6502/llvm-6502
1
0
Fork 0
mirror of https://github.com/c64scene-ar/llvm-6502.git synced 2024-09-28 22:55:52 +00:00
Code Issues Projects Releases Wiki Activity

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

Browse Source 
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.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13017 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2004-04-17 18:36:24 +00:00
parent 4f1134e51a
commit 7980fb9007
1 changed files with 174 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

179
lib/Analysis/ScalarEvolution.cpp
View File

@ -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;
}