Index: head/contrib/llvm/tools/lld/ELF/ICF.cpp
===================================================================
--- head/contrib/llvm/tools/lld/ELF/ICF.cpp	(revision 319884)
+++ head/contrib/llvm/tools/lld/ELF/ICF.cpp	(revision 319885)
@@ -1,383 +1,387 @@
 //===- ICF.cpp ------------------------------------------------------------===//
 //
 //                             The LLVM Linker
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // ICF is short for Identical Code Folding. This is a size optimization to
 // identify and merge two or more read-only sections (typically functions)
 // that happened to have the same contents. It usually reduces output size
 // by a few percent.
 //
 // In ICF, two sections are considered identical if they have the same
 // section flags, section data, and relocations. Relocations are tricky,
 // because two relocations are considered the same if they have the same
 // relocation types, values, and if they point to the same sections *in
 // terms of ICF*.
 //
 // Here is an example. If foo and bar defined below are compiled to the
 // same machine instructions, ICF can and should merge the two, although
 // their relocations point to each other.
 //
 //   void foo() { bar(); }
 //   void bar() { foo(); }
 //
 // If you merge the two, their relocations point to the same section and
 // thus you know they are mergeable, but how do you know they are
 // mergeable in the first place? This is not an easy problem to solve.
 //
 // What we are doing in LLD is to partition sections into equivalence
 // classes. Sections in the same equivalence class when the algorithm
 // terminates are considered identical. Here are details:
 //
 // 1. First, we partition sections using their hash values as keys. Hash
 //    values contain section types, section contents and numbers of
 //    relocations. During this step, relocation targets are not taken into
 //    account. We just put sections that apparently differ into different
 //    equivalence classes.
 //
 // 2. Next, for each equivalence class, we visit sections to compare
 //    relocation targets. Relocation targets are considered equivalent if
 //    their targets are in the same equivalence class. Sections with
 //    different relocation targets are put into different equivalence
 //    clases.
 //
 // 3. If we split an equivalence class in step 2, two relocations
 //    previously target the same equivalence class may now target
 //    different equivalence classes. Therefore, we repeat step 2 until a
 //    convergence is obtained.
 //
 // 4. For each equivalence class C, pick an arbitrary section in C, and
 //    merge all the other sections in C with it.
 //
 // For small programs, this algorithm needs 3-5 iterations. For large
 // programs such as Chromium, it takes more than 20 iterations.
 //
 // This algorithm was mentioned as an "optimistic algorithm" in [1],
 // though gold implements a different algorithm than this.
 //
 // We parallelize each step so that multiple threads can work on different
 // equivalence classes concurrently. That gave us a large performance
 // boost when applying ICF on large programs. For example, MSVC link.exe
 // or GNU gold takes 10-20 seconds to apply ICF on Chromium, whose output
 // size is about 1.5 GB, but LLD can finish it in less than 2 seconds on a
 // 2.8 GHz 40 core machine. Even without threading, LLD's ICF is still
 // faster than MSVC or gold though.
 //
 // [1] Safe ICF: Pointer Safe and Unwinding aware Identical Code Folding
 // in the Gold Linker
 // http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/36912.pdf
 //
 //===----------------------------------------------------------------------===//
 
 #include "ICF.h"
 #include "Config.h"
 #include "SymbolTable.h"
 #include "Threads.h"
 
