[SCCP] Improve worklist management

[nikic]

SCCP currently stores instructions whose lattice value has changed in a worklist, and then updates their users in the main loop. This may result in instructions unnecessarily being visited multiple times (as an instruction will often use multiple other instructions). Additionally, we'd often redundantly visit instructions that were already visited when the containing block first became executable.

Instead, change the worklist to directly store the instructions that need to be revisited. Additionally, do not add instructions to the worklist that will already be covered by the main basic block walk.

This change is conceptually NFC, but is expected to produce minor differences in practice, because the visitation order interacts with the range widening limit.

Compile-time improvement: https://llvm-compile-time-tracker.com/compare.php?from=7354123c34e658e990559a36b1ac7eb0b671e317&to=8f1fac228d7ebbddd8a2aa00e47ee3f5712db125&stat=instructions:u

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-function-specialization

Author: Nikita Popov (nikic)

Changes

SCCP currently stores instructions whose lattice value has changed in a worklist, and then updates their users in the main loop. This may result in instructions unnecessarily being visited multiple times (as an instruction will often use multiple other instructions). Additionally, we'd often redundantly visit instructions that were already visited when the containing block first became executable.

Instead, change the worklist to directly store the instructions that need to be revisited. Additionally, do not add instructions to the worklist that will already be covered by the main basic block walk.

This change is conceptually NFC, but is expected to produce minor difference in practice, because the visitation order interacts with the range widening limit.

Compile-time improvement: https://llvm-compile-time-tracker.com/compare.php?from=7354123c34e658e990559a36b1ac7eb0b671e317&to=8f1fac228d7ebbddd8a2aa00e47ee3f5712db125&stat=instructions:u

---

Full diff: https://github.com/llvm/llvm-project/pull/146321.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/Utils/SCCPSolver.cpp (+66-101)

diff --git a/llvm/lib/Transforms/Utils/SCCPSolver.cpp b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
index cb6aafa279b5f..d72e9e9ea5c56 100644
--- a/llvm/lib/Transforms/Utils/SCCPSolver.cpp
+++ b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
@@ -426,7 +426,10 @@ void SCCPSolver::inferArgAttributes() const {
 class SCCPInstVisitor : public InstVisitor<SCCPInstVisitor> {
   const DataLayout &DL;
   std::function<const TargetLibraryInfo &(Function &)> GetTLI;
-  SmallPtrSet<BasicBlock *, 8> BBExecutable; // The BBs that are executable.
+  /// Basic blocks that are executable (but may not have been visited yet).
+  SmallPtrSet<BasicBlock *, 8> BBExecutable;
+  /// Basic blocks that are executable and have been visited at least once.
+  SmallPtrSet<BasicBlock *, 8> BBVisited;
   DenseMap<Value *, ValueLatticeElement>
       ValueState; // The state each value is in.
 
@@ -466,15 +469,14 @@ class SCCPInstVisitor : public InstVisitor<SCCPInstVisitor> {
   /// constants.
   SmallPtrSet<Function *, 16> TrackingIncomingArguments;
 
-  /// The reason for two worklists is that overdefined is the lowest state
-  /// on the lattice, and moving things to overdefined as fast as possible
-  /// makes SCCP converge much faster.
-  ///
-  /// By having a separate worklist, we accomplish this because everything
-  /// possibly overdefined will become overdefined at the soonest possible
-  /// point.
-  SmallVector<Value *, 64> OverdefinedInstWorkList;
-  SmallVector<Value *, 64> InstWorkList;
+  /// Worklist of instructions to re-visit. This only includes instructions
+  /// in blocks that have already been visited at least once.
+  SmallSetVector<Instruction *, 16> InstWorkList;
+
+  /// Current instruction while visiting a block for the first time, used to
+  /// avoid unnecessary instruction worklist insertions. Null if an instruction
+  /// is visited outside a whole-block visitation.
+  Instruction *CurI = nullptr;
 
   // The BasicBlock work list
   SmallVector<BasicBlock *, 64> BBWorkList;
@@ -497,12 +499,15 @@ class SCCPInstVisitor : public InstVisitor<SCCPInstVisitor> {
     return dyn_cast_or_null<ConstantInt>(getConstant(IV, Ty));
   }
 
-  // pushToWorkList - Helper for markConstant/markOverdefined
-  void pushToWorkList(ValueLatticeElement &IV, Value *V);
+  /// Push instruction \p I to the worklist.
+  void pushToWorkList(Instruction *I);
+
+  /// Push users of value \p V to the worklist.
+  void pushUsersToWorkList(Value *V);
 
-  // Helper to push \p V to the worklist, after updating it to \p IV. Also
-  // prints a debug message with the updated value.
-  void pushToWorkListMsg(ValueLatticeElement &IV, Value *V);
+  /// Like pushUsersToWorkList(), but also prints a debug message with the
+  /// updated value.
+  void pushUsersToWorkListMsg(ValueLatticeElement &IV, Value *V);
 
