[MemProf] Suppress duplicate clones in the LTO backend (#161551)

In some cases due to phase ordering issues with re-cloning during
function assignment, we may end up with duplicate clones in the
summaries (calling the same set of callee clones and/or allocation
hints).

Ideally we would fix this in the thin link, but for now, detect and
suppress these in the LTO backend. In order to satisfy possibly
cross-module references, make each duplicate an alias to the first
identical copy, which gets materialized.

This reduces ThinLTO backend compile times.

Author: teresajohnson

llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp

@@ -29,6 +29,7 @@
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/StringExtras.h"
 #include "llvm/Analysis/MemoryProfileInfo.h"
 #include "llvm/Analysis/ModuleSummaryAnalysis.h"
 #include "llvm/Analysis/OptimizationRemarkEmitter.h"
@@ -40,6 +41,7 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/GraphWriter.h"
 #include "llvm/Support/InterleavedRange.h"
+#include "llvm/Support/SHA1.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/IPO.h"
 #include "llvm/Transforms/Utils/CallPromotionUtils.h"
@@ -60,6 +62,9 @@ STATISTIC(FunctionClonesThinBackend,
           "Number of function clones created during ThinLTO backend");
 STATISTIC(FunctionsClonedThinBackend,
           "Number of functions that had clones created during ThinLTO backend");
+STATISTIC(
+    FunctionCloneDuplicatesThinBackend,
+    "Number of function clone duplicates detected during ThinLTO backend");
 STATISTIC(AllocTypeNotCold, "Number of not cold static allocations (possibly "
                             "cloned) during whole program analysis");
 STATISTIC(AllocTypeCold, "Number of cold static allocations (possibly cloned) "
@@ -5186,19 +5191,127 @@ bool CallsiteContextGraph<DerivedCCG, FuncTy, CallTy>::assignFunctions() {
   return Changed;
 }
 
