[dsymutil] Remove recursion from lookForChildDIEsToKeep (2/2) (NFC)

The functions lookForDIEsToKeep and keepDIEAndDependencies are mutually
recursive and can cause a stackoverflow for large projects. While this
has always been the case, it became a bigger issue when we parallelized
dsymutil, because threads get only a fraction of the stack space.

This patch removes the final recursive call from lookForDIEsToKeep. The
call was used to look at the current DIE's parent chain and mark
everything as kept.

This was tested by running dsymutil on clang built in debug (both with
and without modules) and comparing the MD5 hash of the generated dSYM
companion file.

Differential revision: https://reviews.llvm.org/D70994

Author: JDevlieghere

llvm/tools/dsymutil/DwarfLinker.cpp

@@ -763,9 +763,19 @@ unsigned DwarfLinker::shouldKeepDIE(RelocationManager &RelocMgr,
 namespace {
 /// The  distinct types of work performed by the work loop.
 enum class WorklistItemType {
+  /// Given a DIE, look for DIEs to be kept.
   LookForDIEsToKeep,
+  /// Given a DIE, look for children of this DIE to be kept.
   LookForChildDIEsToKeep,
+  /// Given a DIE, look for DIEs referencing this DIE to be kept.
+  LookForRefDIEsToKeep,
+  /// Given a DIE, look for parent DIEs to be kept.
+  LookForParentDIEsToKeep,
+  /// Given a DIE, update its incompleteness based on whether its children are
+  /// incomplete.
   UpdateChildIncompleteness,
+  /// Given a DIE, update its incompleteness based on whether the DIEs it
+  /// references are incomplete.
   UpdateRefIncompleteness,
 };
 
@@ -777,6 +787,7 @@ struct WorklistItem {
   DWARFDie Die;
   CompileUnit &CU;
   unsigned Flags;
+  unsigned AncestorIdx = 0;
   CompileUnit::DIEInfo *OtherInfo = nullptr;
 
   WorklistItem(DWARFDie Die, CompileUnit &CU, unsigned Flags,
@@ -786,6 +797,10 @@ struct WorklistItem {
   WorklistItem(DWARFDie Die, CompileUnit &CU, WorklistItemType T,
                CompileUnit::DIEInfo *OtherInfo = nullptr)
       : Type(T), Die(Die), CU(CU), OtherInfo(OtherInfo){};
+
+  WorklistItem(unsigned AncestorIdx, CompileUnit &CU, unsigned Flags)
+      : Type(WorklistItemType::LookForParentDIEsToKeep), CU(CU), Flags(Flags),
+        AncestorIdx(AncestorIdx){};
 };
 } // namespace
 
@@ -810,8 +825,8 @@ static void updateChildIncompleteness(const DWARFDie &Die, CompileUnit &CU,
 }
 
 /// Helper that updates the completeness of the current DIE based on the
-/// completeness of the DIEs referencing it. It depends on the incompleteness
-/// of the referencing DIE already being computed.
+/// completeness of the DIEs it references. It depends on the incompleteness of
+/// the referenced DIE already being computed.
 static void updateRefIncompleteness(const DWARFDie &Die, CompileUnit &CU,
                                     CompileUnit::DIEInfo &RefInfo) {
   switch (Die.getTag()) {
@@ -835,6 +850,8 @@ static void updateRefIncompleteness(const DWARFDie &Die, CompileUnit &CU,
     MyInfo.Incomplete = true;
 }
 
+/// Look at the children of the given DIE and decide whether they should be
+/// kept.
 static void lookForChildDIEsToKeep(const DWARFDie &Die, CompileUnit &CU,
                                    unsigned Flags,
                                    SmallVectorImpl<WorklistItem> &Worklist) {
@@ -846,6 +863,8 @@ static void lookForChildDIEsToKeep(const DWARFDie &Die, CompileUnit &CU,
   if (dieNeedsChildrenToBeMeaningful(Die.getTag()))
     Flags &= ~DwarfLinker::TF_ParentWalk;
 
+  // We're finished if this DIE has no children or we're walking the parent
+  // chain.
   if (!Die.hasChildren() || (Flags & DwarfLinker::TF_ParentWalk))
     return;
 
@@ -862,6 +881,94 @@ static void lookForChildDIEsToKeep(const DWARFDie &Die, CompileUnit &CU,
   }
 }
 
