal);
if (Operands[i] == 0) return 0;
}
/// ConstantFold - Constant fold an instruction of the specified type with the
/// specified constant operands. This function may modify the operands vector.
static Constant *ConstantFold(const Instruction *I,
std::vector<Constant*> &Operands) {
if (isa<BinaryOperator>(I) || isa<ShiftInst>(I))
return ConstantExpr::get(I->getOpcode(), Operands[0], Operands[1]);
@ -1647,6 +1609,119 @@ static Constant *EvaluateExpression(Value *V, Constant *PHIVal) {
}
/// getConstantEvolvingPHI - Given an LLVM value and a loop, return a PHI node
/// in the loop that V is derived from. We allow arbitrary operations along the
/// way, but the operands of an operation must either be constants or a value
/// derived from a constant PHI. If this expression does not fit with these
/// constraints, return null.
static PHINode *getConstantEvolvingPHI(Value *V, const Loop *L) {
// If this is not an instruction, or if this is an instruction outside of the
// loop, it can't be derived from a loop PHI.
Instruction *I = dyn_cast<Instruction>(V);
if (I == 0 || !L->contains(I->getParent())) return 0;
if (PHINode *PN = dyn_cast<PHINode>(I))
if (L->getHeader() == I->getParent())
return PN;
else
// We don't currently keep track of the control flow needed to evaluate
// PHIs, so we cannot handle PHIs inside of loops.
return 0;
// If we won't be able to constant fold this expression even if the operands
// are constants, return early.
if (!CanConstantFold(I)) return 0;
// Otherwise, we can evaluate this instruction if all of its operands are
// constant or derived from a PHI node themselves.
PHINode *PHI = 0;
for (unsigned Op = 0, e = I->getNumOperands(); Op != e; ++Op)
if (!(isa<Constant>(I->getOperand(Op)) ||
isa<GlobalValue>(I->getOperand(Op)))) {
PHINode *P = getConstantEvolvingPHI(I->getOperand(Op), L);
if (P == 0) return 0; // Not evolving from PHI
if (PHI == 0)
PHI = P;
else if (PHI != P)
return 0; // Evolving from multiple different PHIs.
}
// This is a expression evolving from a constant PHI!
return PHI;
}
/// EvaluateExpression - Given an expression that passes the
/// getConstantEvolvingPHI predicate, evaluate its value assuming the PHI node
/// in the loop has the value PHIVal. If we can't fold this expression for some
/// reason, return null.
static Constant *EvaluateExpression(Value *V, Constant *PHIVal) {
if (isa<PHINode>(V)) return PHIVal;
if (Constant *C = dyn_cast<Constant>(V)) return C;
if (GlobalValue *GV = dyn_cast<GlobalValue>(V))
return ConstantPointerRef::get(GV);
Instruction *I = cast<Instruction>(V);
std::vector<Constant*> Operands;
Operands.resize(I->getNumOperands());
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
Operands[i] = EvaluateExpression(I->getOperand(i), PHIVal);
if (Operands[i] == 0) return 0;
}
return ConstantFold(I, Operands);
}
/// getConstantEvolutionLoopExitValue - If we know that the specified Phi is
/// in the header of its containing loop, we know the loop executes a
/// constant number of times, and the PHI node is just a recurrence
/// involving constants, fold it.
Constant *ScalarEvolutionsImpl::
getConstantEvolutionLoopExitValue(PHINode *PN, uint64_t Its, const Loop *L) {
std::map<PHINode*, Constant*>::iterator I =
ConstantEvolutionLoopExitValue.find(PN);
if (I != ConstantEvolutionLoopExitValue.end())
return I->second;
if (Its > MaxBruteForceIterations)
return ConstantEvolutionLoopExitValue[PN] = 0; // Not going to evaluate it.
Constant *&RetVal = ConstantEvolutionLoopExitValue[PN];
// Since the loop is canonicalized, the PHI node must have two entries. One
// entry must be a constant (coming in from outside of the loop), and the
// second must be derived from the same PHI.
bool SecondIsBackedge = L->contains(PN->getIncomingBlock(1));
Constant *StartCST =
dyn_cast<Constant>(PN->getIncomingValue(!SecondIsBackedge));
if (StartCST == 0)
return RetVal = 0; // Must be a constant.
Value *BEValue = PN->getIncomingValue(SecondIsBackedge);
PHINode *PN2 = getConstantEvolvingPHI(BEValue, L);
if (PN2 != PN)
return RetVal = 0; // Not derived from same PHI.
// Execute the loop symbolically to determine the exit value.
unsigned IterationNum = 0;
unsigned NumIterations = Its;
if (NumIterations != Its)
return RetVal = 0; // More than 2^32 iterations??
for (Constant *PHIVal = StartCST; ; ++IterationNum) {
if (IterationNum == NumIterations)
return RetVal = PHIVal; // Got exit value!
// Compute the value of the PHI node for the next iteration.
Constant *NextPHI = EvaluateExpression(BEValue, PHIVal);
if (NextPHI == PHIVal)
return RetVal = NextPHI; // Stopped evolving!
if (NextPHI == 0)
return 0; // Couldn't evaluate!
