[LICM] Make promotion faster

Even when MemorySSA-based LICM is used, an AST is still populated
for scalar promotion. As the AST has quadratic complexity, a lot
of time is spent in this step despite the existing access count
limit. This patch optimizes the identification of promotable stores.

The idea here is pretty simple: We're only interested in must-alias
mod sets of loop invariant pointers. As such, only populate the AST
with loop-invariant loads and stores (anything else is definitely
not promotable) and then discard any sets which alias with any of
the remaining, definitely non-promotable accesses.

If we promoted something, check whether this has made some other
accesses loop invariant and thus possible promotion candidates.

This is much faster in practice, because we need to perform AA
queries for O(NumPromotable^2 + NumPromotable*NumNonPromotable)
instead of O(NumTotal^2), and NumPromotable tends to be small.
Additionally, promotable accesses have loop invariant pointers,
for which AA is cheaper.

This has a signicant positive compile-time impact. We save ~1.8%
geomean on CTMark at O3, with 6% on lencod in particular and 25%
on individual files.

Conceptually, this change is NFC, but may not be so in practice,
because the AST is only an approximation, and can produce
different results depending on the order in which accesses are
added. However, there is at least no impact on the number of promotions
(licm.NumPromoted) in test-suite O3 configuration with this change.

Differential Revision: https://reviews.llvm.org/D89264

(cherry picked from commit 3d8f842)

Author: nikic

llvm/include/llvm/Analysis/AliasSetTracker.h

@@ -20,6 +20,7 @@
 #include "llvm/ADT/DenseMapInfo.h"
 #include "llvm/ADT/ilist.h"
 #include "llvm/ADT/ilist_node.h"
+#include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/MemoryLocation.h"
 #include "llvm/IR/Instruction.h"
 #include "llvm/IR/Metadata.h"
@@ -39,7 +40,6 @@ class AliasSetTracker;
 class BasicBlock;
 class LoadInst;
 class Loop;
-class MemorySSA;
 class AnyMemSetInst;
 class AnyMemTransferInst;
 class raw_ostream;
@@ -343,7 +343,6 @@ class AliasSetTracker {
   struct ASTCallbackVHDenseMapInfo : public DenseMapInfo<Value *> {};
 
   AAResults &AA;
-  MemorySSA *MSSA = nullptr;
   Loop *L = nullptr;
   ilist<AliasSet> AliasSets;
 
@@ -357,8 +356,6 @@ class AliasSetTracker {
   /// Create an empty collection of AliasSets, and use the specified alias
   /// analysis object to disambiguate load and store addresses.
   explicit AliasSetTracker(AAResults &AA) : AA(AA) {}
-  explicit AliasSetTracker(AAResults &AA, MemorySSA *MSSA, Loop *L)
-      : AA(AA), MSSA(MSSA), L(L) {}
   ~AliasSetTracker() { clear(); }
 
   /// These methods are used to add different types of instructions to the alias
@@ -383,7 +380,6 @@ class AliasSetTracker {
   void add(BasicBlock &BB);       // Add all instructions in basic block
   void add(const AliasSetTracker &AST); // Add alias relations from another AST
   void addUnknown(Instruction *I);
-  void addAllInstructionsInLoopUsingMSSA();
 
   void clear();
 

llvm/lib/Analysis/AliasSetTracker.cpp

@@ -14,7 +14,6 @@
 #include "llvm/Analysis/GuardUtils.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/MemoryLocation.h"
-#include "llvm/Analysis/MemorySSA.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
@@ -539,15 +538,6 @@ void AliasSetTracker::add(const AliasSetTracker &AST) {
   }
 }
 
-void AliasSetTracker::addAllInstructionsInLoopUsingMSSA() {
-  assert(MSSA && L && "MSSA and L must be available");
-  for (const BasicBlock *BB : L->blocks())
-    if (auto *Accesses = MSSA->getBlockAccesses(BB))
-      for (auto &Access : *Accesses)
-        if (auto *MUD = dyn_cast<MemoryUseOrDef>(&Access))
-          add(MUD->getMemoryInst());
-}
-
 // deleteValue method - This method is used to remove a pointer value from the
 // AliasSetTracker entirely.  It should be used when an instruction is deleted
 // from the program to update the AST.  If you don't use this, you would have

llvm/lib/Transforms/Scalar/LICM.cpp

@@ -185,6 +185,12 @@ static void moveInstructionBefore(Instruction &I, Instruction &Dest,
                                   ICFLoopSafetyInfo &SafetyInfo,
                                   MemorySSAUpdater *MSSAU, ScalarEvolution *SE);
 
