[LICM] Cap the clobbering calls in LICM.

[asbirlea]

Summary

Unlimitted number of calls to getClobberingAccess can lead to high
compile times in pathological cases.
Switching EnableLicmCap flag from bool to int, and enabling to default 100.
(tested to be appropriate for current bechmarks)
We can revisit this value when enabling MemorySSA.

[chandlerc]

LGTM

[asbirlea]

I disregarded the possiblity of threading in the initial diff.
Updated to pass the counter as an argument.
Kindly re-review.

[george.burgess.iv]

Thanks for this!

One currently-actionable nit from me

include/llvm/Transforms/Utils/LoopUtils.h

115

Nit: I'm OK with this as is, but if we grow another MSSA-related optimization limit/counter, please wrap the MSSA-related bits into some sort of options struct (a bit in spirit like llvm::ObjectSizeOpts, but we'd also be tracking the mutable EnableLicmCapCounter/etc in it)

lib/Transforms/Scalar/LICM.cpp

119

nit: enable sounds pretty bool-y for an int. Can we swap to licm-mssa-optimization-cap (or similar)?

[asbirlea]

Address comment (renamed the flag, and the counter to match it).

[asbirlea]

include/llvm/Transforms/Utils/LoopUtils.h

115

Noted, and I agree! I considered adding this in this patch, actually.
Additional refactoring may help somewhat, since the only user of sinkRegion and hoistRegion is LICM. We'd still needs these bits wrapped up for canSinkAndHoist which is used in LoopSink.
Leaving it as a separate cleanup for now.

[george.burgess.iv]

Deferring to Chandler's LGTM for the approach + first patch; changes since then LGTM.

Thanks again!

include/llvm/Transforms/Utils/LoopUtils.h

115

SGTM

include/llvm/Transforms/Utils/LoopUtils.h

 /// uses before definitions, allowing us to sink a loop body in one pass without
 /// iteration. Takes DomTreeNode, AliasAnalysis, LoopInfo, DominatorTree,
 /// DataLayout, TargetLibraryInfo, Loop, AliasSet information for all
 /// instructions of the loop and loop safety information as
 /// arguments. Diagnostics is emitted via \p ORE. It returns changed status.
 bool sinkRegion(DomTreeNode *, AliasAnalysis *, LoopInfo *, DominatorTree *,
                 TargetLibraryInfo *, TargetTransformInfo *, Loop *,
                 AliasSetTracker *, MemorySSAUpdater *, ICFLoopSafetyInfo *,
-                bool, OptimizationRemarkEmitter *);
+                bool, int &, OptimizationRemarkEmitter *);
 
 /// Walk the specified region of the CFG (defined by all blocks
 /// dominated by the specified block, and that are in the current loop) in depth
 /// first order w.r.t the DominatorTree.  This allows us to visit definitions
 /// before uses, allowing us to hoist a loop body in one pass without iteration.
 /// Takes DomTreeNode, AliasAnalysis, LoopInfo, DominatorTree, DataLayout,
 /// TargetLibraryInfo, Loop, AliasSet information for all instructions of the
 /// loop and loop safety information as arguments. Diagnostics is emitted via \p
 /// ORE. It returns changed status.
 bool hoistRegion(DomTreeNode *, AliasAnalysis *, LoopInfo *, DominatorTree *,
                  TargetLibraryInfo *, Loop *, AliasSetTracker *,
-                 MemorySSAUpdater *, ICFLoopSafetyInfo *, bool,
+                 MemorySSAUpdater *, ICFLoopSafetyInfo *, bool, int &,
                  OptimizationRemarkEmitter *);
 
 /// This function deletes dead loops. The caller of this function needs to
 /// guarantee that the loop is infact dead.
 /// The function requires a bunch or prerequisites to be present:
 ///   - The loop needs to be in LCSSA form
 ///   - The loop needs to have a Preheader
 ///   - A unique dedicated exit block must exist