+// Compute a SHA1 hash of the callsite and alloc version information of clone I
+// in the summary, to use in detection of duplicate clones.
+uint64_t ComputeHash(const FunctionSummary *FS, unsigned I) {
+  SHA1 Hasher;
+  // Update hash with any callsites that call non-default (non-zero) callee
+  // versions.
+  for (auto &SN : FS->callsites()) {
+    // In theory all callsites and allocs in this function should have the same
+    // number of clone entries, but handle any discrepancies gracefully below
+    // for NDEBUG builds.
+    assert(
+        SN.Clones.size() > I &&
+        "Callsite summary has fewer entries than other summaries in function");
+    if (SN.Clones.size() <= I || !SN.Clones[I])
+      continue;
+    uint8_t Data[sizeof(SN.Clones[I])];
+    support::endian::write32le(Data, SN.Clones[I]);
+    Hasher.update(Data);
+  }
+  // Update hash with any allocs that have non-default (non-None) hints.
+  for (auto &AN : FS->allocs()) {
+    // In theory all callsites and allocs in this function should have the same
+    // number of clone entries, but handle any discrepancies gracefully below
+    // for NDEBUG builds.
+    assert(AN.Versions.size() > I &&
+           "Alloc summary has fewer entries than other summaries in function");
+    if (AN.Versions.size() <= I ||
+        (AllocationType)AN.Versions[I] == AllocationType::None)
+      continue;
+    Hasher.update(ArrayRef<uint8_t>(&AN.Versions[I], 1));
+  }
+  return support::endian::read64le(Hasher.result().data());
+}
+
 static SmallVector<std::unique_ptr<ValueToValueMapTy>, 4> createFunctionClones(
     Function &F, unsigned NumClones, Module &M, OptimizationRemarkEmitter &ORE,
     std::map<const Function *, SmallPtrSet<const GlobalAlias *, 1>>
-        &FuncToAliasMap) {
+        &FuncToAliasMap,
+    FunctionSummary *FS) {
+  auto TakeDeclNameAndReplace = [](GlobalValue *DeclGV, GlobalValue *NewGV) {
+    // We might have created this when adjusting callsite in another
+    // function. It should be a declaration.
+    assert(DeclGV->isDeclaration());
+    NewGV->takeName(DeclGV);
+    DeclGV->replaceAllUsesWith(NewGV);
+    DeclGV->eraseFromParent();
+  };
+
+  // Handle aliases to this function, and create analogous alias clones to the
+  // provided clone of this function.
+  auto CloneFuncAliases = [&](Function *NewF, unsigned I) {
+    if (!FuncToAliasMap.count(&F))
+      return;
+    for (auto *A : FuncToAliasMap[&F]) {
+      std::string AliasName = getMemProfFuncName(A->getName(), I);
+      auto *PrevA = M.getNamedAlias(AliasName);
+      auto *NewA = GlobalAlias::create(A->getValueType(),
+                                       A->getType()->getPointerAddressSpace(),
+                                       A->getLinkage(), AliasName, NewF);
+      NewA->copyAttributesFrom(A);
+      if (PrevA)
+        TakeDeclNameAndReplace(PrevA, NewA);
+    }
+  };
+
   // The first "clone" is the original copy, we should only call this if we
   // needed to create new clones.
   assert(NumClones > 1);
   SmallVector<std::unique_ptr<ValueToValueMapTy>, 4> VMaps;
   VMaps.reserve(NumClones - 1);
   FunctionsClonedThinBackend++;
+
+  // Map of hash of callsite/alloc versions to the instantiated function clone
+  // (possibly the original) implementing those calls. Used to avoid
+  // instantiating duplicate function clones.
+  // FIXME: Ideally the thin link would not generate such duplicate clones to
+  // start with, but right now it happens due to phase ordering in the function
+  // assignment and possible new clones that produces. We simply make each
+  // duplicate an alias to the matching instantiated clone recorded in the map
+  // (except for available_externally which are made declarations as they would
+  // be aliases in the prevailing module, and available_externally aliases are
+  // not well supported right now).
+  DenseMap<uint64_t, Function *> HashToFunc;
+
+  // Save the hash of the original function version.
+  HashToFunc[ComputeHash(FS, 0)] = &F;
+
   for (unsigned I = 1; I < NumClones; I++) {
     VMaps.emplace_back(std::make_unique<ValueToValueMapTy>());
+    std::string Name = getMemProfFuncName(F.getName(), I);
+    auto Hash = ComputeHash(FS, I);
+    // If this clone would duplicate a previously seen clone, don't generate the
+    // duplicate clone body, just make an alias to satisfy any (potentially
+    // cross-module) references.
+    if (HashToFunc.contains(Hash)) {
+      FunctionCloneDuplicatesThinBackend++;
+      auto *Func = HashToFunc[Hash];
+      if (Func->hasAvailableExternallyLinkage()) {
+        // Skip these as EliminateAvailableExternallyPass does not handle
+        // available_externally aliases correctly and we end up with an
+        // available_externally alias to a declaration. Just create a
+        // declaration for now as we know we will have a definition in another
+        // module.
+        auto Decl = M.getOrInsertFunction(Name, Func->getFunctionType());
+        ORE.emit(OptimizationRemark(DEBUG_TYPE, "MemprofClone", &F)
+                 << "created clone decl " << ore::NV("Decl", Decl.getCallee()));
+        continue;
+      }
+      auto *PrevF = M.getFunction(Name);
+      auto *Alias = GlobalAlias::create(Name, Func);
+      if (PrevF)
+        TakeDeclNameAndReplace(PrevF, Alias);
+      ORE.emit(OptimizationRemark(DEBUG_TYPE, "MemprofClone", &F)
+               << "created clone alias " << ore::NV("Alias", Alias));
+
+      // Now handle aliases to this function, and clone those as well.
+      CloneFuncAliases(Func, I);
+      continue;
+    }
     auto *NewF = CloneFunction(&F, *VMaps.back());
+    HashToFunc[Hash] = NewF;
     FunctionClonesThinBackend++;
     // Strip memprof and callsite metadata from clone as they are no longer
     // needed.
@@ -5208,40 +5321,17 @@ static SmallVector<std::unique_ptr<ValueToValueMapTy>, 4> createFunctionClones(
         Inst.setMetadata(LLVMContext::MD_callsite, nullptr);
       }
     }
-    std::string Name = getMemProfFuncName(F.getName(), I);
     auto *PrevF = M.getFunction(Name);
-    if (PrevF) {
-      // We might have created this when adjusting callsite in another
-      // function. It should be a declaration.
-      assert(PrevF->isDeclaration());
-      NewF->takeName(PrevF);
-      PrevF->replaceAllUsesWith(NewF);
-      PrevF->eraseFromParent();
-    } else
+    if (PrevF)
+      TakeDeclNameAndReplace(PrevF, NewF);
+    else
       NewF->setName(Name);
     updateSubprogramLinkageName(NewF, Name);
     ORE.emit(OptimizationRemark(DEBUG_TYPE, "MemprofClone", &F)
              << "created clone " << ore::NV("NewFunction", NewF));
 
