[IndVarSimplify] use the "canonical" way to infer no-wrap.

[sanjoy]

Summary

rL225282 introduced an ad-hoc way to promote some additions to nuw or
nsw. Since then SCEV has become smarter in directly proving no-wrap;
and using the canonical "ext(A op B) == ext(A) op ext(B)" method of
proving no-wrap is just as powerful now. Rip out the existing
complexity in favor of getting SCEV to do all the heaving lifting
internally.

This change does not add any unit tests because it is supposed to be a
non-functional change. Tests added in rL225282 and rL226075 are valid
tests for this change.

[sanjoy]

improve a comment

[atrick]

Thank You! Thank You!

lib/Transforms/Utils/SimplifyIndVar.cpp

bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
   return true;
 }
 
 /// Annotate BO with nsw / nuw if it provably does not signed-overflow /
 /// unsigned-overflow.  Returns true if anything changed, false otherwise.
 bool SimplifyIndvar::strengthenOverflowingOperation(BinaryOperator *BO,
                                                     Value *IVOperand) {
 
-  // Currently we only handle instructions of the form "add <indvar> <value>"
-  unsigned Op = BO->getOpcode();
-  if (Op != Instruction::Add)
-    return false;
-
-  // If BO is already both nuw and nsw then there is nothing left to do
+  // Fastpath: we don't have any work to do if `BO` is `nuw` and `nsw`.
   if (BO->hasNoUnsignedWrap() && BO->hasNoSignedWrap())
     return false;
 
-  IntegerType *IT = cast<IntegerType>(IVOperand->getType());
-  Value *OtherOperand = nullptr;
-  if (BO->getOperand(0) == IVOperand) {
-    OtherOperand = BO->getOperand(1);
-  } else {
-    assert(BO->getOperand(1) == IVOperand && "only other use!");
-    OtherOperand = BO->getOperand(0);
-  }
+  const SCEV *(ScalarEvolution::*GetExprForBO)(const SCEV *, const SCEV *,
+                                               SCEV::NoWrapFlags);
 
-  bool Changed = false;
-  const SCEV *OtherOpSCEV = SE->getSCEV(OtherOperand);
-  if (OtherOpSCEV == SE->getCouldNotCompute())
+  switch (BO->getOpcode()) {
+  default:
     return false;
 
-  const SCEV *IVOpSCEV = SE->getSCEV(IVOperand);
-  const SCEV *ZeroSCEV = SE->getConstant(IVOpSCEV->getType(), 0);
+  case Instruction::Add:
+    GetExprForBO = &ScalarEvolution::getAddExpr;
+    break;
 
-  if (!BO->hasNoSignedWrap()) {
-    // Upgrade the add to an "add nsw" if we can prove that it will never
-    // sign-overflow or sign-underflow.
+  case Instruction::Sub:
+    GetExprForBO = &ScalarEvolution::getMinusSCEV;
+    break;
 
-    const SCEV *SignedMax =
-      SE->getConstant(APInt::getSignedMaxValue(IT->getBitWidth()));
-    const SCEV *SignedMin =
-      SE->getConstant(APInt::getSignedMinValue(IT->getBitWidth()));
+  case Instruction::Mul:
+    GetExprForBO = &ScalarEvolution::getMulExpr;
+    break;
+  }
 
