ELF: Move build id computation to Writer. NFCI.

[pcc]

Summary

With partitions, each partition should have the same build id. This means
that the build id needs to be only computed once, otherwise we will end up
with different build ids in each partition as a result of the file contents
changing. This change moves the computation of the build id into Writer so
that it only happens once.

[grimar]

I think this one is probably OK. But I'd leave it to Rui.

[ruiu]

LGTM

Sorry, I missed this one.

lld/ELF/SyntheticSections.h

 
 public:
   BuildIdSection();
   void writeTo(uint8_t *Buf) override;
   size_t getSize() const override { return HeaderSize + HashSize; }
   void writeBuildId(llvm::ArrayRef<uint8_t> Buf);
 
 private:
-  void computeHash(llvm::ArrayRef<uint8_t> Buf,
-                   std::function<void(uint8_t *, ArrayRef<uint8_t>)> Hash);
-
   size_t HashSize;
   uint8_t *HashBuf;
 };
 
 // BssSection is used to reserve space for copy relocations and common symbols.
 // We create three instances of this class for .bss, .bss.rel.ro and "COMMON",
 // that are used for writable symbols, read-only symbols and common symbols,
 // respectively.

lld/ELF/SyntheticSections.cpp

 #include "llvm/ADT/StringExtras.h"
 #include "llvm/BinaryFormat/Dwarf.h"
 #include "llvm/DebugInfo/DWARF/DWARFDebugPubTable.h"
 #include "llvm/Object/ELFObjectFile.h"
 #include "llvm/Support/Compression.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/LEB128.h"
 #include "llvm/Support/MD5.h"
-#include "llvm/Support/RandomNumberGenerator.h"
-#include "llvm/Support/SHA1.h"
-#include "llvm/Support/xxhash.h"
 #include <cstdlib>
 #include <thread>
 
 using namespace llvm;
 using namespace llvm::dwarf;
 using namespace llvm::ELF;
 using namespace llvm::object;
 using namespace llvm::support;

 void BuildIdSection::writeTo(uint8_t *Buf) {
   write32(Buf, 4);                      // Name size
   write32(Buf + 4, HashSize);           // Content size
   write32(Buf + 8, NT_GNU_BUILD_ID);    // Type
   memcpy(Buf + 12, "GNU", 4);           // Name string
   HashBuf = Buf + 16;
 }
 
-// Split one uint8 array into small pieces of uint8 arrays.
-static std::vector<ArrayRef<uint8_t>> split(ArrayRef<uint8_t> Arr,
-                                            size_t ChunkSize) {
-  std::vector<ArrayRef<uint8_t>> Ret;
-  while (Arr.size() > ChunkSize) {
-    Ret.push_back(Arr.take_front(ChunkSize));
-    Arr = Arr.drop_front(ChunkSize);
-  }
-  if (!Arr.empty())
-    Ret.push_back(Arr);
-  return Ret;
-}
-
-// Computes a hash value of Data using a given hash function.
-// In order to utilize multiple cores, we first split data into 1MB
-// chunks, compute a hash for each chunk, and then compute a hash value
-// of the hash values.
-void BuildIdSection::computeHash(
-    llvm::ArrayRef<uint8_t> Data,
-    std::function<void(uint8_t *Dest, ArrayRef<uint8_t> Arr)> HashFn) {
-  std::vector<ArrayRef<uint8_t>> Chunks = split(Data, 1024 * 1024);
-  std::vector<uint8_t> Hashes(Chunks.size() * HashSize);
-
-  // Compute hash values.
-  parallelForEachN(0, Chunks.size(), [&](size_t I) {
-    HashFn(Hashes.data() + I * HashSize, Chunks[I]);
-  });
-
-  // Write to the final output buffer.
-  HashFn(HashBuf, Hashes);
+void BuildIdSection::writeBuildId(ArrayRef<uint8_t> Buf) {
+  assert(Buf.size() == HashSize);
+  memcpy(HashBuf, Buf.data(), HashSize);
 }
 
 BssSection::BssSection(StringRef Name, uint64_t Size, uint32_t Alignment)
     : SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_NOBITS, Alignment, Name) {
   this->Bss = true;
   this->Size = Size;
 }
 
-void BuildIdSection::writeBuildId(ArrayRef<uint8_t> Buf) {
-  switch (Config->BuildId) {
-  case BuildIdKind::Fast:
-    computeHash(Buf, [](uint8_t *Dest, ArrayRef<uint8_t> Arr) {
-      write64le(Dest, xxHash64(Arr));
-    });
-    break;
-  case BuildIdKind::Md5:
-    computeHash(Buf, [](uint8_t *Dest, ArrayRef<uint8_t> Arr) {
-      memcpy(Dest, MD5::hash(Arr).data(), 16);
-    });
-    break;
-  case BuildIdKind::Sha1:
-    computeHash(Buf, [](uint8_t *Dest, ArrayRef<uint8_t> Arr) {
-      memcpy(Dest, SHA1::hash(Arr).data(), 20);
-    });
-    break;
-  case BuildIdKind::Uuid:
-    if (auto EC = getRandomBytes(HashBuf, HashSize))
-      error("entropy source failure: " + EC.message());
-    break;
-  case BuildIdKind::Hexstring:
-    memcpy(HashBuf, Config->BuildIdVector.data(), Config->BuildIdVector.size());
-    break;
-  default:
-    llvm_unreachable("unknown BuildIdKind");
-  }
-}
-
 EhFrameSection::EhFrameSection()
     : SyntheticSection(SHF_ALLOC, SHT_PROGBITS, 1, ".eh_frame") {}
 