     // Now handle aliases to this function, and clone those as well.
-    if (!FuncToAliasMap.count(&F))
-      continue;
-    for (auto *A : FuncToAliasMap[&F]) {
-      std::string Name = getMemProfFuncName(A->getName(), I);
-      auto *PrevA = M.getNamedAlias(Name);
-      auto *NewA = GlobalAlias::create(A->getValueType(),
-                                       A->getType()->getPointerAddressSpace(),
-                                       A->getLinkage(), Name, NewF);
-      NewA->copyAttributesFrom(A);
-      if (PrevA) {
-        // We might have created this when adjusting callsite in another
-        // function. It should be a declaration.
-        assert(PrevA->isDeclaration());
-        NewA->takeName(PrevA);
-        PrevA->replaceAllUsesWith(NewA);
-        PrevA->eraseFromParent();
-      }
-    }
+    CloneFuncAliases(NewF, I);
   }
   return VMaps;
 }
@@ -5401,7 +5491,7 @@ bool MemProfContextDisambiguation::applyImport(Module &M) {
     SmallVector<std::unique_ptr<ValueToValueMapTy>, 4> VMaps;
     bool ClonesCreated = false;
     unsigned NumClonesCreated = 0;
-    auto CloneFuncIfNeeded = [&](unsigned NumClones) {
+    auto CloneFuncIfNeeded = [&](unsigned NumClones, FunctionSummary *FS) {
       // We should at least have version 0 which is the original copy.
       assert(NumClones > 0);
       // If only one copy needed use original.
@@ -5415,7 +5505,7 @@ bool MemProfContextDisambiguation::applyImport(Module &M) {
         assert(NumClonesCreated == NumClones);
         return;
       }
-      VMaps = createFunctionClones(F, NumClones, M, ORE, FuncToAliasMap);
+      VMaps = createFunctionClones(F, NumClones, M, ORE, FuncToAliasMap, FS);
       // The first "clone" is the original copy, which doesn't have a VMap.
       assert(VMaps.size() == NumClones - 1);
       Changed = true;
@@ -5424,9 +5514,9 @@ bool MemProfContextDisambiguation::applyImport(Module &M) {
     };
 
     auto CloneCallsite = [&](const CallsiteInfo &StackNode, CallBase *CB,
-                             Function *CalledFunction) {
+                             Function *CalledFunction, FunctionSummary *FS) {
       // Perform cloning if not yet done.
-      CloneFuncIfNeeded(/*NumClones=*/StackNode.Clones.size());
+      CloneFuncIfNeeded(/*NumClones=*/StackNode.Clones.size(), FS);
 
       assert(!isMemProfClone(*CalledFunction));
 
