+/// Try to reuse the switch table index compare. Following pattern:
+/// \code
+/// if (idx < tablesize)
+/// r = table[idx]; // table does not contain default_value
+/// else
+/// r = default_value;
+/// if (r != default_value)
+/// ...
+/// \endcode
+/// Is optimized to:
+/// \code
+/// cond = idx < tablesize;
+/// if (cond)
+/// r = table[idx];
+/// else
+/// r = default_value;
+/// if (cond)
+/// ...
+/// \endcode
+/// Jump threading will then eliminate the second if(cond).
+static void reuseTableCompare(User *PhiUser, BasicBlock *PhiBlock,
+ BranchInst *RangeCheckBranch, Constant *DefaultValue,
+ const SmallVectorImpl<std::pair<ConstantInt*, Constant*> >& Values) {
+
+ ICmpInst *CmpInst = dyn_cast<ICmpInst>(PhiUser);
+ if (!CmpInst)
+ return;
+
+ // We require that the compare is in the same block as the phi so that jump
+ // threading can do its work afterwards.
+ if (CmpInst->getParent() != PhiBlock)
+ return;
+
+ Constant *CmpOp1 = dyn_cast<Constant>(CmpInst->getOperand(1));
+ if (!CmpOp1)
+ return;
+
+ Value *RangeCmp = RangeCheckBranch->getCondition();
+ Constant *TrueConst = ConstantInt::getTrue(RangeCmp->getType());
+ Constant *FalseConst = ConstantInt::getFalse(RangeCmp->getType());
+
+ // Check if the compare with the default value is constant true or false.
+ Constant *DefaultConst = ConstantExpr::getICmp(CmpInst->getPredicate(),
+ DefaultValue, CmpOp1, true);
+ if (DefaultConst != TrueConst && DefaultConst != FalseConst)
+ return;
+
+ // Check if the compare with the case values is distinct from the default
+ // compare result.
+ for (auto ValuePair : Values) {
+ Constant *CaseConst = ConstantExpr::getICmp(CmpInst->getPredicate(),
+ ValuePair.second, CmpOp1, true);
+ if (!CaseConst || CaseConst == DefaultConst)
+ return;
+ assert((CaseConst == TrueConst || CaseConst == FalseConst) &&
+ "Expect true or false as compare result.");
+ }
+
+ // Check if the branch instruction dominates the phi node. It's a simple
+ // dominance check, but sufficient for our needs.
+ // Although this check is invariant in the calling loops, it's better to do it
+ // at this late stage. Practically we do it at most once for a switch.
+ BasicBlock *BranchBlock = RangeCheckBranch->getParent();
+ for (auto PI = pred_begin(PhiBlock), E = pred_end(PhiBlock); PI != E; ++PI) {
+ BasicBlock *Pred = *PI;
+ if (Pred != BranchBlock && Pred->getUniquePredecessor() != BranchBlock)
+ return;
+ }
+
+ if (DefaultConst == FalseConst) {
+ // The compare yields the same result. We can replace it.
+ CmpInst->replaceAllUsesWith(RangeCmp);
+ ++NumTableCmpReuses;
+ } else {
+ // The compare yields the same result, just inverted. We can replace it.
+ Value *InvertedTableCmp = BinaryOperator::CreateXor(RangeCmp,
+ ConstantInt::get(RangeCmp->getType(), 1), "inverted.cmp",
+ RangeCheckBranch);
+ CmpInst->replaceAllUsesWith(InvertedTableCmp);
+ ++NumTableCmpReuses;
+ }
+}
+