   // markConstant - Make a value be marked as "constant".  If the value
   // is not already a constant, add it to the instruction work list so that
@@ -661,46 +666,9 @@ class SCCPInstVisitor : public InstVisitor<SCCPInstVisitor> {
   // successors are reachable from a given terminator instruction.
   void getFeasibleSuccessors(Instruction &TI, SmallVectorImpl<bool> &Succs);
 
-  // OperandChangedState - This method is invoked on all of the users of an
-  // instruction that was just changed state somehow.  Based on this
-  // information, we need to update the specified user of this instruction.
-  void operandChangedState(Instruction *I) {
-    if (BBExecutable.count(I->getParent())) // Inst is executable?
-      visit(*I);
-  }
-
   // Add U as additional user of V.
   void addAdditionalUser(Value *V, User *U) { AdditionalUsers[V].insert(U); }
 
-  // Mark I's users as changed, including AdditionalUsers.
-  void markUsersAsChanged(Value *I) {
-    // Functions include their arguments in the use-list. Changed function
-    // values mean that the result of the function changed. We only need to
-    // update the call sites with the new function result and do not have to
-    // propagate the call arguments.
-    if (isa<Function>(I)) {
-      for (User *U : I->users()) {
-        if (auto *CB = dyn_cast<CallBase>(U))
-          handleCallResult(*CB);
-      }
-    } else {
-      for (User *U : I->users())
-        if (auto *UI = dyn_cast<Instruction>(U))
-          operandChangedState(UI);
-    }
-
-    auto Iter = AdditionalUsers.find(I);
-    if (Iter != AdditionalUsers.end()) {
-      // Copy additional users before notifying them of changes, because new
-      // users may be added, potentially invalidating the iterator.
-      SmallVector<Instruction *, 2> ToNotify;
-      for (User *U : Iter->second)
-        if (auto *UI = dyn_cast<Instruction>(U))
-          ToNotify.push_back(UI);
-      for (Instruction *UI : ToNotify)
-        operandChangedState(UI);
-    }
-  }
   void handleCallOverdefined(CallBase &CB);
   void handleCallResult(CallBase &CB);
   void handleCallArguments(CallBase &CB);
@@ -985,19 +953,41 @@ bool SCCPInstVisitor::markBlockExecutable(BasicBlock *BB) {
   return true;
 }
 
-void SCCPInstVisitor::pushToWorkList(ValueLatticeElement &IV, Value *V) {
-  if (IV.isOverdefined()) {
-    if (OverdefinedInstWorkList.empty() || OverdefinedInstWorkList.back() != V)
-      OverdefinedInstWorkList.push_back(V);
+void SCCPInstVisitor::pushToWorkList(Instruction *I) {
+  // If we're currently visiting a block, do not push any instructions in the
+  // same blocks that are after the current one, as they will be visited
+  // anyway. We do have to push updates to earlier instructions (e.g. phi
+  // nodes or loads of tracked globals).
+  if (CurI && I->getParent() == CurI->getParent() && !I->comesBefore(CurI))
     return;
+  // Only push instructions in already visited blocks. Otherwise we'll handle
+  // it when we visit the block for the first time.
+  if (BBVisited.count(I->getParent()))
+    InstWorkList.insert(I);
+}
+
+void SCCPInstVisitor::pushUsersToWorkList(Value *V) {
+  for (User *U : V->users())
+    if (auto *UI = dyn_cast<Instruction>(U))
+      pushToWorkList(UI);
+
+  auto Iter = AdditionalUsers.find(V);
+  if (Iter != AdditionalUsers.end()) {
+    // Copy additional users before notifying them of changes, because new
+    // users may be added, potentially invalidating the iterator.
+    SmallVector<Instruction *, 2> ToNotify;
+    for (User *U : Iter->second)
+      if (auto *UI = dyn_cast<Instruction>(U))
+        ToNotify.push_back(UI);
+    for (Instruction *UI : ToNotify)
+      pushToWorkList(UI);
   }
-  if (InstWorkList.empty() || InstWorkList.back() != V)
-    InstWorkList.push_back(V);
 }
 
-void SCCPInstVisitor::pushToWorkListMsg(ValueLatticeElement &IV, Value *V) {
+void SCCPInstVisitor::pushUsersToWorkListMsg(ValueLatticeElement &IV,
+                                             Value *V) {
   LLVM_DEBUG(dbgs() << "updated " << IV << ": " << *V << '\n');
-  pushToWorkList(IV, V);
+  pushUsersToWorkList(V);
 }
 
 bool SCCPInstVisitor::markConstant(ValueLatticeElement &IV, Value *V,
@@ -1005,7 +995,7 @@ bool SCCPInstVisitor::markConstant(ValueLatticeElement &IV, Value *V,
   if (!IV.markConstant(C, MayIncludeUndef))
     return false;
   LLVM_DEBUG(dbgs() << "markConstant: " << *C << ": " << *V << '\n');
-  pushToWorkList(IV, V);
+  pushUsersToWorkList(V);
   return true;
 }
 
@@ -1014,7 +1004,7 @@ bool SCCPInstVisitor::markNotConstant(ValueLatticeElement &IV, Value *V,
   if (!IV.markNotConstant(C))
     return false;
   LLVM_DEBUG(dbgs() << "markNotConstant: " << *C << ": " << *V << '\n');
-  pushToWorkList(IV, V);
+  pushUsersToWorkList(V);
   return true;
 }
 