 /// target executes at most once per execution of the loop body.  This is used
 /// to assess the legality of duplicating atomic loads.  Generally, this is
 /// true when moving out of loop and not true when moving into loops.
 /// If \p ORE is set use it to emit optimization remarks.
 bool canSinkOrHoistInst(Instruction &I, AAResults *AA, DominatorTree *DT,
                         Loop *CurLoop, AliasSetTracker *CurAST,
                         MemorySSAUpdater *MSSAU, bool TargetExecutesOncePerLoop,
                         bool NoOfMemAccessesTooLarge,
+                        int *LicmMssaOptCap = nullptr,
                         OptimizationRemarkEmitter *ORE = nullptr);
 
 /// Returns a Min/Max operation corresponding to MinMaxRecurrenceKind.
 Value *createMinMaxOp(IRBuilder<> &Builder,
                       RecurrenceDescriptor::MinMaxRecurrenceKind RK,
                       Value *Left, Value *Right);
 
 /// Generates an ordered vector reduction using extracts to reduce the value.

lib/Transforms/Scalar/LICM.cpp

 
 // Default value of zero implies we use the regular alias set tracker mechanism
 // instead of the cross product using AA to identify aliasing of the memory
 // location we are interested in.
 static cl::opt<int>
 LICMN2Theshold("licm-n2-threshold", cl::Hidden, cl::init(0),
                cl::desc("How many instruction to cross product using AA"));
 
-// Experimental option to allow imprecision in LICM (use MemorySSA cap) in
-// pathological cases, in exchange for faster compile. This is to be removed
-// if MemorySSA starts to address the same issue. This flag applies only when
-// LICM uses MemorySSA instead on AliasSetTracker. When the flag is disabled
-// (default), LICM calls MemorySSAWalker's getClobberingMemoryAccess, which
-// gets perfect accuracy. When flag is enabled, LICM will call into MemorySSA's
-// getDefiningAccess, which may not be precise, since optimizeUses is capped.
-static cl::opt<bool> EnableLicmCap(
-    "enable-licm-cap", cl::init(false), cl::Hidden,
-    cl::desc("Enable imprecision in LICM (uses MemorySSA cap) in "
-             "pathological cases, in exchange for faster compile"));
+// Experimental option to allow imprecision in LICM in pathological cases, in
+// exchange for faster compile. This is to be removed if MemorySSA starts to
+// address the same issue. This flag applies only when LICM uses MemorySSA
+// instead on AliasSetTracker. LICM calls MemorySSAWalker's
+// getClobberingMemoryAccess, up to the value of the Cap, getting perfect
+// accuracy. Afterwards, LICM will call into MemorySSA's getDefiningAccess,
+// which may not be precise, since optimizeUses is capped. The result is
+// correct, but we may not get as "far up" as possible to get which access is
+// clobbering the one queried.
+static cl::opt<int> LicmMssaOptCap(
+    "licm-mssa-optimization-cap", cl::init(100), cl::Hidden,
+    cl::desc("Enable imprecision in LICM in pathological cases, in exchange "
+             "for faster compile. Caps the MemorySSA clobbering calls."));
 
 // Experimentally, memory promotion carries less importance than sinking and
 // hoisting. Limit when we do promotion when using MemorySSA, in order to save
 // compile time.
 static cl::opt<unsigned> AccessCapForMSSAPromotion(
     "max-acc-licm-promotion", cl::init(250), cl::Hidden,
     cl::desc("[LICM & MemorySSA] When MSSA in LICM is disabled, this has no "
              "effect. When MSSA in LICM is enabled, then this is the maximum "
static bool isSafeToExecuteUnconditionally(Instruction &Inst,
                                            const Loop *CurLoop,
                                            const LoopSafetyInfo *SafetyInfo,
                                            OptimizationRemarkEmitter *ORE,
                                            const Instruction *CtxI = nullptr);
 static bool pointerInvalidatedByLoop(MemoryLocation MemLoc,
                                      AliasSetTracker *CurAST, Loop *CurLoop,
                                      AliasAnalysis *AA);
 static bool pointerInvalidatedByLoopWithMSSA(MemorySSA *MSSA, MemoryUse *MU,
-                                             Loop *CurLoop);
+                                             Loop *CurLoop,
+                                             int &LicmMssaOptCounter);
 static Instruction *CloneInstructionInExitBlock(
     Instruction &I, BasicBlock &ExitBlock, PHINode &PN, const LoopInfo *LI,
     const LoopSafetyInfo *SafetyInfo, MemorySSAUpdater *MSSAU);
 
