[PATCH] D62792: [SimplifyIndVar] Simplify non-overflowing saturating add/sub

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: reames, spatel, lebedev.ri.
Herald added subscribers: llvm-commits, sanjoy, javed.absar, hiraditya.
Herald added a project: LLVM.

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 <https://reviews.llvm.org/D62703>, just based on a different underlying analysis (SCEV vs LVI) that catches different cases.

One case this doesn't handle yet is if the IV calculation itself uses a saturating counter.


Repository:
  rL LLVM

https://reviews.llvm.org/D62792

Files:
  llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
  llvm/test/Transforms/IndVarSimplify/eliminate-sat.ll


Index: llvm/test/Transforms/IndVarSimplify/eliminate-sat.ll
===================================================================
--- llvm/test/Transforms/IndVarSimplify/eliminate-sat.ll
+++ llvm/test/Transforms/IndVarSimplify/eliminate-sat.ll
@@ -12,8 +12,8 @@
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[I_INC:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[SAT:%.*]] = call i32 @llvm.uadd.sat.i32(i32 [[I]], i32 1)
-; CHECK-NEXT:    store volatile i32 [[SAT]], i32* [[P:%.*]]
+; CHECK-NEXT:    [[SAT1:%.*]] = add nuw nsw i32 [[I]], 1
+; CHECK-NEXT:    store volatile i32 [[SAT1]], i32* [[P:%.*]]
 ; CHECK-NEXT:    [[I_INC]] = add nuw nsw i32 [[I]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i32 [[I_INC]], 100
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP]], label [[END:%.*]]
@@ -41,8 +41,8 @@
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[I_INC:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[SAT:%.*]] = call i32 @llvm.sadd.sat.i32(i32 [[I]], i32 1)
-; CHECK-NEXT:    store volatile i32 [[SAT]], i32* [[P:%.*]]
+; CHECK-NEXT:    [[SAT1:%.*]] = add nuw nsw i32 [[I]], 1
+; CHECK-NEXT:    store volatile i32 [[SAT1]], i32* [[P:%.*]]
 ; CHECK-NEXT:    [[I_INC]] = add nuw nsw i32 [[I]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i32 [[I_INC]], 100
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP]], label [[END:%.*]]
@@ -70,8 +70,8 @@
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ [[I_INC:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[SAT:%.*]] = call i32 @llvm.usub.sat.i32(i32 [[I]], i32 1)
-; CHECK-NEXT:    store volatile i32 [[SAT]], i32* [[P:%.*]]
+; CHECK-NEXT:    [[SAT1:%.*]] = sub nuw nsw i32 [[I]], 1
+; CHECK-NEXT:    store volatile i32 [[SAT1]], i32* [[P:%.*]]
 ; CHECK-NEXT:    [[I_INC]] = add nuw nsw i32 [[I]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i32 [[I_INC]], 100
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP]], label [[END:%.*]]
@@ -99,8 +99,8 @@
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[I_INC:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[SAT:%.*]] = call i32 @llvm.ssub.sat.i32(i32 [[I]], i32 1)
-; CHECK-NEXT:    store volatile i32 [[SAT]], i32* [[P:%.*]]
+; CHECK-NEXT:    [[SAT1:%.*]] = sub nsw i32 [[I]], 1
+; CHECK-NEXT:    store volatile i32 [[SAT1]], i32* [[P:%.*]]
 ; CHECK-NEXT:    [[I_INC]] = add nuw nsw i32 [[I]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i32 [[I_INC]], 100
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP]], label [[END:%.*]]
Index: llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
+++ llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -81,6 +81,7 @@
     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 @@
   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
@@ -613,6 +633,10 @@
     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;


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