 #include "llvm/ADT/Hashing.h"
 #include "llvm/Object/ELF.h"
 #include "llvm/Support/ELF.h"
 #include <algorithm>
 #include <atomic>
 
 using namespace lld;
 using namespace lld::elf;
 using namespace llvm;
 using namespace llvm::ELF;
 using namespace llvm::object;
 
 namespace {
 template <class ELFT> class ICF {
 public:
   void run();
 
 private:
   void segregate(size_t Begin, size_t End, bool Constant);
 
   template <class RelTy>
   bool constantEq(ArrayRef<RelTy> RelsA, ArrayRef<RelTy> RelsB);
 
   template <class RelTy>
   bool variableEq(const InputSection<ELFT> *A, ArrayRef<RelTy> RelsA,
                   const InputSection<ELFT> *B, ArrayRef<RelTy> RelsB);
 
   bool equalsConstant(const InputSection<ELFT> *A, const InputSection<ELFT> *B);
   bool equalsVariable(const InputSection<ELFT> *A, const InputSection<ELFT> *B);
 
   size_t findBoundary(size_t Begin, size_t End);
 
   void forEachClassRange(size_t Begin, size_t End,
                          std::function<void(size_t, size_t)> Fn);
 
   void forEachClass(std::function<void(size_t, size_t)> Fn);
 
   std::vector<InputSection<ELFT> *> Sections;
 
   // We repeat the main loop while `Repeat` is true.
   std::atomic<bool> Repeat;
 
   // The main loop counter.
   int Cnt = 0;
 
   // We have two locations for equivalence classes. On the first iteration
   // of the main loop, Class[0] has a valid value, and Class[1] contains
   // garbage. We read equivalence classes from slot 0 and write to slot 1.
   // So, Class[0] represents the current class, and Class[1] represents
   // the next class. On each iteration, we switch their roles and use them
   // alternately.
   //
   // Why are we doing this? Recall that other threads may be working on
   // other equivalence classes in parallel. They may read sections that we
   // are updating. We cannot update equivalence classes in place because
   // it breaks the invariance that all possibly-identical sections must be
   // in the same equivalence class at any moment. In other words, the for
   // loop to update equivalence classes is not atomic, and that is
   // observable from other threads. By writing new classes to other
   // places, we can keep the invariance.
   //
   // Below, `Current` has the index of the current class, and `Next` has
   // the index of the next class. If threading is enabled, they are either
   // (0, 1) or (1, 0).
   //
   // Note on single-thread: if that's the case, they are always (0, 0)
   // because we can safely read the next class without worrying about race
   // conditions. Using the same location makes this algorithm converge
   // faster because it uses results of the same iteration earlier.
   int Current = 0;
   int Next = 0;
 };
 }
 
 // Returns a hash value for S. Note that the information about
 // relocation targets is not included in the hash value.
 template <class ELFT> static uint32_t getHash(InputSection<ELFT> *S) {
   return hash_combine(S->Flags, S->getSize(), S->NumRelocations);
 }
 
 // Returns true if section S is subject of ICF.
 template <class ELFT> static bool isEligible(InputSection<ELFT> *S) {
   // .init and .fini contains instructions that must be executed to
   // initialize and finalize the process. They cannot and should not
   // be merged.
   return S->Live && (S->Flags & SHF_ALLOC) && !(S->Flags & SHF_WRITE) &&
          S->Name != ".init" && S->Name != ".fini";
 }
 
 // Split an equivalence class into smaller classes.
 template <class ELFT>
 void ICF<ELFT>::segregate(size_t Begin, size_t End, bool Constant) {
   // This loop rearranges sections in [Begin, End) so that all sections
   // that are equal in terms of equals{Constant,Variable} are contiguous
   // in [Begin, End).
   //
   // The algorithm is quadratic in the worst case, but that is not an
   // issue in practice because the number of the distinct sections in
   // each range is usually very small.
 
   while (Begin < End) {
     // Divide [Begin, End) into two. Let Mid be the start index of the
     // second group.
     auto Bound = std::stable_partition(
         Sections.begin() + Begin + 1, Sections.begin() + End,
         [&](InputSection<ELFT> *S) {
           if (Constant)
             return equalsConstant(Sections[Begin], S);
           return equalsVariable(Sections[Begin], S);
         });
     size_t Mid = Bound - Sections.begin();
 
