[llvm] [LIR][SCEVExpander] Restore original flags when aborting transform (PR #82362)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

SCEVExpanderCleaner will currently remove instructions created by SCEVExpander, but not restore poison generating flags that it may have dropped. As such, running LIR can currently spuriously drop flags without performing any transforms.

Fix this by keeping track of original instruction flags in SCEVExpander.

Fixes https://github.com/llvm/llvm-project/issues/82337.

---
Full diff: https://github.com/llvm/llvm-project/pull/82362.diff


3 Files Affected:

- (modified) llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h (+20) 
- (modified) llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp (+42) 
- (modified) llvm/test/Transforms/LoopIdiom/pr82337.ll (+3-3) 


``````````diff
diff --git a/llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h b/llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h
index 035705b7f4b78b..955e6d967088ac 100644
--- a/llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h
+++ b/llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h
@@ -41,6 +41,17 @@ struct SCEVOperand {
   const SCEV* S;
 };
 
+struct PoisonFlags {
+  unsigned NUW : 1;
+  unsigned NSW : 1;
+  unsigned Exact : 1;
+  unsigned Disjoint : 1;
+  unsigned NNeg : 1;
+
+  PoisonFlags(const Instruction *I);
+  void apply(Instruction *I);
+};
+
 /// This class uses information about analyze scalars to rewrite expressions
 /// in canonical form.
 ///
@@ -48,6 +59,8 @@ struct SCEVOperand {
 /// and destroy it when finished to allow the release of the associated
 /// memory.
 class SCEVExpander : public SCEVVisitor<SCEVExpander, Value *> {
+  friend class SCEVExpanderCleaner;
+
   ScalarEvolution &SE;
   const DataLayout &DL;
 
@@ -70,6 +83,10 @@ class SCEVExpander : public SCEVVisitor<SCEVExpander, Value *> {
   /// InsertedValues/InsertedPostIncValues.
   SmallPtrSet<Value *, 16> ReusedValues;
 
+  /// Original flags of instructions for which they were modified. Used
+  /// by SCEVExpanderCleaner to undo changes.
+  DenseMap<AssertingVH<Instruction>, PoisonFlags> OrigFlags;
+
   // The induction variables generated.
   SmallVector<WeakVH, 2> InsertedIVs;
 
@@ -188,6 +205,7 @@ class SCEVExpander : public SCEVVisitor<SCEVExpander, Value *> {
     InsertedValues.clear();
     InsertedPostIncValues.clear();
     ReusedValues.clear();
+    OrigFlags.clear();
     ChainedPhis.clear();
     InsertedIVs.clear();
   }
@@ -483,6 +501,8 @@ class SCEVExpander : public SCEVVisitor<SCEVExpander, Value *> {
 
   void rememberInstruction(Value *I);
 
+  void rememberFlags(Instruction *I);
+
   bool isNormalAddRecExprPHI(PHINode *PN, Instruction *IncV, const Loop *L);
 
   bool isExpandedAddRecExprPHI(PHINode *PN, Instruction *IncV, const Loop *L);
diff --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index 3a28909473d95f..2e6e5c310faab7 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -43,6 +43,37 @@ cl::opt<unsigned> llvm::SCEVCheapExpansionBudget(
 
 using namespace PatternMatch;
 
+PoisonFlags::PoisonFlags(const Instruction *I) {
+  NUW = false;
+  NSW = false;
+  Exact = false;
+  Disjoint = false;
+  NNeg = false;
+  if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(I)) {
+    NUW = OBO->hasNoUnsignedWrap();
+    NSW = OBO->hasNoUnsignedWrap();
+  }
+  if (auto *PEO = dyn_cast<PossiblyExactOperator>(I))
+    Exact = PEO->isExact();
+  if (auto *PDI = dyn_cast<PossiblyDisjointInst>(I))
+    Disjoint = PDI->isDisjoint();
+  if (auto *PNI = dyn_cast<PossiblyNonNegInst>(I))