 static void eraseInstruction(Instruction &I, ICFLoopSafetyInfo &SafetyInfo,
                              AliasSetTracker *AST, MemorySSAUpdater *MSSAU);
 
 static void moveInstructionBefore(Instruction &I, Instruction &Dest,
bool LoopInvariantCodeMotion::runOnLoop(
     MemorySSA *MSSA, OptimizationRemarkEmitter *ORE, bool DeleteAST) {
   bool Changed = false;
 
   assert(L->isLCSSAForm(*DT) && "Loop is not in LCSSA form.");
 
   std::unique_ptr<AliasSetTracker> CurAST;
   std::unique_ptr<MemorySSAUpdater> MSSAU;
   bool NoOfMemAccTooLarge = false;
+  int LicmMssaOptCounter = 0;
 
   if (!MSSA) {
     LLVM_DEBUG(dbgs() << "LICM: Using Alias Set Tracker.\n");
     CurAST = collectAliasInfoForLoop(L, LI, AA);
   } else {
     LLVM_DEBUG(dbgs() << "LICM: Using MemorySSA.\n");
     MSSAU = make_unique<MemorySSAUpdater>(MSSA);
 
bool LoopInvariantCodeMotion::runOnLoop(
   // Traverse the body of the loop in depth first order on the dominator tree so
   // that we are guaranteed to see definitions before we see uses.  This allows
   // us to sink instructions in one pass, without iteration.  After sinking
   // instructions, we perform another pass to hoist them out of the loop.
   //
   if (L->hasDedicatedExits())
     Changed |= sinkRegion(DT->getNode(L->getHeader()), AA, LI, DT, TLI, TTI, L,
                           CurAST.get(), MSSAU.get(), &SafetyInfo,
-                          NoOfMemAccTooLarge, ORE);
+                          NoOfMemAccTooLarge, LicmMssaOptCounter, ORE);
   if (Preheader)
     Changed |= hoistRegion(DT->getNode(L->getHeader()), AA, LI, DT, TLI, L,
                            CurAST.get(), MSSAU.get(), &SafetyInfo,
-                           NoOfMemAccTooLarge, ORE);
+                           NoOfMemAccTooLarge, LicmMssaOptCounter, ORE);
 
   // Now that all loop invariants have been removed from the loop, promote any
   // memory references to scalars that we can.
   // Don't sink stores from loops without dedicated block exits. Exits
   // containing indirect branches are not transformed by loop simplify,
   // make sure we catch that. An additional load may be generated in the
   // preheader for SSA updater, so also avoid sinking when no preheader
   // is available.

 /// first order w.r.t the DominatorTree.  This allows us to visit uses before
 /// definitions, allowing us to sink a loop body in one pass without iteration.
 ///
 bool llvm::sinkRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
                       DominatorTree *DT, TargetLibraryInfo *TLI,
                       TargetTransformInfo *TTI, Loop *CurLoop,
                       AliasSetTracker *CurAST, MemorySSAUpdater *MSSAU,
                       ICFLoopSafetyInfo *SafetyInfo, bool NoOfMemAccTooLarge,
-                      OptimizationRemarkEmitter *ORE) {
+                      int &LicmMssaOptCounter, OptimizationRemarkEmitter *ORE) {
 
   // Verify inputs.
   assert(N != nullptr && AA != nullptr && LI != nullptr && DT != nullptr &&
          CurLoop != nullptr && SafetyInfo != nullptr &&
          "Unexpected input to sinkRegion.");
   assert(((CurAST != nullptr) ^ (MSSAU != nullptr)) &&
          "Either AliasSetTracker or MemorySSA should be initialized.");
 