+/// Look at DIEs referenced by the given DIE and decide whether they should be
+/// kept. All DIEs referenced though attributes should be kept.
+static void lookForRefDIEsToKeep(const DWARFDie &Die, CompileUnit &CU,
+                                 unsigned Flags, DwarfLinker &Linker,
+                                 const UnitListTy &Units,
+                                 const DebugMapObject &DMO,
+                                 SmallVectorImpl<WorklistItem> &Worklist) {
+  bool UseOdr = (Flags & DwarfLinker::TF_DependencyWalk)
+                    ? (Flags & DwarfLinker::TF_ODR)
+                    : CU.hasODR();
+  DWARFUnit &Unit = CU.getOrigUnit();
+  DWARFDataExtractor Data = Unit.getDebugInfoExtractor();
+  const auto *Abbrev = Die.getAbbreviationDeclarationPtr();
+  uint64_t Offset = Die.getOffset() + getULEB128Size(Abbrev->getCode());
+
+  SmallVector<std::pair<DWARFDie, CompileUnit &>, 4> ReferencedDIEs;
+  for (const auto &AttrSpec : Abbrev->attributes()) {
+    DWARFFormValue Val(AttrSpec.Form);
+    if (!Val.isFormClass(DWARFFormValue::FC_Reference) ||
+        AttrSpec.Attr == dwarf::DW_AT_sibling) {
+      DWARFFormValue::skipValue(AttrSpec.Form, Data, &Offset,
+                                Unit.getFormParams());
+      continue;
+    }
+
+    Val.extractValue(Data, &Offset, Unit.getFormParams(), &Unit);
+    CompileUnit *ReferencedCU;
+    if (auto RefDie =
+            resolveDIEReference(Linker, DMO, Units, Val, Die, ReferencedCU)) {
+      uint32_t RefIdx = ReferencedCU->getOrigUnit().getDIEIndex(RefDie);
+      CompileUnit::DIEInfo &Info = ReferencedCU->getInfo(RefIdx);
+      bool IsModuleRef = Info.Ctxt && Info.Ctxt->getCanonicalDIEOffset() &&
+                         Info.Ctxt->isDefinedInClangModule();
+      // If the referenced DIE has a DeclContext that has already been
+      // emitted, then do not keep the one in this CU. We'll link to
+      // the canonical DIE in cloneDieReferenceAttribute.
+      //
+      // FIXME: compatibility with dsymutil-classic. UseODR shouldn't
+      // be necessary and could be advantageously replaced by
+      // ReferencedCU->hasODR() && CU.hasODR().
+      //
+      // FIXME: compatibility with dsymutil-classic. There is no
+      // reason not to unique ref_addr references.
+      if (AttrSpec.Form != dwarf::DW_FORM_ref_addr && (UseOdr || IsModuleRef) &&
+          Info.Ctxt &&
+          Info.Ctxt != ReferencedCU->getInfo(Info.ParentIdx).Ctxt &&
+          Info.Ctxt->getCanonicalDIEOffset() && isODRAttribute(AttrSpec.Attr))
+        continue;
+
+      // Keep a module forward declaration if there is no definition.
+      if (!(isODRAttribute(AttrSpec.Attr) && Info.Ctxt &&
+            Info.Ctxt->getCanonicalDIEOffset()))
+        Info.Prune = false;
+      ReferencedDIEs.emplace_back(RefDie, *ReferencedCU);
+    }
+  }
+
+  unsigned ODRFlag = UseOdr ? DwarfLinker::TF_ODR : 0;
+
+  // Add referenced DIEs in reverse order to the worklist to effectively
+  // process them in order.
+  for (auto &P : reverse(ReferencedDIEs)) {