+    NNeg = PNI->hasNonNeg();
+}
+
+void PoisonFlags::apply(Instruction *I) {
+  if (isa<OverflowingBinaryOperator>(I)) {
+    I->setHasNoUnsignedWrap(NUW);
+    I->setHasNoUnsignedWrap(NSW);
+  }
+  if (isa<PossiblyExactOperator>(I))
+    I->setIsExact(Exact);
+  if (auto *PDI = dyn_cast<PossiblyDisjointInst>(I))
+    PDI->setIsDisjoint(Disjoint);
+  if (auto *PNI = dyn_cast<PossiblyNonNegInst>(I))
+    PNI->setNonNeg(NNeg);
+}
+
 /// ReuseOrCreateCast - Arrange for there to be a cast of V to Ty at IP,
 /// reusing an existing cast if a suitable one (= dominating IP) exists, or
 /// creating a new one.
@@ -724,6 +755,7 @@ bool SCEVExpander::hoistIVInc(Instruction *IncV, Instruction *InsertPos,
   auto FixupPoisonFlags = [this](Instruction *I) {
     // Drop flags that are potentially inferred from old context and infer flags
     // in new context.
+    rememberFlags(I);
     I->dropPoisonGeneratingFlags();
     if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(I))
       if (auto Flags = SE.getStrengthenedNoWrapFlagsFromBinOp(OBO)) {
@@ -1472,6 +1504,7 @@ Value *SCEVExpander::expand(const SCEV *S) {
     V = fixupLCSSAFormFor(V);
   } else {
     for (Instruction *I : DropPoisonGeneratingInsts) {
+      rememberFlags(I);
       I->dropPoisonGeneratingFlagsAndMetadata();
       // See if we can re-infer from first principles any of the flags we just
       // dropped.
@@ -1512,6 +1545,11 @@ void SCEVExpander::rememberInstruction(Value *I) {
   DoInsert(I);
 }
 
+void SCEVExpander::rememberFlags(Instruction *I) {
+  // If we already have flags for the instruction, keep the existing ones.
+  OrigFlags.try_emplace(I, PoisonFlags(I));
+}
+
 void SCEVExpander::replaceCongruentIVInc(
     PHINode *&Phi, PHINode *&OrigPhi, Loop *L, const DominatorTree *DT,
     SmallVectorImpl<WeakTrackingVH> &DeadInsts) {
@@ -2309,6 +2347,10 @@ void SCEVExpanderCleaner::cleanup() {
   if (ResultUsed)
     return;
 
+  // Restore original poison flags.
+  for (auto [I, Flags] : Expander.OrigFlags)
+    Flags.apply(I);
+
   auto InsertedInstructions = Expander.getAllInsertedInstructions();
 #ifndef NDEBUG
   SmallPtrSet<Instruction *, 8> InsertedSet(InsertedInstructions.begin(),
diff --git a/llvm/test/Transforms/LoopIdiom/pr82337.ll b/llvm/test/Transforms/LoopIdiom/pr82337.ll
index e8a6e1704f7c17..da9eb14af3f0a4 100644
--- a/llvm/test/Transforms/LoopIdiom/pr82337.ll
+++ b/llvm/test/Transforms/LoopIdiom/pr82337.ll
@@ -1,15 +1,15 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
 ; RUN: opt -S -passes=loop-idiom < %s | FileCheck %s
 
-; FIXME: The poison flags should be preserved, as no transform takes place.
+; The poison flags should be preserved, as no transform takes place.
 define void @test(ptr %p.end, ptr %p.start) {
 ; CHECK-LABEL: define void @test(
 ; CHECK-SAME: ptr [[P_END:%.*]], ptr [[P_START:%.*]]) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[P_END_INT:%.*]] = ptrtoint ptr [[P_END]] to i64
 ; CHECK-NEXT:    [[P_START_INT:%.*]] = ptrtoint ptr [[P_START]] to i64
-; CHECK-NEXT:    [[DIST:%.*]] = sub i64 [[P_END_INT]], [[P_START_INT]]
-; CHECK-NEXT:    [[LEN:%.*]] = lshr i64 [[DIST]], 5
+; CHECK-NEXT:    [[DIST:%.*]] = sub nuw i64 [[P_END_INT]], [[P_START_INT]]
+; CHECK-NEXT:    [[LEN:%.*]] = lshr exact i64 [[DIST]], 5
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr [[P_END]], [[P_START]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[EXIT:%.*]], label [[PREHEADER:%.*]]
 ; CHECK:       preheader:

``````````

</details>


https://github.com/llvm/llvm-project/pull/82362