+static void foreachMemoryAccess(MemorySSA *MSSA, Loop *L,
+                                function_ref<void(Instruction *)> Fn);
+static SmallVector<SmallSetVector<Value *, 8>, 0>
+collectPromotionCandidates(MemorySSA *MSSA, AliasAnalysis *AA, Loop *L,
+                           SmallVectorImpl<Instruction *> &MaybePromotable);
+
 namespace {
 struct LoopInvariantCodeMotion {
   bool runOnLoop(Loop *L, AAResults *AA, LoopInfo *LI, DominatorTree *DT,
@@ -203,9 +209,6 @@ struct LoopInvariantCodeMotion {
 
   std::unique_ptr<AliasSetTracker>
   collectAliasInfoForLoop(Loop *L, LoopInfo *LI, AAResults *AA);
-  std::unique_ptr<AliasSetTracker>
-  collectAliasInfoForLoopWithMSSA(Loop *L, AAResults *AA,
-                                  MemorySSAUpdater *MSSAU);
 };
 
 struct LegacyLICMPass : public LoopPass {
@@ -430,31 +433,48 @@ bool LoopInvariantCodeMotion::runOnLoop(
       PredIteratorCache PIC;
 
       bool Promoted = false;
-
-      // Build an AST using MSSA.
-      if (!CurAST.get())
-        CurAST = collectAliasInfoForLoopWithMSSA(L, AA, MSSAU.get());
-
-      // Loop over all of the alias sets in the tracker object.
-      for (AliasSet &AS : *CurAST) {
-        // We can promote this alias set if it has a store, if it is a "Must"
-        // alias set, if the pointer is loop invariant, and if we are not
-        // eliminating any volatile loads or stores.
-        if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
-            !L->isLoopInvariant(AS.begin()->getValue()))
-          continue;
-
-        assert(
-            !AS.empty() &&
-            "Must alias set should have at least one pointer element in it!");
-
-        SmallSetVector<Value *, 8> PointerMustAliases;
-        for (const auto &ASI : AS)
-          PointerMustAliases.insert(ASI.getValue());
-
-        Promoted |= promoteLoopAccessesToScalars(
-            PointerMustAliases, ExitBlocks, InsertPts, MSSAInsertPts, PIC, LI,
-            DT, TLI, L, CurAST.get(), MSSAU.get(), &SafetyInfo, ORE);
+      if (CurAST.get()) {
+        // Loop over all of the alias sets in the tracker object.
+        for (AliasSet &AS : *CurAST) {
+          // We can promote this alias set if it has a store, if it is a "Must"
+          // alias set, if the pointer is loop invariant, and if we are not
+          // eliminating any volatile loads or stores.
+          if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
+              !L->isLoopInvariant(AS.begin()->getValue()))
+            continue;
+
+          assert(
+              !AS.empty() &&
+              "Must alias set should have at least one pointer element in it!");
+
+          SmallSetVector<Value *, 8> PointerMustAliases;
+          for (const auto &ASI : AS)
+            PointerMustAliases.insert(ASI.getValue());
+
+          Promoted |= promoteLoopAccessesToScalars(
+              PointerMustAliases, ExitBlocks, InsertPts, MSSAInsertPts, PIC, LI,
+              DT, TLI, L, CurAST.get(), MSSAU.get(), &SafetyInfo, ORE);
+        }
+      } else {
+        SmallVector<Instruction *, 16> MaybePromotable;
+        foreachMemoryAccess(MSSA, L, [&](Instruction *I) {
+          MaybePromotable.push_back(I);
+        });
+
+        // Promoting one set of accesses may make the pointers for another set
+        // loop invariant, so run this in a loop (with the MaybePromotable set
+        // decreasing in size over time).
+        bool LocalPromoted;
+        do {
+          LocalPromoted = false;
+          for (const SmallSetVector<Value *, 8> &PointerMustAliases :
+               collectPromotionCandidates(MSSA, AA, L, MaybePromotable)) {
+            LocalPromoted |= promoteLoopAccessesToScalars(
+                PointerMustAliases, ExitBlocks, InsertPts, MSSAInsertPts, PIC,
+                LI, DT, TLI, L, /*AST*/nullptr, MSSAU.get(), &SafetyInfo, ORE);
+          }
+          Promoted |= LocalPromoted;
+        } while (LocalPromoted);
       }
 