     // Now we split [Begin, End) into [Begin, Mid) and [Mid, End) by
     // updating the sections in [Begin, End). We use Mid as an equivalence
     // class ID because every group ends with a unique index.
     for (size_t I = Begin; I < Mid; ++I)
       Sections[I]->Class[Next] = Mid;
 
     // If we created a group, we need to iterate the main loop again.
     if (Mid != End)
       Repeat = true;
 
     Begin = Mid;
   }
 }
 
 // Compare two lists of relocations.
 template <class ELFT>
 template <class RelTy>
 bool ICF<ELFT>::constantEq(ArrayRef<RelTy> RelsA, ArrayRef<RelTy> RelsB) {
   auto Eq = [](const RelTy &A, const RelTy &B) {
     return A.r_offset == B.r_offset &&
            A.getType(Config->Mips64EL) == B.getType(Config->Mips64EL) &&
            getAddend<ELFT>(A) == getAddend<ELFT>(B);
   };
 
   return RelsA.size() == RelsB.size() &&
          std::equal(RelsA.begin(), RelsA.end(), RelsB.begin(), Eq);
 }
 
 // Compare "non-moving" part of two InputSections, namely everything
 // except relocation targets.
 template <class ELFT>
 bool ICF<ELFT>::equalsConstant(const InputSection<ELFT> *A,
                                const InputSection<ELFT> *B) {
   if (A->NumRelocations != B->NumRelocations || A->Flags != B->Flags ||
       A->getSize() != B->getSize() || A->Data != B->Data)
     return false;
 
   if (A->AreRelocsRela)
     return constantEq(A->relas(), B->relas());
   return constantEq(A->rels(), B->rels());
 }
 
 // Compare two lists of relocations. Returns true if all pairs of
 // relocations point to the same section in terms of ICF.
 template <class ELFT>
 template <class RelTy>
 bool ICF<ELFT>::variableEq(const InputSection<ELFT> *A, ArrayRef<RelTy> RelsA,
                            const InputSection<ELFT> *B, ArrayRef<RelTy> RelsB) {
   auto Eq = [&](const RelTy &RA, const RelTy &RB) {
     // The two sections must be identical.
     SymbolBody &SA = A->getFile()->getRelocTargetSym(RA);
     SymbolBody &SB = B->getFile()->getRelocTargetSym(RB);
     if (&SA == &SB)
       return true;
 
-    // Or, the two sections must be in the same equivalence class.
     auto *DA = dyn_cast<DefinedRegular<ELFT>>(&SA);
     auto *DB = dyn_cast<DefinedRegular<ELFT>>(&SB);
     if (!DA || !DB)
       return false;
     if (DA->Value != DB->Value)
       return false;
 
+    // Either both symbols must be absolute...
+    if (!DA->Section || !DB->Section)
+      return !DA->Section && !DB->Section;
+
+    // Or the two sections must be in the same equivalence class.
     auto *X = dyn_cast<InputSection<ELFT>>(DA->Section);
     auto *Y = dyn_cast<InputSection<ELFT>>(DB->Section);
     if (!X || !Y)
       return false;
 
     // Ineligible sections are in the special equivalence class 0.
     // They can never be the same in terms of the equivalence class.
     if (X->Class[Current] == 0)
       return false;
 
     return X->Class[Current] == Y->Class[Current];
   };
 
   return std::equal(RelsA.begin(), RelsA.end(), RelsB.begin(), Eq);
 }
 
 // Compare "moving" part of two InputSections, namely relocation targets.
 template <class ELFT>
 bool ICF<ELFT>::equalsVariable(const InputSection<ELFT> *A,
                                const InputSection<ELFT> *B) {
   if (A->AreRelocsRela)
     return variableEq(A, A->relas(), B, B->relas());
   return variableEq(A, A->rels(), B, B->rels());
 }
 
 template <class ELFT> size_t ICF<ELFT>::findBoundary(size_t Begin, size_t End) {
   uint32_t Class = Sections[Begin]->Class[Current];
   for (size_t I = Begin + 1; I < End; ++I)
     if (Class != Sections[I]->Class[Current])
       return I;
   return End;
 }
 
