[ELF] Use parallel_for_each for writing.

This changes improves performance of lld, when self-hosting lld, when compared
with the bfd linker. BFD linker on average takes 8 seconds in elapsed time.
lld takes 3 seconds elapased time average. Without this change, lld takes ~5
seconds average. The runtime comparisons were done on a release build and
measured by running linking thrice.

lld self-host without the change
----------------------------------
real    0m3.196s
user    0m4.580s
sys     0m0.832s

lld self-host with lld
-----------------------
user    0m3.024s
user    0m3.252s
sys     0m0.796s

time taken to build lld with bfd
--------------------------------
real    0m8.419s
user    0m7.748s
sys     0m0.632s

llvm-svn: 232460

diff --git a/lld/lib/ReaderWriter/ELF/OutputELFWriter.h b/lld/lib/ReaderWriter/ELF/OutputELFWriter.h
index e26bb68..c137905 100644 (file)
--- a/lld/lib/ReaderWriter/ELF/OutputELFWriter.h
+++ b/lld/lib/ReaderWriter/ELF/OutputELFWriter.h
@@ -586,8 +586,10 @@ std::error_code OutputELFWriter<ELFT>::writeOutput(const File &file,
   _elfHeader->write(this, _layout, *buffer);
   _programHeader->write(this, _layout, *buffer);
 
-  for (auto section : _layout.sections())
-    section->write(this, _layout, *buffer);
+  auto sections = _layout.sections();
+  parallel_for_each(
+      sections.begin(), sections.end(),
+      [&](Chunk<ELFT> *section) { section->write(this, _layout, *buffer); });
   writeTask.end();
 
   ScopedTask commitTask(getDefaultDomain(), "ELF Writer commit to disk");

diff --git a/lld/lib/ReaderWriter/ELF/SectionChunks.h b/lld/lib/ReaderWriter/ELF/SectionChunks.h
index db1f298..2e71cb2 100644 (file)
--- a/lld/lib/ReaderWriter/ELF/SectionChunks.h
+++ b/lld/lib/ReaderWriter/ELF/SectionChunks.h
@@ -234,17 +234,17 @@ public:
   /// routine gets called after the linker fixes up the virtual address
   /// of the section
   virtual void assignVirtualAddress(uint64_t addr) override {
-    for (auto &ai : _atoms) {
+    parallel_for_each(_atoms.begin(), _atoms.end(), [&](AtomLayout *ai) {
       ai->_virtualAddr = addr + ai->_fileOffset;
-    }
+    });
   }
 
   /// \brief Set the file offset of each Atom in the section. This routine
   /// gets called after the linker fixes up the section offset
   void assignFileOffsets(uint64_t offset) override {
-    for (auto &ai : _atoms) {
+    parallel_for_each(_atoms.begin(), _atoms.end(), [&](AtomLayout *ai) {
       ai->_fileOffset = offset + ai->_fileOffset;
-    }
+    });
   }
 
   /// \brief Find the Atom address given a name, this is needed to properly