       // Once we have promoted values across the loop body we have to
@@ -2218,6 +2238,77 @@ bool llvm::promoteLoopAccessesToScalars(
   return true;
 }
 
+static void foreachMemoryAccess(MemorySSA *MSSA, Loop *L,
+                                function_ref<void(Instruction *)> Fn) {
+  for (const BasicBlock *BB : L->blocks())
+    if (const auto *Accesses = MSSA->getBlockAccesses(BB))
+      for (const auto &Access : *Accesses)
+        if (const auto *MUD = dyn_cast<MemoryUseOrDef>(&Access))
+          Fn(MUD->getMemoryInst());
+}
+
+static SmallVector<SmallSetVector<Value *, 8>, 0>
+collectPromotionCandidates(MemorySSA *MSSA, AliasAnalysis *AA, Loop *L,
+                           SmallVectorImpl<Instruction *> &MaybePromotable) {
+  AliasSetTracker AST(*AA);
+
+  auto IsPotentiallyPromotable = [L](const Instruction *I) {
+    if (const auto *SI = dyn_cast<StoreInst>(I))
+      return L->isLoopInvariant(SI->getPointerOperand());
+    if (const auto *LI = dyn_cast<LoadInst>(I))
+      return L->isLoopInvariant(LI->getPointerOperand());
+    return false;
+  };
+
+  // Populate AST with potentially promotable accesses and remove them from
+  // MaybePromotable, so they will not be checked again on the next iteration.
+  SmallPtrSet<Value *, 16> AttemptingPromotion;
+  llvm::erase_if(MaybePromotable, [&](Instruction *I) {
+    if (IsPotentiallyPromotable(I)) {
+      AttemptingPromotion.insert(I);
+      AST.add(I);
+      return true;
+    }
+    return false;
+  });
+
+  // We're only interested in must-alias sets that contain a mod.
+  SmallVector<const AliasSet *, 8> Sets;
+  for (AliasSet &AS : AST)
+    if (!AS.isForwardingAliasSet() && AS.isMod() && AS.isMustAlias())
+      Sets.push_back(&AS);
+
+  if (Sets.empty())
+    return {}; // Nothing to promote...
+
+  // Discard any sets for which there is an aliasing non-promotable access.
+  foreachMemoryAccess(MSSA, L, [&](Instruction *I) {
+    if (AttemptingPromotion.contains(I))
+      return;
+
+    if (Optional<MemoryLocation> Loc = MemoryLocation::getOrNone(I)) {
+      llvm::erase_if(Sets, [&](const AliasSet *AS) {
+        return AS->aliasesPointer(Loc->Ptr, Loc->Size, Loc->AATags, *AA)
+               != NoAlias;
+      });
+    } else {
+      llvm::erase_if(Sets, [&](const AliasSet *AS) {
+        return AS->aliasesUnknownInst(I, *AA);
+      });
+    }
+  });
+
+  SmallVector<SmallSetVector<Value *, 8>, 0> Result;
+  for (const AliasSet *Set : Sets) {
+    SmallSetVector<Value *, 8> PointerMustAliases;
+    for (const auto &ASI : *Set)
+      PointerMustAliases.insert(ASI.getValue());
+    Result.push_back(std::move(PointerMustAliases));
+  }
+
+  return Result;
+}
+
 /// Returns an owning pointer to an alias set which incorporates aliasing info
 /// from L and all subloops of L.
 std::unique_ptr<AliasSetTracker>
@@ -2238,15 +2329,6 @@ LoopInvariantCodeMotion::collectAliasInfoForLoop(Loop *L, LoopInfo *LI,
   return CurAST;
 }
 
-std::unique_ptr<AliasSetTracker>
-LoopInvariantCodeMotion::collectAliasInfoForLoopWithMSSA(
-    Loop *L, AAResults *AA, MemorySSAUpdater *MSSAU) {
-  auto *MSSA = MSSAU->getMemorySSA();
-  auto CurAST = std::make_unique<AliasSetTracker>(*AA, MSSA, L);
-  CurAST->addAllInstructionsInLoopUsingMSSA();
-  return CurAST;
-}
-
 static bool pointerInvalidatedByLoop(MemoryLocation MemLoc,
                                      AliasSetTracker *CurAST, Loop *CurLoop,
                                      AAResults *AA) {