 // Search for an existing CIE record or create a new one.
 // CIE records from input object files are uniquified by their contents
 // and where their relocations point to.
 template <class ELFT, class RelTy>
 CieRecord *EhFrameSection::addCie(EhSectionPiece &Cie, ArrayRef<RelTy> Rels) {

lld/ELF/Writer.cpp

 #include "SyntheticSections.h"
 #include "Target.h"
 #include "lld/Common/Filesystem.h"
 #include "lld/Common/Memory.h"
 #include "lld/Common/Strings.h"
 #include "lld/Common/Threads.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringSwitch.h"
+#include "llvm/Support/RandomNumberGenerator.h"
+#include "llvm/Support/SHA1.h"
+#include "llvm/Support/xxhash.h"
 #include <climits>
 
 using namespace llvm;
 using namespace llvm::ELF;
 using namespace llvm::object;
 using namespace llvm::support;
 using namespace llvm::support::endian;
 
for (OutputSection *Sec : OutputSections)
     if (Sec->Type == SHT_REL || Sec->Type == SHT_RELA)
       Sec->writeTo<ELFT>(Out::BufferStart + Sec->Offset);
 
   for (OutputSection *Sec : OutputSections)
     if (Sec->Type != SHT_REL && Sec->Type != SHT_RELA)
       Sec->writeTo<ELFT>(Out::BufferStart + Sec->Offset);
 }
 
+// Split one uint8 array into small pieces of uint8 arrays.
+static std::vector<ArrayRef<uint8_t>> split(ArrayRef<uint8_t> Arr,
+                                            size_t ChunkSize) {
+  std::vector<ArrayRef<uint8_t>> Ret;
+  while (Arr.size() > ChunkSize) {
+    Ret.push_back(Arr.take_front(ChunkSize));
+    Arr = Arr.drop_front(ChunkSize);
+  }
+  if (!Arr.empty())
+    Ret.push_back(Arr);
+  return Ret;
+}
+
+// Computes a hash value of Data using a given hash function.
+// In order to utilize multiple cores, we first split data into 1MB
+// chunks, compute a hash for each chunk, and then compute a hash value
+// of the hash values.
+static void
+computeHash(llvm::MutableArrayRef<uint8_t> HashBuf,
+            llvm::ArrayRef<uint8_t> Data,
+            std::function<void(uint8_t *Dest, ArrayRef<uint8_t> Arr)> HashFn) {
+  std::vector<ArrayRef<uint8_t>> Chunks = split(Data, 1024 * 1024);
+  std::vector<uint8_t> Hashes(Chunks.size() * HashBuf.size());
+
+  // Compute hash values.
+  parallelForEachN(0, Chunks.size(), [&](size_t I) {
+    HashFn(Hashes.data() + I * HashBuf.size(), Chunks[I]);
+  });
+
+  // Write to the final output buffer.
+  HashFn(HashBuf.data(), Hashes);
+}
+
+static std::vector<uint8_t> computeBuildId(llvm::ArrayRef<uint8_t> Buf) {
+  std::vector<uint8_t> BuildId;
+  switch (Config->BuildId) {
+  case BuildIdKind::Fast:
+    BuildId.resize(8);
+    computeHash(BuildId, Buf, [](uint8_t *Dest, ArrayRef<uint8_t> Arr) {
+      write64le(Dest, xxHash64(Arr));
+    });
+    break;
+  case BuildIdKind::Md5:
+    BuildId.resize(16);
+    computeHash(BuildId, Buf, [](uint8_t *Dest, ArrayRef<uint8_t> Arr) {
+      memcpy(Dest, MD5::hash(Arr).data(), 16);
+    });
+    break;
+  case BuildIdKind::Sha1:
+    BuildId.resize(20);
+    computeHash(BuildId, Buf, [](uint8_t *Dest, ArrayRef<uint8_t> Arr) {
+      memcpy(Dest, SHA1::hash(Arr).data(), 20);
+    });
+    break;
+  case BuildIdKind::Uuid:
+    BuildId.resize(16);
+    if (auto EC = llvm::getRandomBytes(BuildId.data(), 16))
+      error("entropy source failure: " + EC.message());
+    break;
+  case BuildIdKind::Hexstring:
+    BuildId = Config->BuildIdVector;
+    break;
+  default:
+    llvm_unreachable("unknown BuildIdKind");
+  }
+  return BuildId;
+}
+
 template <class ELFT> void Writer<ELFT>::writeBuildId() {
   if (!In.BuildId || !In.BuildId->getParent())
     return;
 
   // Compute a hash of all sections of the output file.
-  In.BuildId->writeBuildId({Out::BufferStart, size_t(FileSize)});
+  std::vector<uint8_t> BuildId =
+      computeBuildId({Out::BufferStart, size_t(FileSize)});
+  In.BuildId->writeBuildId(BuildId);
 }
 
 template void elf::writeResult<ELF32LE>();
 template void elf::writeResult<ELF32BE>();
 template void elf::writeResult<ELF64LE>();
 template void elf::writeResult<ELF64BE>();