for (BasicBlock::iterator II = BB->end(); II != BB->begin();) {
       // Check to see if we can sink this instruction to the exit blocks
       // of the loop.  We can do this if the all users of the instruction are
       // outside of the loop.  In this case, it doesn't even matter if the
       // operands of the instruction are loop invariant.
       //
       bool FreeInLoop = false;
       if (isNotUsedOrFreeInLoop(I, CurLoop, SafetyInfo, TTI, FreeInLoop) &&
           canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, MSSAU, true,
-                             NoOfMemAccTooLarge, ORE) &&
+                             NoOfMemAccTooLarge, &LicmMssaOptCounter, ORE) &&
           !I.mayHaveSideEffects()) {
         if (sink(I, LI, DT, CurLoop, SafetyInfo, MSSAU, ORE, FreeInLoop)) {
           if (!FreeInLoop) {
             ++II;
             eraseInstruction(I, *SafetyInfo, CurAST, MSSAU);
           }
           Changed = true;
         }

 /// the specified block, and that are in the current loop) in depth first
 /// order w.r.t the DominatorTree.  This allows us to visit definitions before
 /// uses, allowing us to hoist a loop body in one pass without iteration.
 ///
 bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
                        DominatorTree *DT, TargetLibraryInfo *TLI, Loop *CurLoop,
                        AliasSetTracker *CurAST, MemorySSAUpdater *MSSAU,
                        ICFLoopSafetyInfo *SafetyInfo, bool NoOfMemAccTooLarge,
+                       int &LicmMssaOptCounter,
                        OptimizationRemarkEmitter *ORE) {
   // Verify inputs.
   assert(N != nullptr && AA != nullptr && LI != nullptr && DT != nullptr &&
          CurLoop != nullptr && SafetyInfo != nullptr &&
          "Unexpected input to hoistRegion.");
   assert(((CurAST != nullptr) ^ (MSSAU != nullptr)) &&
          "Either AliasSetTracker or MemorySSA should be initialized.");
 
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
       // Try hoisting the instruction out to the preheader.  We can only do
       // this if all of the operands of the instruction are loop invariant and
       // if it is safe to hoist the instruction.
       // TODO: It may be safe to hoist if we are hoisting to a conditional block
       // and we have accurately duplicated the control flow from the loop header
       // to that block.
       if (CurLoop->hasLoopInvariantOperands(&I) &&
           canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, MSSAU, true,
-                             NoOfMemAccTooLarge, ORE) &&
+                             NoOfMemAccTooLarge, &LicmMssaOptCounter, ORE) &&
           isSafeToExecuteUnconditionally(
               I, DT, CurLoop, SafetyInfo, ORE,
               CurLoop->getLoopPreheader()->getTerminator())) {
         hoist(I, DT, CurLoop, CFH.getOrCreateHoistedBlock(BB), SafetyInfo,
               MSSAU, ORE);
         HoistedInstructions.push_back(&I);
         Changed = true;
         continue;
bool isOnlyMemoryAccess(const Instruction *I, const Loop *L,
   return true;
 }
 }
 
 bool llvm::canSinkOrHoistInst(Instruction &I, AAResults *AA, DominatorTree *DT,
                               Loop *CurLoop, AliasSetTracker *CurAST,
                               MemorySSAUpdater *MSSAU,
                               bool TargetExecutesOncePerLoop,
-                              bool NoOfMemAccTooLarge,
+                              bool NoOfMemAccTooLarge, int *LicmMssaOptCounter,
                               OptimizationRemarkEmitter *ORE) {
   // If we don't understand the instruction, bail early.
   if (!isHoistableAndSinkableInst(I))
     return false;
 
   MemorySSA *MSSA = MSSAU ? MSSAU->getMemorySSA() : nullptr;
+  if (MSSA)
+    assert(LicmMssaOptCounter != nullptr && "Counter cannot be null.");
 
   // Loads have extra constraints we have to verify before we can hoist them.
   if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
     if (!LI->isUnordered())
       return false; // Don't sink/hoist volatile or ordered atomic loads!
 