 // Sections in the same equivalence class are contiguous in Sections
 // vector. Therefore, Sections vector can be considered as contiguous
 // groups of sections, grouped by the class.
 //
 // This function calls Fn on every group that starts within [Begin, End).
 // Note that a group must starts in that range but doesn't necessarily
 // have to end before End.
 template <class ELFT>
 void ICF<ELFT>::forEachClassRange(size_t Begin, size_t End,
                                   std::function<void(size_t, size_t)> Fn) {
   if (Begin > 0)
     Begin = findBoundary(Begin - 1, End);
 
   while (Begin < End) {
     size_t Mid = findBoundary(Begin, Sections.size());
     Fn(Begin, Mid);
     Begin = Mid;
   }
 }
 
 // Call Fn on each equivalence class.
 template <class ELFT>
 void ICF<ELFT>::forEachClass(std::function<void(size_t, size_t)> Fn) {
   // If threading is disabled or the number of sections are
   // too small to use threading, call Fn sequentially.
   if (!Config->Threads || Sections.size() < 1024) {
     forEachClassRange(0, Sections.size(), Fn);
     ++Cnt;
     return;
   }
 
   Current = Cnt % 2;
   Next = (Cnt + 1) % 2;
 
   // Split sections into 256 shards and call Fn in parallel.
   size_t NumShards = 256;
   size_t Step = Sections.size() / NumShards;
   forLoop(0, NumShards,
           [&](size_t I) { forEachClassRange(I * Step, (I + 1) * Step, Fn); });
   forEachClassRange(Step * NumShards, Sections.size(), Fn);
   ++Cnt;
 }
 
 // The main function of ICF.
 template <class ELFT> void ICF<ELFT>::run() {
   // Collect sections to merge.
   for (InputSectionBase<ELFT> *Sec : Symtab<ELFT>::X->Sections)
     if (auto *S = dyn_cast<InputSection<ELFT>>(Sec))
       if (isEligible(S))
         Sections.push_back(S);
 
   // Initially, we use hash values to partition sections.
   for (InputSection<ELFT> *S : Sections)
     // Set MSB to 1 to avoid collisions with non-hash IDs.
     S->Class[0] = getHash(S) | (1 << 31);
 
   // From now on, sections in Sections vector are ordered so that sections
   // in the same equivalence class are consecutive in the vector.
   std::stable_sort(Sections.begin(), Sections.end(),
                    [](InputSection<ELFT> *A, InputSection<ELFT> *B) {
                      return A->Class[0] < B->Class[0];
                    });
 
   // Compare static contents and assign unique IDs for each static content.
   forEachClass([&](size_t Begin, size_t End) { segregate(Begin, End, true); });
 
   // Split groups by comparing relocations until convergence is obtained.
   do {
     Repeat = false;
     forEachClass(
         [&](size_t Begin, size_t End) { segregate(Begin, End, false); });
   } while (Repeat);
 
   log("ICF needed " + Twine(Cnt) + " iterations");
 
   // Merge sections by the equivalence class.
   forEachClass([&](size_t Begin, size_t End) {
     if (End - Begin == 1)
       return;
 
     log("selected " + Sections[Begin]->Name);
     for (size_t I = Begin + 1; I < End; ++I) {
       log("  removed " + Sections[I]->Name);
       Sections[Begin]->replace(Sections[I]);
     }
   });
 }
 
 // ICF entry point function.
 template <class ELFT> void elf::doIcf() { ICF<ELFT>().run(); }
 
 template void elf::doIcf<ELF32LE>();
 template void elf::doIcf<ELF32BE>();
 template void elf::doIcf<ELF64LE>();
 template void elf::doIcf<ELF64BE>();