[SimplifyIndVar] Simplify non-overflowing saturating add/sub

If we can detect that saturating math that depends on an IV cannot
overflow, replace it with simple math. This is similar to the CVP
optimization from D62703, just based on a different underlying
analysis (SCEV vs LVI) that catches different cases.

Differential Revision: https://reviews.llvm.org/D62792

llvm-svn: 363489

1 files changed, 24 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
index f1048d98f00..cbb114f9a47 100644
--- a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -81,6 +81,7 @@ namespace {
     bool replaceIVUserWithLoopInvariant(Instruction *UseInst);
 
     bool eliminateOverflowIntrinsic(WithOverflowInst *WO);
+    bool eliminateSaturatingIntrinsic(SaturatingInst *SI);
     bool eliminateTrunc(TruncInst *TI);
     bool eliminateIVUser(Instruction *UseInst, Instruction *IVOperand);
     bool makeIVComparisonInvariant(ICmpInst *ICmp, Value *IVOperand);
@@ -477,6 +478,25 @@ bool SimplifyIndvar::eliminateOverflowIntrinsic(WithOverflowInst *WO) {
   return true;
 }
 
+bool SimplifyIndvar::eliminateSaturatingIntrinsic(SaturatingInst *SI) {
+  const SCEV *LHS = SE->getSCEV(SI->getLHS());
+  const SCEV *RHS = SE->getSCEV(SI->getRHS());
+  if (!willNotOverflow(SE, SI->getBinaryOp(), SI->isSigned(), LHS, RHS))
+    return false;
+
+  BinaryOperator *BO = BinaryOperator::Create(
+      SI->getBinaryOp(), SI->getLHS(), SI->getRHS(), SI->getName(), SI);
+  if (SI->isSigned())
+    BO->setHasNoSignedWrap();
+  else
+    BO->setHasNoUnsignedWrap();
+
+  SI->replaceAllUsesWith(BO);
+  DeadInsts.emplace_back(SI);
+  Changed = true;
+  return true;
+}
+
 bool SimplifyIndvar::eliminateTrunc(TruncInst *TI) {
   // It is always legal to replace
   //   icmp <pred> i32 trunc(iv), n
@@ -614,6 +634,10 @@ bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
     if (eliminateOverflowIntrinsic(WO))
       return true;
 
+  if (auto *SI = dyn_cast<SaturatingInst>(UseInst))
+    if (eliminateSaturatingIntrinsic(SI))
+      return true;
+
   if (auto *TI = dyn_cast<TruncInst>(UseInst))
     if (eliminateTrunc(TI))
       return true;