+    // Add a worklist item before every child to calculate incompleteness right
+    // after the current child is processed.
+    uint32_t RefIdx = P.second.getOrigUnit().getDIEIndex(P.first);
+    CompileUnit::DIEInfo &Info = P.second.getInfo(RefIdx);
+    Worklist.emplace_back(Die, CU, WorklistItemType::UpdateRefIncompleteness,
+                          &Info);
+    Worklist.emplace_back(P.first, P.second,
+                          DwarfLinker::TF_Keep |
+                              DwarfLinker::TF_DependencyWalk | ODRFlag);
+  }
+}
+
+/// Look at the parent of the given DIE and decide whether they should be kept.
+static void lookForParentDIEsToKeep(unsigned AncestorIdx, CompileUnit &CU,
+                                    unsigned Flags,
+                                    SmallVectorImpl<WorklistItem> &Worklist) {
+  // Stop if we encounter an ancestor that's already marked as kept.
+  if (CU.getInfo(AncestorIdx).Keep)
+    return;
+
+  DWARFUnit &Unit = CU.getOrigUnit();
+  DWARFDie ParentDIE = Unit.getDIEAtIndex(AncestorIdx);
+  Worklist.emplace_back(CU.getInfo(AncestorIdx).ParentIdx, CU, Flags);
+  Worklist.emplace_back(ParentDIE, CU, Flags);
+}
+
 /// Recursively walk the \p DIE tree and look for DIEs to keep. Store that
 /// information in \p CU's DIEInfo.
 ///
@@ -875,6 +982,17 @@ static void lookForChildDIEsToKeep(const DWARFDie &Die, CompileUnit &CU,
 /// TF_DependencyWalk flag tells us which kind of traversal we are currently
 /// doing.
 ///
+/// The recursive algorithm is implemented iteratively as a work list because
+/// very deep recursion could exhaust the stack for large projects. The work
+/// list acts as a scheduler for different types of work that need to be
+/// performed.
+///
+/// The recursive nature of the algorithm is simulated by running the "main"
+/// algorithm (LookForDIEsToKeep) followed by either looking at more DIEs
+/// (LookForChildDIEsToKeep, LookForRefDIEsToKeep, LookForParentDIEsToKeep) or
+/// fixing up a computed property (UpdateChildIncompleteness,
+/// UpdateRefIncompleteness).
+///
 /// The return value indicates whether the DIE is incomplete.
 void DwarfLinker::lookForDIEsToKeep(RelocationManager &RelocMgr,
                                     RangesTy &Ranges, const UnitListTy &Units,
@@ -900,6 +1018,14 @@ void DwarfLinker::lookForDIEsToKeep(RelocationManager &RelocMgr,
     case WorklistItemType::LookForChildDIEsToKeep:
       lookForChildDIEsToKeep(Current.Die, Current.CU, Current.Flags, Worklist);
       continue;
+    case WorklistItemType::LookForRefDIEsToKeep:
+      lookForRefDIEsToKeep(Current.Die, Current.CU, Current.Flags, *this, Units,
+                           DMO, Worklist);
+      continue;
+    case WorklistItemType::LookForParentDIEsToKeep:
+      lookForParentDIEsToKeep(Current.AncestorIdx, Current.CU, Current.Flags,
+                              Worklist);
+      continue;
     case WorklistItemType::LookForDIEsToKeep:
       break;
     }
@@ -933,12 +1059,6 @@ void DwarfLinker::lookForDIEsToKeep(RelocationManager &RelocMgr,
 