@@ -5448,6 +5538,10 @@ bool MemProfContextDisambiguation::applyImport(Module &M) {
       // below.
       auto CalleeOrigName = CalledFunction->getName();
       for (unsigned J = 0; J < StackNode.Clones.size(); J++) {
+        // If the VMap is empty, this clone was a duplicate of another and was
+        // created as an alias or a declaration.
+        if (J > 0 && VMaps[J - 1]->empty())
+          continue;
         // Do nothing if this version calls the original version of its
         // callee.
         if (!StackNode.Clones[J])
@@ -5591,7 +5685,7 @@ bool MemProfContextDisambiguation::applyImport(Module &M) {
 #endif
 
           // Perform cloning if not yet done.
-          CloneFuncIfNeeded(/*NumClones=*/AllocNode.Versions.size());
+          CloneFuncIfNeeded(/*NumClones=*/AllocNode.Versions.size(), FS);
 
           OrigAllocsThinBackend++;
           AllocVersionsThinBackend += AllocNode.Versions.size();
@@ -5624,6 +5718,10 @@ bool MemProfContextDisambiguation::applyImport(Module &M) {
 
           // Update the allocation types per the summary info.
           for (unsigned J = 0; J < AllocNode.Versions.size(); J++) {
+            // If the VMap is empty, this clone was a duplicate of another and
+            // was created as an alias or a declaration.
+            if (J > 0 && VMaps[J - 1]->empty())
+              continue;
             // Ignore any that didn't get an assigned allocation type.
             if (AllocNode.Versions[J] == (uint8_t)AllocationType::None)
               continue;
@@ -5670,7 +5768,7 @@ bool MemProfContextDisambiguation::applyImport(Module &M) {
             // we don't need to do ICP, but might need to clone this
             // function as it is the target of other cloned calls.
             if (NumClones)
-              CloneFuncIfNeeded(NumClones);
+              CloneFuncIfNeeded(NumClones, FS);
           }
 
           else {
@@ -5690,7 +5788,7 @@ bool MemProfContextDisambiguation::applyImport(Module &M) {
             }
 #endif
 
-            CloneCallsite(StackNode, CB, CalledFunction);
+            CloneCallsite(StackNode, CB, CalledFunction, FS);
           }
         } else if (CB->isTailCall() && CalledFunction) {
           // Locate the synthesized callsite info for the callee VI, if any was
@@ -5700,7 +5798,7 @@ bool MemProfContextDisambiguation::applyImport(Module &M) {
           if (CalleeVI && MapTailCallCalleeVIToCallsite.count(CalleeVI)) {
             auto Callsite = MapTailCallCalleeVIToCallsite.find(CalleeVI);
             assert(Callsite != MapTailCallCalleeVIToCallsite.end());
-            CloneCallsite(Callsite->second, CB, CalledFunction);
+            CloneCallsite(Callsite->second, CB, CalledFunction, FS);
           }
         }
       }
@@ -5846,6 +5944,10 @@ void MemProfContextDisambiguation::performICP(
       // check.
       CallBase *CBClone = CB;
       for (unsigned J = 0; J < NumClones; J++) {
+        // If the VMap is empty, this clone was a duplicate of another and was
+        // created as an alias or a declaration.
+        if (J > 0 && VMaps[J - 1]->empty())
+          continue;
         // Copy 0 is the original function.
         if (J > 0)
           CBClone = cast<CallBase>((*VMaps[J - 1])[CB]);
@@ -5891,6 +5993,10 @@ void MemProfContextDisambiguation::performICP(
     // TotalCount and the number promoted.
     CallBase *CBClone = CB;
     for (unsigned J = 0; J < NumClones; J++) {
+      // If the VMap is empty, this clone was a duplicate of another and was
+      // created as an alias or a declaration.
+      if (J > 0 && VMaps[J - 1]->empty())
+        continue;
       // Copy 0 is the original function.
       if (J > 0)
         CBClone = cast<CallBase>((*VMaps[J - 1])[CB]);