@@ -1023,7 +1013,7 @@ bool SCCPInstVisitor::markConstantRange(ValueLatticeElement &IV, Value *V,
   if (!IV.markConstantRange(CR))
     return false;
   LLVM_DEBUG(dbgs() << "markConstantRange: " << CR << ": " << *V << '\n');
-  pushToWorkList(IV, V);
+  pushUsersToWorkList(V);
   return true;
 }
 
@@ -1036,7 +1026,7 @@ bool SCCPInstVisitor::markOverdefined(ValueLatticeElement &IV, Value *V) {
              << "Function '" << F->getName() << "'\n";
              else dbgs() << *V << '\n');
   // Only instructions go on the work list
-  pushToWorkList(IV, V);
+  pushUsersToWorkList(V);
   return true;
 }
 
@@ -1144,7 +1134,7 @@ bool SCCPInstVisitor::mergeInValue(ValueLatticeElement &IV, Value *V,
                                    ValueLatticeElement MergeWithV,
                                    ValueLatticeElement::MergeOptions Opts) {
   if (IV.mergeIn(MergeWithV, Opts)) {
-    pushToWorkList(IV, V);
+    pushUsersToWorkList(V);
     LLVM_DEBUG(dbgs() << "Merged " << MergeWithV << " into " << *V << " : "
                       << IV << "\n");
     return true;
@@ -1164,7 +1154,7 @@ bool SCCPInstVisitor::markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest) {
                       << " -> " << Dest->getName() << '\n');
 
     for (PHINode &PN : Dest->phis())
-      visitPHINode(PN);
+      pushToWorkList(&PN);
   }
   return true;
 }
@@ -1540,7 +1530,7 @@ void SCCPInstVisitor::visitSelectInst(SelectInst &I) {
   bool Changed = State.mergeIn(TVal);
   Changed |= State.mergeIn(FVal);
   if (Changed)
-    pushToWorkListMsg(State, &I);
+    pushUsersToWorkListMsg(State, &I);
 }
 
 // Handle Unary Operators.
@@ -2039,53 +2029,28 @@ void SCCPInstVisitor::handleCallResult(CallBase &CB) {
 
 void SCCPInstVisitor::solve() {
   // Process the work lists until they are empty!
-  while (!BBWorkList.empty() || !InstWorkList.empty() ||
-         !OverdefinedInstWorkList.empty()) {
-    // Process the overdefined instruction's work list first, which drives other
-    // things to overdefined more quickly.
-    while (!OverdefinedInstWorkList.empty()) {
-      Value *I = OverdefinedInstWorkList.pop_back_val();
-      Invalidated.erase(I);
-
-      LLVM_DEBUG(dbgs() << "\nPopped off OI-WL: " << *I << '\n');
-
-      // "I" got into the work list because it either made the transition from
-      // bottom to constant, or to overdefined.
-      //
-      // Anything on this worklist that is overdefined need not be visited
-      // since all of its users will have already been marked as overdefined
-      // Update all of the users of this instruction's value.
-      //
-      markUsersAsChanged(I);
-    }
-
+  while (!BBWorkList.empty() || !InstWorkList.empty()) {
     // Process the instruction work list.
     while (!InstWorkList.empty()) {
-      Value *I = InstWorkList.pop_back_val();
+      Instruction *I = InstWorkList.pop_back_val();
       Invalidated.erase(I);
 
       LLVM_DEBUG(dbgs() << "\nPopped off I-WL: " << *I << '\n');
 
-      // "I" got into the work list because it made the transition from undef to
-      // constant.
-      //
-      // Anything on this worklist that is overdefined need not be visited
-      // since all of its users will have already been marked as overdefined.
-      // Update all of the users of this instruction's value.
-      //
-      if (I->getType()->isStructTy() || !getValueState(I).isOverdefined())
-        markUsersAsChanged(I);
+      visit(I);
     }
 
     // Process the basic block work list.
     while (!BBWorkList.empty()) {
       BasicBlock *BB = BBWorkList.pop_back_val();
+      BBVisited.insert(BB);
 
       LLVM_DEBUG(dbgs() << "\nPopped off BBWL: " << *BB << '\n');
-
-      // Notify all instructions in this basic block that they are newly
-      // executable.
-      visit(BB);
+      for (Instruction &I : *BB) {
+        CurI = &I;
+        visit(I);
+      }
+      CurI = nullptr;
     }
   }
 }

[dtcxzyw]

Is it still valuable to have a separate worklist for insts that contain an operand just marked as Overdefined?

[nikic]

I see a negative effects from using a separate overdefined worklist: https://llvm-compile-time-tracker.com/compare.php?from=8f1fac228d7ebbddd8a2aa00e47ee3f5712db125&to=715c96e212becce2bb05cffd7a468891547ef992&stat=instructions:u

[nikic]

Clang test failure is unrelated. llvm-opt-benchmark looks better on average at a glance.

[dtcxzyw]

LGTM. Thanks!

[fhahn]

Very nice, thanks!