     // Loads from constant memory are always safe to move, even if they end up
     // in the same alias set as something that ends up being modified.
if (isLoadInvariantInLoop(LI, DT, CurLoop))
       return true;
 
     bool Invalidated;
     if (CurAST)
       Invalidated = pointerInvalidatedByLoop(MemoryLocation::get(LI), CurAST,
                                              CurLoop, AA);
     else
       Invalidated = pointerInvalidatedByLoopWithMSSA(
-          MSSA, cast<MemoryUse>(MSSA->getMemoryAccess(LI)), CurLoop);
+          MSSA, cast<MemoryUse>(MSSA->getMemoryAccess(LI)), CurLoop,
+          *LicmMssaOptCounter);
     // Check loop-invariant address because this may also be a sinkable load
     // whose address is not necessarily loop-invariant.
     if (ORE && Invalidated && CurLoop->isLoopInvariant(LI->getPointerOperand()))
       ORE->emit([&]() {
         return OptimizationRemarkMissed(
                    DEBUG_TYPE, "LoadWithLoopInvariantAddressInvalidated", LI)
                << "failed to move load with loop-invariant address "
                   "because the loop may invalidate its value";
if (AliasAnalysis::onlyReadsMemory(Behavior)) {
           if (Op->getType()->isPointerTy()) {
             bool Invalidated;
             if (CurAST)
               Invalidated = pointerInvalidatedByLoop(
                   MemoryLocation(Op, LocationSize::unknown(), AAMDNodes()),
                   CurAST, CurLoop, AA);
             else
               Invalidated = pointerInvalidatedByLoopWithMSSA(
-                  MSSA, cast<MemoryUse>(MSSA->getMemoryAccess(CI)), CurLoop);
+                  MSSA, cast<MemoryUse>(MSSA->getMemoryAccess(CI)), CurLoop,
+                  *LicmMssaOptCounter);
             if (Invalidated)
               return false;
           }
         return true;
       }
 
       // If this call only reads from memory and there are no writes to memory
       // in the loop, we can hoist or sink the call as appropriate.
if (CurAST) {
       if (!UniqueI)
         // other memory op, give up
         return false;
       assert(UniqueI == SI && "AS must contain SI");
       return true;
     } else { // MSSAU
       if (isOnlyMemoryAccess(SI, CurLoop, MSSAU))
         return true;
-      if (!EnableLicmCap) {
+      if (*LicmMssaOptCounter < LicmMssaOptCap) {
         auto *Source = MSSA->getSkipSelfWalker()->getClobberingMemoryAccess(SI);
+        (*LicmMssaOptCounter)++;
         // If there are no clobbering Defs in the loop, we still need to check
         // for interfering Uses. If there are more accesses than the Promotion
         // cap, give up, we're not walking a list that long. Otherwise, walk the
         // list, check each Use if it's optimized to an access outside the loop.
         // If yes, store is safe to hoist. This is fairly restrictive, but
         // conservatively correct.
         // TODO: Cache set of Uses on the first walk in runOnLoop, update when
         // moving accesses. Can also extend to dominating uses.
for (Instruction &I : *BB) {
         return true;
       }
     }
   LLVM_DEBUG(dbgs() << "Aliasing okay for " << *(MemLoc.Ptr) << "\n");
   return false;
 }
 
 static bool pointerInvalidatedByLoopWithMSSA(MemorySSA *MSSA, MemoryUse *MU,
-                                             Loop *CurLoop) {
+                                             Loop *CurLoop,
+                                             int &LicmMssaOptCounter) {
   MemoryAccess *Source;
-  // See declaration of EnableLicmCap for usage details.
-  if (EnableLicmCap)
+  // See declaration of LicmMssaOptCap for usage details.
+  if (LicmMssaOptCounter >= LicmMssaOptCap)
     Source = MU->getDefiningAccess();
-  else
+  else {
     Source = MSSA->getSkipSelfWalker()->getClobberingMemoryAccess(MU);
+    LicmMssaOptCounter++;
+  }
   return !MSSA->isLiveOnEntryDef(Source) &&
          CurLoop->contains(Source->getBlock());
 }
 
 /// Little predicate that returns true if the specified basic block is in
 /// a subloop of the current one, not the current one itself.
 ///
 static bool inSubLoop(BasicBlock *BB, Loop *CurLoop, LoopInfo *LI) {
   assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
   return LI->getLoopFor(BB) != CurLoop;
 }