+
+/// FoldICmpDivCst - Fold "icmp pred, ([su]div X, DivRHS), CmpRHS" where DivRHS
+/// and CmpRHS are both known to be integer constants.
+Instruction *InstCombiner::FoldICmpDivCst(ICmpInst &ICI, BinaryOperator *DivI,
+ ConstantInt *DivRHS) {
+ ConstantInt *CmpRHS = cast<ConstantInt>(ICI.getOperand(1));
+ const APInt &CmpRHSV = CmpRHS->getValue();
+
+ // FIXME: If the operand types don't match the type of the divide
+ // then don't attempt this transform. The code below doesn't have the
+ // logic to deal with a signed divide and an unsigned compare (and
+ // vice versa). This is because (x /s C1) <s C2 produces different
+ // results than (x /s C1) <u C2 or (x /u C1) <s C2 or even
+ // (x /u C1) <u C2. Simply casting the operands and result won't
+ // work. :( The if statement below tests that condition and bails
+ // if it finds it.
+ bool DivIsSigned = DivI->getOpcode() == Instruction::SDiv;
+ if (!ICI.isEquality() && DivIsSigned != ICI.isSignedPredicate())
+ return 0;
+ if (DivRHS->isZero())
+ return 0; // The ProdOV computation fails on divide by zero.
+
+ // Compute Prod = CI * DivRHS. We are essentially solving an equation
+ // of form X/C1=C2. We solve for X by multiplying C1 (DivRHS) and
+ // C2 (CI). By solving for X we can turn this into a range check
+ // instead of computing a divide.
+ ConstantInt *Prod = Multiply(CmpRHS, DivRHS);
+
+ // Determine if the product overflows by seeing if the product is
+ // not equal to the divide. Make sure we do the same kind of divide
+ // as in the LHS instruction that we're folding.
+ bool ProdOV = (DivIsSigned ? ConstantExpr::getSDiv(Prod, DivRHS) :
+ ConstantExpr::getUDiv(Prod, DivRHS)) != CmpRHS;
+
+ // Get the ICmp opcode
+ ICmpInst::Predicate Pred = ICI.getPredicate();
+
+ // Figure out the interval that is being checked. For example, a comparison
+ // like "X /u 5 == 0" is really checking that X is in the interval [0, 5).
+ // Compute this interval based on the constants involved and the signedness of
+ // the compare/divide. This computes a half-open interval, keeping track of
+ // whether either value in the interval overflows. After analysis each
+ // overflow variable is set to 0 if it's corresponding bound variable is valid
+ // -1 if overflowed off the bottom end, or +1 if overflowed off the top end.
+ int LoOverflow = 0, HiOverflow = 0;
+ ConstantInt *LoBound = 0, *HiBound = 0;
+
+
+ if (!DivIsSigned) { // udiv
+ // e.g. X/5 op 3 --> [15, 20)
+ LoBound = Prod;
+ HiOverflow = LoOverflow = ProdOV;
+ if (!HiOverflow)
+ HiOverflow = AddWithOverflow(HiBound, LoBound, DivRHS, false);
+ } else if (DivRHS->getValue().isPositive()) { // Divisor is > 0.
+ if (CmpRHSV == 0) { // (X / pos) op 0
+ // Can't overflow. e.g. X/2 op 0 --> [-1, 2)
+ LoBound = cast<ConstantInt>(ConstantExpr::getNeg(SubOne(DivRHS)));
+ HiBound = DivRHS;
+ } else if (CmpRHSV.isPositive()) { // (X / pos) op pos
+ LoBound = Prod; // e.g. X/5 op 3 --> [15, 20)
+ HiOverflow = LoOverflow = ProdOV;
+ if (!HiOverflow)
+ HiOverflow = AddWithOverflow(HiBound, Prod, DivRHS, true);
+ } else { // (X / pos) op neg
+ // e.g. X/5 op -3 --> [-15-4, -15+1) --> [-19, -14)
+ Constant *DivRHSH = ConstantExpr::getNeg(SubOne(DivRHS));
+ LoOverflow = AddWithOverflow(LoBound, Prod,
+ cast<ConstantInt>(DivRHSH), true) ? -1 : 0;
+ HiBound = AddOne(Prod);
+ HiOverflow = ProdOV ? -1 : 0;
+ }
+ } else { // Divisor is < 0.
