[IndVars] Eliminate op.with.overflow when possible

Summary:
If we can prove that an op.with.overflow intrinsic does not overflow, we
can get rid of the intrinsic, and replace it with non-wrapping
arithmetic.

Reviewers: atrick, regehr

Subscribers: sanjoy, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D18685

llvm-svn: 265913

1 files changed, 107 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
index c95f10e1b7b..e542d403c7e 100644
--- a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -71,6 +71,7 @@ namespace {
 
     bool eliminateIdentitySCEV(Instruction *UseInst, Instruction *IVOperand);
 
+    bool eliminateOverflowIntrinsic(CallInst *CI);
     bool eliminateIVUser(Instruction *UseInst, Instruction *IVOperand);
     void eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand);
     void eliminateIVRemainder(BinaryOperator *Rem, Value *IVOperand,
@@ -318,6 +319,108 @@ void SimplifyIndvar::eliminateIVRemainder(BinaryOperator *Rem,
   DeadInsts.emplace_back(Rem);
 }
 
+bool SimplifyIndvar::eliminateOverflowIntrinsic(CallInst *CI) {
+  auto *F = CI->getCalledFunction();
+  if (!F)
+    return false;
+
+  typedef const SCEV *(ScalarEvolution::*OperationFunctionTy)(
+      const SCEV *, const SCEV *, SCEV::NoWrapFlags);
+  typedef const SCEV *(ScalarEvolution::*ExtensionFunctionTy)(
+      const SCEV *, Type *);
+
+  OperationFunctionTy Operation;
+  ExtensionFunctionTy Extension;
+
+  Instruction::BinaryOps RawOp;
+
+  // We always have exactly one of nsw or nuw.  If NoSignedOverflow is false, we
+  // have nuw.
+  bool NoSignedOverflow;
+
+  switch (F->getIntrinsicID()) {
+  default:
+    return false;
+
+  case Intrinsic::sadd_with_overflow:
+    Operation = &ScalarEvolution::getAddExpr;
+    Extension = &ScalarEvolution::getSignExtendExpr;
+    RawOp = Instruction::Add;
+    NoSignedOverflow = true;
+    break;
+
+  case Intrinsic::uadd_with_overflow:
+    Operation = &ScalarEvolution::getAddExpr;
+    Extension = &ScalarEvolution::getZeroExtendExpr;
+    RawOp = Instruction::Add;
+    NoSignedOverflow = false;
+    break;
+
+  case Intrinsic::ssub_with_overflow:
+    Operation = &ScalarEvolution::getMinusSCEV;
+    Extension = &ScalarEvolution::getSignExtendExpr;
+    RawOp = Instruction::Sub;
+    NoSignedOverflow = true;
+    break;
+
+  case Intrinsic::usub_with_overflow:
+    Operation = &ScalarEvolution::getMinusSCEV;
+    Extension = &ScalarEvolution::getZeroExtendExpr;
+    RawOp = Instruction::Sub;
+    NoSignedOverflow = false;
+    break;
+  }
+
+  const SCEV *LHS = SE->getSCEV(CI->getArgOperand(0));
+  const SCEV *RHS = SE->getSCEV(CI->getArgOperand(1));
+
+  auto *NarrowTy = cast<IntegerType>(LHS->getType());
+  auto *WideTy =
+    IntegerType::get(NarrowTy->getContext(), NarrowTy->getBitWidth() * 2);
+
+  const SCEV *A =
+      (SE->*Extension)((SE->*Operation)(LHS, RHS, SCEV::FlagAnyWrap), WideTy);
+  const SCEV *B =
+      (SE->*Operation)((SE->*Extension)(LHS, WideTy),
+                       (SE->*Extension)(RHS, WideTy), SCEV::FlagAnyWrap);
+
+  if (A != B)
+    return false;
+
+  // Proved no overflow, nuke the overflow check and, if possible, the overflow
+  // intrinsic as well.
+
+  BinaryOperator *NewResult = BinaryOperator::Create(
+      RawOp, CI->getArgOperand(0), CI->getArgOperand(1), "", CI);
+
+  if (NoSignedOverflow)
+    NewResult->setHasNoSignedWrap(true);
+  else
+    NewResult->setHasNoUnsignedWrap(true);
+
+  SmallVector<ExtractValueInst *, 4> ToDelete;
+
+  for (auto *U : CI->users()) {
+    if (auto *EVI = dyn_cast<ExtractValueInst>(U)) {
+      if (EVI->getIndices()[0] == 1)
+        EVI->replaceAllUsesWith(ConstantInt::getFalse(CI->getContext()));
+      else {
+        assert(EVI->getIndices()[0] == 0 && "Only two possibilities!");
+        EVI->replaceAllUsesWith(NewResult);
+      }
+      ToDelete.push_back(EVI);
+    }
+  }
+
+  for (auto *EVI : ToDelete)
+    EVI->eraseFromParent();
+
+  if (CI->use_empty())
+    CI->eraseFromParent();
+
+  return true;
+}
+
 /// Eliminate an operation that consumes a simple IV and has no observable
 /// side-effect given the range of IV values.  IVOperand is guaranteed SCEVable,
 /// but UseInst may not be.
@@ -335,6 +438,10 @@ bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
     }
   }
 
+  if (auto *CI = dyn_cast<CallInst>(UseInst))
+    if (eliminateOverflowIntrinsic(CI))
+      return true;
+
   if (eliminateIdentitySCEV(UseInst, IVOperand))
     return true;