[IRCE] Refactor parseRangeCheckICmp to compute SCEVs instead of Values

[aleksandr.popov]

Summary

The motivation is to make an opportunity to compute and return
expressions after parsing ICmp into a range check (e.g. Length + 1)

[mkazantsev]

Looks pretty straightforward.

llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp

368–369

*Index = nullptr, just to avoid errors in the future.

llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp

 class InductiveRangeCheck {
 
   const SCEV *Begin = nullptr;
   const SCEV *Step = nullptr;
   const SCEV *End = nullptr;
   Use *CheckUse = nullptr;
 
   static bool parseRangeCheckICmp(Loop *L, ICmpInst *ICI, ScalarEvolution &SE,
-                                  Value *&Index, Value *&Length);
+                                  const SCEV *&Index, const SCEV *&End);
 
   static void
   extractRangeChecksFromCond(Loop *L, ScalarEvolution &SE, Use &ConditionUse,
                              SmallVectorImpl<InductiveRangeCheck> &Checks,
                              SmallPtrSetImpl<Value *> &Visited);
 
 public:
   const SCEV *getBegin() const { return Begin; }

 INITIALIZE_PASS_DEPENDENCY(BranchProbabilityInfoWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
 INITIALIZE_PASS_END(IRCELegacyPass, "irce", "Inductive range check elimination",
                     false, false)
 
 /// Parse a single ICmp instruction, `ICI`, into a range check.  If `ICI` cannot
-/// be interpreted as a range check, return false and set `Index` and `Length`
-/// to `nullptr`.  Otherwise set `Index` to the value being range checked, and
-/// set `Length` to the upper limit `Index` is being range checked.
-bool
-InductiveRangeCheck::parseRangeCheckICmp(Loop *L, ICmpInst *ICI,
-                                         ScalarEvolution &SE, Value *&Index,
-                                         Value *&Length) {
+/// be interpreted as a range check, return false and set `Index` and `End`
+/// to `nullptr`.  Otherwise set `Index` to the SCEV being range checked, and
+/// set `End` to the upper limit `Index` is being range checked.
+bool InductiveRangeCheck::parseRangeCheckICmp(Loop *L, ICmpInst *ICI,
+                                              ScalarEvolution &SE,
+                                              const SCEV *&Index,
+                                              const SCEV *&End) {
   auto IsLoopInvariant = [&SE, L](Value *V) {
     return SE.isLoopInvariant(SE.getSCEV(V), L);
   };
 
   ICmpInst::Predicate Pred = ICI->getPredicate();
   Value *LHS = ICI->getOperand(0);
   Value *RHS = ICI->getOperand(1);
 
   switch (Pred) {
   default:
     return false;
 
   case ICmpInst::ICMP_SLE:
     std::swap(LHS, RHS);
     [[fallthrough]];
   case ICmpInst::ICMP_SGE:
     if (match(RHS, m_ConstantInt<0>())) {
-      Index = LHS;
+      Index = SE.getSCEV(LHS);
       return true; // Lower.
     }
     return false;
 
   case ICmpInst::ICMP_SLT:
     std::swap(LHS, RHS);
     [[fallthrough]];
   case ICmpInst::ICMP_SGT:
     if (match(RHS, m_ConstantInt<-1>())) {
-      Index = LHS;
+      Index = SE.getSCEV(LHS);
       return true; // Lower.
     }
 
     if (IsLoopInvariant(LHS)) {
-      Index = RHS;
-      Length = LHS;
+      Index = SE.getSCEV(RHS);
+      End = SE.getSCEV(LHS);
       return true; // Upper.
     }
     return false;
 
   case ICmpInst::ICMP_ULT:
     std::swap(LHS, RHS);
     [[fallthrough]];
   case ICmpInst::ICMP_UGT:
     if (IsLoopInvariant(LHS)) {
-      Index = RHS;
-      Length = LHS;
+      Index = SE.getSCEV(RHS);
+      End = SE.getSCEV(LHS);
       return true; // Both lower and upper.
     }
     return false;
   }
 
   llvm_unreachable("default clause returns!");
 }
 
extractRangeChecksFromCond(L, SE, cast<User>(Condition)->getOperandUse(1),
                                Checks, Visited);
     return;
   }
 
   ICmpInst *ICI = dyn_cast<ICmpInst>(Condition);
   if (!ICI)
     return;
 
-  Value *Length = nullptr, *Index;
-  if (!parseRangeCheckICmp(L, ICI, SE, Index, Length))
+  const SCEV *End = nullptr, *Index = nullptr;
+  if (!parseRangeCheckICmp(L, ICI, SE, Index, End))
     return;
 
-  const auto *IndexAddRec = dyn_cast<SCEVAddRecExpr>(SE.getSCEV(Index));
+  const auto *IndexAddRec = dyn_cast<SCEVAddRecExpr>(Index);
   bool IsAffineIndex =
       IndexAddRec && (IndexAddRec->getLoop() == L) && IndexAddRec->isAffine();
 
   if (!IsAffineIndex)
     return;
 
-  const SCEV *End = nullptr;
   // We strengthen "0 <= I" to "0 <= I < INT_SMAX" and "I < L" to "0 <= I < L".
   // We can potentially do much better here.
-  if (Length)
-    End = SE.getSCEV(Length);
-  else {
+  if (!End) {
     // So far we can only reach this point for Signed range check. This may
     // change in future. In this case we will need to pick Unsigned max for the
     // unsigned range check.
     unsigned BitWidth = cast<IntegerType>(IndexAddRec->getType())->getBitWidth();
     const SCEV *SIntMax = SE.getConstant(APInt::getSignedMaxValue(BitWidth));
     End = SIntMax;
   }