+ if (CmpRHSV == 0) { // (X / neg) op 0
+ // e.g. X/-5 op 0 --> [-4, 5)
+ LoBound = AddOne(DivRHS);
+ HiBound = cast<ConstantInt>(ConstantExpr::getNeg(DivRHS));
+ if (HiBound == DivRHS) { // -INTMIN = INTMIN
+ HiOverflow = 1; // [INTMIN+1, overflow)
+ HiBound = 0; // e.g. X/INTMIN = 0 --> X > INTMIN
+ }
+ } else if (CmpRHSV.isPositive()) { // (X / neg) op pos
+ // e.g. X/-5 op 3 --> [-19, -14)
+ HiOverflow = LoOverflow = ProdOV ? -1 : 0;
+ if (!LoOverflow)
+ LoOverflow = AddWithOverflow(LoBound, Prod, AddOne(DivRHS), true) ?-1:0;
+ HiBound = AddOne(Prod);
+ } else { // (X / neg) op neg
+ // e.g. X/-5 op -3 --> [15, 20)
+ LoBound = Prod;
+ LoOverflow = HiOverflow = ProdOV ? 1 : 0;
+ HiBound = Subtract(Prod, DivRHS);
+ }
+
+ // Dividing by a negative swaps the condition. LT <-> GT
+ Pred = ICmpInst::getSwappedPredicate(Pred);
+ }
+
+ Value *X = DivI->getOperand(0);
+ switch (Pred) {
+ default: assert(0 && "Unhandled icmp opcode!");
+ case ICmpInst::ICMP_EQ:
+ if (LoOverflow && HiOverflow)
+ return ReplaceInstUsesWith(ICI, ConstantInt::getFalse());
+ else if (HiOverflow)
+ return new ICmpInst(DivIsSigned ? ICmpInst::ICMP_SGE :
+ ICmpInst::ICMP_UGE, X, LoBound);
+ else if (LoOverflow)
+ return new ICmpInst(DivIsSigned ? ICmpInst::ICMP_SLT :
+ ICmpInst::ICMP_ULT, X, HiBound);
+ else
+ return InsertRangeTest(X, LoBound, HiBound, DivIsSigned, true, ICI);
+ case ICmpInst::ICMP_NE:
+ if (LoOverflow && HiOverflow)
+ return ReplaceInstUsesWith(ICI, ConstantInt::getTrue());
+ else if (HiOverflow)
+ return new ICmpInst(DivIsSigned ? ICmpInst::ICMP_SLT :
+ ICmpInst::ICMP_ULT, X, LoBound);
+ else if (LoOverflow)
+ return new ICmpInst(DivIsSigned ? ICmpInst::ICMP_SGE :
+ ICmpInst::ICMP_UGE, X, HiBound);
+ else
+ return InsertRangeTest(X, LoBound, HiBound, DivIsSigned, false, ICI);
+ case ICmpInst::ICMP_ULT:
+ case ICmpInst::ICMP_SLT:
+ if (LoOverflow == +1) // Low bound is greater than input range.
+ return ReplaceInstUsesWith(ICI, ConstantInt::getTrue());
+ if (LoOverflow == -1) // Low bound is less than input range.
+ return ReplaceInstUsesWith(ICI, ConstantInt::getFalse());
+ return new ICmpInst(Pred, X, LoBound);
+ case ICmpInst::ICMP_UGT:
+ case ICmpInst::ICMP_SGT:
+ if (HiOverflow == +1) // High bound greater than input range.
+ return ReplaceInstUsesWith(ICI, ConstantInt::getFalse());
+ else if (HiOverflow == -1) // High bound less than input range.
+ return ReplaceInstUsesWith(ICI, ConstantInt::getTrue());
+ if (Pred == ICmpInst::ICMP_UGT)
+ return new ICmpInst(ICmpInst::ICMP_UGE, X, HiBound);
+ else
+ return new ICmpInst(ICmpInst::ICMP_SGE, X, HiBound);
+ }
+}
+
+