     // If it is a newly kept DIE mark it as well as all its dependencies as
     // kept.
-    bool UseOdr = (Current.Flags & TF_DependencyWalk)
-                      ? (Current.Flags & TF_ODR)
-                      : Current.CU.hasODR();
-    unsigned ODRFlag = UseOdr ? TF_ODR : 0;
-
-    DWARFUnit &Unit = Current.CU.getOrigUnit();
     MyInfo.Keep = true;
 
     // We're looking for incomplete types.
@@ -947,74 +1067,19 @@ void DwarfLinker::lookForDIEsToKeep(RelocationManager &RelocMgr,
         Current.Die.getTag() != dwarf::DW_TAG_member &&
         dwarf::toUnsigned(Current.Die.find(dwarf::DW_AT_declaration), 0);
 
-    // First mark all the parent chain as kept.
-    unsigned AncestorIdx = MyInfo.ParentIdx;
-    while (!Current.CU.getInfo(AncestorIdx).Keep) {
-      lookForDIEsToKeep(
-          RelocMgr, Ranges, Units, Unit.getDIEAtIndex(AncestorIdx), DMO,
-          Current.CU, TF_ParentWalk | TF_Keep | TF_DependencyWalk | ODRFlag);
-      AncestorIdx = Current.CU.getInfo(AncestorIdx).ParentIdx;
-    }
-
-    // Then we need to mark all the DIEs referenced by this DIE's attributes
-    // as kept.
-    DWARFDataExtractor Data = Unit.getDebugInfoExtractor();
-    const auto *Abbrev = Current.Die.getAbbreviationDeclarationPtr();
-    uint64_t Offset =
-        Current.Die.getOffset() + getULEB128Size(Abbrev->getCode());
-
-    // Mark all DIEs referenced through attributes as kept.
-    SmallVector<std::pair<DWARFDie, CompileUnit &>, 4> ReferencedDIEs;
-    for (const auto &AttrSpec : Abbrev->attributes()) {
-      DWARFFormValue Val(AttrSpec.Form);
-      if (!Val.isFormClass(DWARFFormValue::FC_Reference) ||
-          AttrSpec.Attr == dwarf::DW_AT_sibling) {
-        DWARFFormValue::skipValue(AttrSpec.Form, Data, &Offset,
-                                  Unit.getFormParams());
-        continue;
-      }
-
-      Val.extractValue(Data, &Offset, Unit.getFormParams(), &Unit);
-      CompileUnit *ReferencedCU;
-      if (auto RefDie = resolveDIEReference(*this, DMO, Units, Val,
-                                            Current.Die, ReferencedCU)) {
-        uint32_t RefIdx = ReferencedCU->getOrigUnit().getDIEIndex(RefDie);
-        CompileUnit::DIEInfo &Info = ReferencedCU->getInfo(RefIdx);
-        bool IsModuleRef = Info.Ctxt && Info.Ctxt->getCanonicalDIEOffset() &&
-                            Info.Ctxt->isDefinedInClangModule();
-        // If the referenced DIE has a DeclContext that has already been
-        // emitted, then do not keep the one in this CU. We'll link to
-        // the canonical DIE in cloneDieReferenceAttribute.
-        // FIXME: compatibility with dsymutil-classic. UseODR shouldn't
-        // be necessary and could be advantageously replaced by
-        // ReferencedCU->hasODR() && CU.hasODR().
-        // FIXME: compatibility with dsymutil-classic. There is no
-        // reason not to unique ref_addr references.
-        if (AttrSpec.Form != dwarf::DW_FORM_ref_addr &&
-            (UseOdr || IsModuleRef) && Info.Ctxt &&
-            Info.Ctxt != ReferencedCU->getInfo(Info.ParentIdx).Ctxt &&
-            Info.Ctxt->getCanonicalDIEOffset() &&
-            isODRAttribute(AttrSpec.Attr))
-          continue;
+    // After looking at the parent chain, look for referenced DIEs. Because of
+    // the LIFO worklist we need to schedule that work before any subsequent
+    // items are added to the worklist.
+    Worklist.emplace_back(Current.Die, Current.CU, Current.Flags,
+                          WorklistItemType::LookForRefDIEsToKeep);
 
-        // Keep a module forward declaration if there is no definition.
-        if (!(isODRAttribute(AttrSpec.Attr) && Info.Ctxt &&
-              Info.Ctxt->getCanonicalDIEOffset()))
-          Info.Prune = false;
-        ReferencedDIEs.emplace_back(RefDie, *ReferencedCU);
-      }
-    }
+    bool UseOdr = (Current.Flags & TF_DependencyWalk) ? (Current.Flags & TF_ODR)
+                                                      : Current.CU.hasODR();
+    unsigned ODRFlag = UseOdr ? TF_ODR : 0;
+    unsigned ParFlags = TF_ParentWalk | TF_Keep | TF_DependencyWalk | ODRFlag;
 
-    // Add referenced DIEs in reverse order to the worklist to effectively
-    // process them in order.
-    for (auto& P : reverse(ReferencedDIEs)) {
-      // Add a worklist item before every child to calculate incompleteness right
-      // after the current child is processed.
-      uint32_t RefIdx = P.second.getOrigUnit().getDIEIndex(P.first);
-      CompileUnit::DIEInfo &Info = P.second.getInfo(RefIdx);
-      Worklist.emplace_back(Current.Die, Current.CU, WorklistItemType::UpdateRefIncompleteness, &Info);
-      Worklist.emplace_back(P.first, P.second, TF_Keep | TF_DependencyWalk | ODRFlag);
-    }
+    // Now schedule the parent walk.
+    Worklist.emplace_back(MyInfo.ParentIdx, Current.CU, ParFlags);
   }
 }