PHIVal = NextPHI;
}
}
/// ComputeIterationCountExhaustively - If the trip is known to execute a
/// constant number of times (the condition evolves only from constants),
/// try to evaluate a few iterations of the loop until we get the exit
@ -1679,16 +1754,18 @@ ComputeIterationCountExhaustively(const Loop *L, Value *Cond, bool ExitWhen) {
ConstantBool *CondVal =
dyn_cast_or_null<ConstantBool>(EvaluateExpression(Cond, PHIVal));
if (!CondVal) return UnknownValue; // Couldn't symbolically evaluate.
if (CondVal->getValue() == ExitWhen) {
ConstantEvolutionLoopExitValue[PN] = PHIVal;
++NumBruteForceTripCountsComputed;
return SCEVConstant::get(ConstantUInt::get(Type::UIntTy, IterationNum));
}
// Otherwise, compute the value of the PHI node for the next iteration.
Constant *Next = EvaluateExpression(BEValue, PHIVal);
if (Next == 0 || Next == PHIVal)
// Compute the value of the PHI node for the next iteration.
Constant *NextPHI = EvaluateExpression(BEValue, PHIVal);
if (NextPHI == 0 || NextPHI == PHIVal)
return UnknownValue; // Couldn't evaluate or not making progress...
PHIVal = Next;
PHIVal = NextPHI;
}
// Too many iterations were needed to evaluate.
@ -1701,7 +1778,67 @@ ComputeIterationCountExhaustively(const Loop *L, Value *Cond, bool ExitWhen) {
SCEVHandle ScalarEvolutionsImpl::getSCEVAtScope(SCEV *V, const Loop *L) {
// FIXME: this should be turned into a virtual method on SCEV!
if (isa<SCEVConstant>(V) || isa<SCEVUnknown>(V)) return V;
if (isa<SCEVConstant>(V)) return V;
// If this instruction is evolves from a constant-evolving PHI, compute the
// exit value from the loop without using SCEVs.
if (SCEVUnknown *SU = dyn_cast<SCEVUnknown>(V)) {
if (Instruction *I = dyn_cast<Instruction>(SU->getValue())) {
const Loop *LI = this->LI[I->getParent()];
if (LI && LI->getParentLoop() == L) // Looking for loop exit value.
if (PHINode *PN = dyn_cast<PHINode>(I))
if (PN->getParent() == LI->getHeader()) {
// Okay, there is no closed form solution for the PHI node. Check
// to see if the loop that contains it has a known iteration count.
// If so, we may be able to force computation of the exit value.
SCEVHandle IterationCount = getIterationCount(LI);
if (SCEVConstant *ICC = dyn_cast<SCEVConstant>(IterationCount)) {
// Okay, we know how many times the containing loop executes. If
// this is a constant evolving PHI node, get the final value at
// the specified iteration number.
Constant *RV = getConstantEvolutionLoopExitValue(PN,
ICC->getValue()->getRawValue(),
LI);
if (RV) return SCEVUnknown::get(RV);
}
}
// Okay, this is a some expression that we cannot symbolically evaluate
// into a SCEV. Check to see if it's possible to symbolically evaluate
// the arguments into constants, and if see, try to constant propagate the
// result. This is particularly useful for computing loop exit values.
if (CanConstantFold(I)) {
std::vector<Constant*> Operands;
Operands.reserve(I->getNumOperands());
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
Value *Op = I->getOperand(i);
if (Constant *C = dyn_cast<Constant>(Op)) {
Operands.push_back(C);
} else if (GlobalValue *GV = dyn_cast<GlobalValue>(Op)) {
Operands.push_back(ConstantPointerRef::get(GV));
} else {
SCEVHandle OpV = getSCEVAtScope(getSCEV(Op), L);
if (SCEVConstant *SC = dyn_cast<SCEVConstant>(OpV))
Operands.push_back(ConstantExpr::getCast(SC->getValue(),
Op->getType()));
else if (SCEVUnknown *SU = dyn_cast<SCEVUnknown>(OpV)) {
if (Constant *C = dyn_cast<Constant>(SU->getValue()))
Operands.push_back(ConstantExpr::getCast(C, Op->getType()));
else
return V;
} else {
return V;
}
}
}
return SCEVUnknown::get(ConstantFold(I, Operands));
}
}
// This is some other type of SCEVUnknown, just return it.
return V;
}
if (SCEVCommutativeExpr *Comm = dyn_cast<SCEVCommutativeExpr>(V)) {
// Avoid performing the look-up in the common case where the specified
// expression has no loop-variant portions.
@ -1711,7 +1848,7 @@ SCEVHandle ScalarEvolutionsImpl::getSCEVAtScope(SCEV *V, const Loop *L) {
if (OpAtScope == UnknownValue) return UnknownValue;
// Okay, at least one of these operands is loop variant but might be
// foldable. Build a new instance of the folded commutative expression.
std::vector<SCEVHandle> NewOps(Comm->op_begin(), Comm->op_begin()+i-1);
std::vector<SCEVHandle> NewOps(Comm->op_begin(), Comm->op_begin()+i);
NewOps.push_back(OpAtScope);
for (++i; i != e; ++i) {