+/// transformSExtICmp - Transform (sext icmp) to bitwise / integer operations
+/// in order to eliminate the icmp.
+Instruction *InstCombiner::transformSExtICmp(ICmpInst *ICI, Instruction &CI) {
+ Value *Op0 = ICI->getOperand(0), *Op1 = ICI->getOperand(1);
+ ICmpInst::Predicate Pred = ICI->getPredicate();
+
+ if (ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
+ // (x <s 0) ? -1 : 0 -> ashr x, 31 -> all ones if negative
+ // (x >s -1) ? -1 : 0 -> not (ashr x, 31) -> all ones if positive
+ if ((Pred == ICmpInst::ICMP_SLT && Op1C->isZero()) ||
+ (Pred == ICmpInst::ICMP_SGT && Op1C->isAllOnesValue())) {
+
+ Value *Sh = ConstantInt::get(Op0->getType(),
+ Op0->getType()->getScalarSizeInBits()-1);
+ Value *In = Builder->CreateAShr(Op0, Sh, Op0->getName()+".lobit");
+ if (In->getType() != CI.getType())
+ In = Builder->CreateIntCast(In, CI.getType(), true/*SExt*/);
+
+ if (Pred == ICmpInst::ICMP_SGT)
+ In = Builder->CreateNot(In, In->getName()+".not");
+ return ReplaceInstUsesWith(CI, In);
+ }
+
+ // If we know that only one bit of the LHS of the icmp can be set and we
+ // have an equality comparison with zero or a power of 2, we can transform
+ // the icmp and sext into bitwise/integer operations.
+ if (ICI->hasOneUse() &&
+ ICI->isEquality() && (Op1C->isZero() || Op1C->getValue().isPowerOf2())){
+ unsigned BitWidth = Op1C->getType()->getBitWidth();
+ APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
+ ComputeMaskedBits(Op0, KnownZero, KnownOne);
+
+ APInt KnownZeroMask(~KnownZero);
+ if (KnownZeroMask.isPowerOf2()) {
+ Value *In = ICI->getOperand(0);
+
+ // If the icmp tests for a known zero bit we can constant fold it.
+ if (!Op1C->isZero() && Op1C->getValue() != KnownZeroMask) {
+ Value *V = Pred == ICmpInst::ICMP_NE ?
+ ConstantInt::getAllOnesValue(CI.getType()) :
+ ConstantInt::getNullValue(CI.getType());
+ return ReplaceInstUsesWith(CI, V);
+ }
+
+ if (!Op1C->isZero() == (Pred == ICmpInst::ICMP_NE)) {
+ // sext ((x & 2^n) == 0) -> (x >> n) - 1
+ // sext ((x & 2^n) != 2^n) -> (x >> n) - 1
+ unsigned ShiftAmt = KnownZeroMask.countTrailingZeros();
+ // Perform a right shift to place the desired bit in the LSB.
+ if (ShiftAmt)
+ In = Builder->CreateLShr(In,
+ ConstantInt::get(In->getType(), ShiftAmt));
+
+ // At this point "In" is either 1 or 0. Subtract 1 to turn
+ // {1, 0} -> {0, -1}.
+ In = Builder->CreateAdd(In,
+ ConstantInt::getAllOnesValue(In->getType()),
+ "sext");
+ } else {
+ // sext ((x & 2^n) != 0) -> (x << bitwidth-n) a>> bitwidth-1
+ // sext ((x & 2^n) == 2^n) -> (x << bitwidth-n) a>> bitwidth-1
+ unsigned ShiftAmt = KnownZeroMask.countLeadingZeros();
+ // Perform a left shift to place the desired bit in the MSB.
+ if (ShiftAmt)
+ In = Builder->CreateShl(In,
+ ConstantInt::get(In->getType(), ShiftAmt));
+
+ // Distribute the bit over the whole bit width.
+ In = Builder->CreateAShr(In, ConstantInt::get(In->getType(),
+ BitWidth - 1), "sext");
+ }
+
+ if (CI.getType() == In->getType())
+ return ReplaceInstUsesWith(CI, In);
+ return CastInst::CreateIntegerCast(In, CI.getType(), true/*SExt*/);
+ }
+ }
+ }
+
+ // vector (x <s 0) ? -1 : 0 -> ashr x, 31 -> all ones if signed.
+ if (VectorType *VTy = dyn_cast<VectorType>(CI.getType())) {
+ if (Pred == ICmpInst::ICMP_SLT && match(Op1, m_Zero()) &&
+ Op0->getType() == CI.getType()) {
+ Type *EltTy = VTy->getElementType();
+
+ // splat the shift constant to a constant vector.
+ Constant *VSh = ConstantInt::get(VTy, EltTy->getScalarSizeInBits()-1);
+ Value *In = Builder->CreateAShr(Op0, VSh, Op0->getName()+".lobit");
+ return ReplaceInstUsesWith(CI, In);
+ }
+ }
+
+ return 0;
+}
+