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Add the ability to compute trip counts that are only controlled by constants

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even if the loop is using expressions that we can't compute as a closed-form.
This allows us to calculate that this function always returns 55:

int test() {
  double X;
  int Count = 0;
  for (X = 100; X > 1; X = sqrt(X), ++Count)
    /*empty*/;
  return Count;
}

And allows us to compute trip counts for loops like:

        int h = 1;
         do h = 3 * h + 1; while (h <= 256);

(which occurs in bzip2), and for this function, which occurs after inlining
and other optimizations:

int popcount()
{
   int x = 666;
  int result = 0;
  while (x != 0) {
    result = result + (x & 0x1);
    x = x >> 1;
  }
  return result;
}

We still cannot compute the exit values of result or h in the two loops above,
which means we cannot delete the loop, but we are getting closer.  Being able to
compute a constant trip count for these two loops will allow us to unroll them
completely though.

llvm-svn: 13017
This commit is contained in:
Chris Lattner 2004-04-17 18:36:24 +00:00
parent bcb690dc9b
commit 9a73de2ba2
1 changed files with 174 additions and 5 deletions
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179
lib/Analysis/ScalarEvolution.cpp
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@ -72,9 +72,11 @@
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Assembly/Writer.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/ConstantRange.h"
#include "llvm/Support/InstIterator.h"
#include "Support/CommandLine.h"
#include "Support/Statistic.h"
#include <cmath>
using namespace llvm;
@ -92,6 +94,14 @@ namespace {
Statistic<>
NumTripCountsNotComputed("scalar-evolution",
"Number of loops without predictable loop counts");
Statistic<>
NumBruteForceTripCountsComputed("scalar-evolution",
"Number of loops with trip counts computed by force");
cl::opt<unsigned>
MaxBruteForceIterations("scalar-evolution-max-iterations", cl::ReallyHidden,
cl::desc("Maximum number of iterations SCEV will symbolically execute a constant derived loop"),
cl::init(100));
}
//===----------------------------------------------------------------------===//
@ -1170,6 +1180,14 @@ namespace {
/// will iterate.
SCEVHandle ComputeIterationCount(const Loop *L);
/// 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
/// condition gets a value of ExitWhen (true or false). If we cannot
/// evaluate the trip count of the loop, return UnknownValue.
SCEVHandle ComputeIterationCountExhaustively(const Loop *L, Value *Cond,
bool ExitWhen);
/// HowFarToZero - Return the number of times a backedge comparing the
/// specified value to zero will execute. If not computable, return
/// UnknownValue
@ -1444,7 +1462,9 @@ SCEVHandle ScalarEvolutionsImpl::ComputeIterationCount(const Loop *L) {
if (ExitBr == 0) return UnknownValue;
assert(ExitBr->isConditional() && "If unconditional, it can't be in loop!");
SetCondInst *ExitCond = dyn_cast<SetCondInst>(ExitBr->getCondition());
if (ExitCond == 0) return UnknownValue;
if (ExitCond == 0) // Not a setcc
return ComputeIterationCountExhaustively(L, ExitBr->getCondition(),
ExitBr->getSuccessor(0) == ExitBlock);
SCEVHandle LHS = getSCEV(ExitCond->getOperand(0));
SCEVHandle RHS = getSCEV(ExitCond->getOperand(1));
@ -1505,13 +1525,17 @@ SCEVHandle ScalarEvolutionsImpl::ComputeIterationCount(const Loop *L) {
switch (Cond) {
case Instruction::SetNE: // while (X != Y)
// Convert to: while (X-Y != 0)
if (LHS->getType()->isInteger())
return HowFarToZero(getMinusSCEV(LHS, RHS), L);
if (LHS->getType()->isInteger()) {
SCEVHandle TC = HowFarToZero(getMinusSCEV(LHS, RHS), L);
if (!isa<SCEVCouldNotCompute>(TC)) return TC;
}
break;
case Instruction::SetEQ:
// Convert to: while (X-Y == 0) // while (X == Y)
if (LHS->getType()->isInteger())
return HowFarToNonZero(getMinusSCEV(LHS, RHS), L);
if (LHS->getType()->isInteger()) {
SCEVHandle TC = HowFarToNonZero(getMinusSCEV(LHS, RHS), L);
if (!isa<SCEVCouldNotCompute>(TC)) return TC;
}
break;
default:
#if 0
@ -1523,6 +1547,151 @@ SCEVHandle ScalarEvolutionsImpl::ComputeIterationCount(const Loop *L) {
#endif
break;
}
return ComputeIterationCountExhaustively(L, ExitCond,
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;
// 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);
}
/// 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;
}
if (isa<BinaryOperator>(I) || isa<ShiftInst>(I))
return ConstantExpr::get(I->getOpcode(), Operands[0], Operands[1]);
switch (I->getOpcode()) {
case Instruction::Cast:
return ConstantExpr::getCast(Operands[0], I->getType());
case Instruction::Select:
return ConstantExpr::getSelect(Operands[0], Operands[1], Operands[2]);
case Instruction::Call:
if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(Operands[0])) {
Operands.erase(Operands.begin());
return ConstantFoldCall(cast<Function>(CPR->getValue()), Operands);
}
return 0;
case Instruction::GetElementPtr:
Constant *Base = Operands[0];
Operands.erase(Operands.begin());
return ConstantExpr::getGetElementPtr(Base, Operands);
}
return 0;
}
/// 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
/// condition gets a value of ExitWhen (true or false). If we cannot
/// evaluate the trip count of the loop, return UnknownValue.
SCEVHandle ScalarEvolutionsImpl::
ComputeIterationCountExhaustively(const Loop *L, Value *Cond, bool ExitWhen) {
PHINode *PN = getConstantEvolvingPHI(Cond, L);
if (PN == 0) return UnknownValue;
// 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 UnknownValue; // Must be a constant.
Value *BEValue = PN->getIncomingValue(SecondIsBackedge);
PHINode *PN2 = getConstantEvolvingPHI(BEValue, L);
if (PN2 != PN) return UnknownValue; // Not derived from same PHI.
// Okay, we find a PHI node that defines the trip count of this loop. Execute
// the loop symbolically to determine when the condition gets a value of
// "ExitWhen".
unsigned IterationNum = 0;
unsigned MaxIterations = MaxBruteForceIterations; // Limit analysis.
for (Constant *PHIVal = StartCST;
IterationNum != MaxIterations; ++IterationNum) {
ConstantBool *CondVal =
dyn_cast_or_null<ConstantBool>(EvaluateExpression(Cond, PHIVal));
if (!CondVal) return UnknownValue; // Couldn't symbolically evaluate.
if (CondVal->getValue() == ExitWhen) {
++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)
return UnknownValue; // Couldn't evaluate or not making progress...
PHIVal = Next;
}
// Too many iterations were needed to evaluate.
return UnknownValue;
}