[ELF] Use zlib::compress which operates on std::unique_ptr<uint8_t[]>

[MaskRay]

Summary

zlib::compress on SmallVectorImpl<char> uses compressBound() to compute
the upper bound of the compressed buffer (larger than the source),
allocates initialized (zeroed) SmallVectorImpl<char>, which makes every
allocated page resident.

However, the actual size of the compressed buffer is usually much
smaller. Making every page resident is wasteful.

std::unique_ptr<uint8_t[]> backed buffer is uninitialized and avoids
redundant resident pages.

For a test binary with ~1.9GiB uncompressed debug info, this optimization
decreases max RSS by ~1.5GiB.

[MaskRay]

https://reviews.llvm.org/D50223 has been changed to improve zlib::compress's current SmallVectorImpl interface. This is no longer relevant.

ELF/OutputSections.h

public:
 
   void sort(llvm::function_ref<int(InputSectionBase *S)> Order);
   void sortInitFini();
   void sortCtorsDtors();
 
 private:
   // Used for implementation of --compress-debug-sections option.
   std::vector<uint8_t> ZDebugHeader;
-  llvm::SmallVector<char, 1> CompressedData;
+  std::unique_ptr<uint8_t[]> CompressedData;
+  size_t CompressedSize;
 
   uint32_t getFiller();
 };
 
 int getPriority(StringRef S);
 
 std::vector<InputSection *> getInputSections(OutputSection* OS);
 

ELF/OutputSections.cpp

template <class ELFT> void OutputSection::maybeCompress() {
   auto *Hdr = reinterpret_cast<Elf_Chdr *>(ZDebugHeader.data());
   Hdr->ch_type = ELFCOMPRESS_ZLIB;
   Hdr->ch_size = Size;
   Hdr->ch_addralign = Alignment;
 
   // Write section contents to a temporary buffer and compress it.
   std::vector<uint8_t> Buf(Size);
   writeTo<ELFT>(Buf.data());
-  if (Error E = zlib::compress(toStringRef(Buf), CompressedData))
+  if (Error E =
+          zlib::compress(toStringRef(Buf), CompressedData, CompressedSize))
     fatal("compress failed: " + llvm::toString(std::move(E)));
 
   // Update section headers.
-  Size = sizeof(Elf_Chdr) + CompressedData.size();
+  Size = sizeof(Elf_Chdr) + CompressedSize;
   Flags |= SHF_COMPRESSED;
 }
 
 static void writeInt(uint8_t *Buf, uint64_t Data, uint64_t Size) {
   if (Size == 1)
     *Buf = Data;
   else if (Size == 2)
     write16(Buf, Data);
template <class ELFT> void OutputSection::writeTo(uint8_t *Buf) {
   if (Type == SHT_NOBITS)
     return;
 
   Loc = Buf;
 
   // If -compress-debug-section is specified and if this is a debug seciton,
   // we've already compressed section contents. If that's the case,
   // just write it down.
-  if (!CompressedData.empty()) {
+  if (CompressedData) {
     memcpy(Buf, ZDebugHeader.data(), ZDebugHeader.size());
-    memcpy(Buf + ZDebugHeader.size(), CompressedData.data(),
-           CompressedData.size());
+    memcpy(Buf + ZDebugHeader.size(), CompressedData.get(), CompressedSize);
     return;
   }
 
   // Write leading padding.
   std::vector<InputSection *> Sections = getInputSections(this);
   uint32_t Filler = getFiller();
   if (Filler)
     fill(Buf, Sections.empty() ? Size : Sections[0]->OutSecOff, Filler);