-    // The addition "IVOperand + OtherOp" does not sign-overflow if the result
-    // is sign-representable in 2's complement in the given bit-width.
-    //
-    // If OtherOp is SLT 0, then for an IVOperand in [SignedMin - OtherOp,
-    // SignedMax], "IVOperand + OtherOp" is in [SignedMin, SignedMax + OtherOp].
-    // Everything in [SignedMin, SignedMax + OtherOp] is representable since
-    // SignedMax + OtherOp is at least -1.
-    //
-    // If OtherOp is SGE 0, then for an IVOperand in [SignedMin, SignedMax -
-    // OtherOp], "IVOperand + OtherOp" is in [SignedMin + OtherOp, SignedMax].
-    // Everything in [SignedMin + OtherOp, SignedMax] is representable since
-    // SignedMin + OtherOp is at most -1.
-    //
-    // It follows that for all values of IVOperand in [SignedMin - smin(0,
-    // OtherOp), SignedMax - smax(0, OtherOp)] the result of the add is
-    // representable (i.e. there is no sign-overflow).
-
-    const SCEV *UpperDelta = SE->getSMaxExpr(ZeroSCEV, OtherOpSCEV);
-    const SCEV *UpperLimit = SE->getMinusSCEV(SignedMax, UpperDelta);
-
-    bool NeverSignedOverflows =
-      SE->isKnownPredicate(ICmpInst::ICMP_SLE, IVOpSCEV, UpperLimit);
-
-    if (NeverSignedOverflows) {
-      const SCEV *LowerDelta = SE->getSMinExpr(ZeroSCEV, OtherOpSCEV);
-      const SCEV *LowerLimit = SE->getMinusSCEV(SignedMin, LowerDelta);
-
-      bool NeverSignedUnderflows =
-        SE->isKnownPredicate(ICmpInst::ICMP_SGE, IVOpSCEV, LowerLimit);
-      if (NeverSignedUnderflows) {
-        BO->setHasNoSignedWrap(true);
-        Changed = true;
-      }
-    }
-  }
+  unsigned BitWidth = cast<IntegerType>(BO->getType())->getBitWidth();
+  Type *WideTy = IntegerType::get(BO->getContext(), BitWidth * 2);
+  const SCEV *LHS = SE->getSCEV(BO->getOperand(0));
+  const SCEV *RHS = SE->getSCEV(BO->getOperand(1));
+
+  bool Changed = false;
+
+  if (!BO->hasNoUnsignedWrap()) {
+    const SCEV *ExtendAfterOp = SE->getZeroExtendExpr(SE->getSCEV(BO), WideTy);
+    const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
+      SE->getZeroExtendExpr(LHS, WideTy), SE->getZeroExtendExpr(RHS, WideTy),
+      SCEV::FlagAnyWrap);
+    if (ExtendAfterOp == OpAfterExtend) {
+      BO->setHasNoUnsignedWrap();
+      SE->forgetValue(BO);
 
-  if (!BO->hasNoUnsignedWrap()) {
-    // Upgrade the add computing "IVOperand + OtherOp" to an "add nuw" if we can
-    // prove that it will never unsigned-overflow (i.e. the result will always
-    // be representable in the given bit-width).
-    //
-    // "IVOperand + OtherOp" is unsigned-representable in 2's complement iff it
-    // does not produce a carry.  "IVOperand + OtherOp" produces no carry iff
-    // IVOperand ULE (UnsignedMax - OtherOp).
-
-    const SCEV *UnsignedMax =
-      SE->getConstant(APInt::getMaxValue(IT->getBitWidth()));
-    const SCEV *UpperLimit = SE->getMinusSCEV(UnsignedMax, OtherOpSCEV);
-
-    bool NeverUnsignedOverflows =
-        SE->isKnownPredicate(ICmpInst::ICMP_ULE, IVOpSCEV, UpperLimit);
-
-    if (NeverUnsignedOverflows) {
-      BO->setHasNoUnsignedWrap(true);
+  if (!BO->hasNoSignedWrap()) {
+    const SCEV *ExtendAfterOp = SE->getSignExtendExpr(SE->getSCEV(BO), WideTy);
+    const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
+      SE->getSignExtendExpr(LHS, WideTy), SE->getSignExtendExpr(RHS, WideTy),
+      SCEV::FlagAnyWrap);
+    if (ExtendAfterOp == OpAfterExtend) {
+      BO->setHasNoSignedWrap();
+      SE->forgetValue(BO);
       Changed = true;
     }
   }
 
   return Changed;
 }
 
 /// \brief Split sadd.with.overflow into add + sadd.with.overflow to allow