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Implement several new SCEV folding rules for UDiv SCEVs. 

This fixes an old FIXME, and is needed by some upcoming changes.

llvm-svn: 71247
This commit is contained in:
Dan Gohman 2009-05-08 20:18:49 +00:00
parent 51c4bc320e
commit c3a3cb47d2
1 changed files with 54 additions and 2 deletions
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56
llvm/lib/Analysis/ScalarEvolution.cpp
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@ -1306,7 +1306,61 @@ SCEVHandle ScalarEvolution::getUDivExpr(const SCEVHandle &LHS,
if (const SCEVConstant *RHSC = dyn_cast<SCEVConstant>(RHS)) {
if (RHSC->getValue()->equalsInt(1))
return LHS; // X udiv 1 --> x
if (RHSC->isZero())
return getIntegerSCEV(0, LHS->getType()); // value is undefined
// Determine if the division can be folded into the operands of
// its operands.
// TODO: Generalize this to non-constants by using known-bits information.
const Type *Ty = LHS->getType();
unsigned LZ = RHSC->getValue()->getValue().countLeadingZeros();
unsigned MaxShiftAmt = getTypeSizeInBits(Ty) - LZ;
// For non-power-of-two values, effectively round the value up to the
// nearest power of two.
if (!RHSC->getValue()->getValue().isPowerOf2())
++MaxShiftAmt;
const IntegerType *ExtTy =
IntegerType::get(getTypeSizeInBits(Ty) + MaxShiftAmt);
// {X,+,N}/C --> {X/C,+,N/C} if safe and N/C can be folded.
if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(LHS))
if (const SCEVConstant *Step =
dyn_cast<SCEVConstant>(AR->getStepRecurrence(*this)))
if (!Step->getValue()->getValue()
.urem(RHSC->getValue()->getValue()) &&
getTruncateExpr(getZeroExtendExpr(AR, ExtTy), Ty) == AR) {
std::vector<SCEVHandle> Operands;
for (unsigned i = 0, e = AR->getNumOperands(); i != e; ++i)
Operands.push_back(getUDivExpr(AR->getOperand(i), RHS));
return getAddRecExpr(Operands, AR->getLoop());
}
// (A*B)/C --> A*(B/C) if safe and B/C can be folded.
if (const SCEVMulExpr *M = dyn_cast<SCEVMulExpr>(LHS))
if (getTruncateExpr(getZeroExtendExpr(M, ExtTy), Ty) == M)
// Find an operand that's safely divisible.
for (unsigned i = 0, e = M->getNumOperands(); i != e; ++i) {
SCEVHandle Op = M->getOperand(i);
SCEVHandle Div = getUDivExpr(Op, RHSC);
if (!isa<SCEVUDivExpr>(Div) && getMulExpr(Div, RHSC) == Op) {
std::vector<SCEVHandle> Operands = M->getOperands();
Operands[i] = Div;
return getMulExpr(Operands);
}
}
// (A+B)/C --> (A/C + B/C) if safe and A/C and B/C can be folded.
if (const SCEVAddRecExpr *A = dyn_cast<SCEVAddRecExpr>(LHS))
if (getTruncateExpr(getZeroExtendExpr(A, ExtTy), Ty) == A) {
std::vector<SCEVHandle> Operands;
for (unsigned i = 0, e = A->getNumOperands(); i != e; ++i) {
SCEVHandle Op = getUDivExpr(A->getOperand(i), RHS);
if (isa<SCEVUDivExpr>(Op) || getMulExpr(Op, RHS) != A->getOperand(i))
break;
Operands.push_back(Op);
}
if (Operands.size() == A->getNumOperands())
return getAddExpr(Operands);
}
// Fold if both operands are constant.
if (const SCEVConstant *LHSC = dyn_cast<SCEVConstant>(LHS)) {
Constant *LHSCV = LHSC->getValue();
Constant *RHSCV = RHSC->getValue();
@ -1314,8 +1368,6 @@ SCEVHandle ScalarEvolution::getUDivExpr(const SCEVHandle &LHS,
}
}
// FIXME: implement folding of (X*4)/4 when we know X*4 doesn't overflow.
SCEVUDivExpr *&Result = (*SCEVUDivs)[std::make_pair(LHS, RHS)];
if (Result == 0) Result = new SCEVUDivExpr(LHS, RHS);
return Result;