[lld-macho] Use parallel algorithms in favor of ThreadPool (#99471)

In https://reviews.llvm.org/D115416, it was decided that an explicit
thread pool should be used instead of the simpler fork-join model of the
`parallelFor*` family of functions. Since then, more parallelism has
been added to LLD, but these changes always used the latter strategy,
similarly to other ports of LLD.

This meant that we ended up spawning twice the requested amount of
threads; one set for the `llvm/Support/Parallel.h` executor, and one for
the thread pool.

Since that decision, 3b4d800 has landed, which allows us to
explicitly enqueue jobs on the executor pool of the parallel algorithms,
which should be enough to achieve sharded output writing and
parallelized input file parsing. Now only the construction of the map
file is left that should be done *concurrently* with different linking
steps, this commit proposes explicitly spawning a dedicated worker
thread for it.

Author: BertalanD

lld/MachO/Writer.cpp

@@ -28,8 +28,8 @@
 #include "llvm/Support/LEB128.h"
 #include "llvm/Support/Parallel.h"
 #include "llvm/Support/Path.h"
-#include "llvm/Support/ThreadPool.h"
 #include "llvm/Support/TimeProfiler.h"
+#include "llvm/Support/thread.h"
 #include "llvm/Support/xxhash.h"
 
 #include <algorithm>
@@ -66,7 +66,6 @@ class Writer {
 
   template <class LP> void run();
 
-  DefaultThreadPool threadPool;
   std::unique_ptr<FileOutputBuffer> &buffer;
   uint64_t addr = 0;
   uint64_t fileOff = 0;
@@ -1121,14 +1120,12 @@ void Writer::finalizeLinkEditSegment() {
       symtabSection,     indirectSymtabSection,
       dataInCodeSection, functionStartsSection,
   };
-  SmallVector<std::shared_future<void>> threadFutures;
-  threadFutures.reserve(linkEditSections.size());
-  for (LinkEditSection *osec : linkEditSections)
-    if (osec)
-      threadFutures.emplace_back(threadPool.async(
-          [](LinkEditSection *osec) { osec->finalizeContents(); }, osec));
-  for (std::shared_future<void> &future : threadFutures)
-    future.wait();
+
+  parallelForEach(linkEditSections.begin(), linkEditSections.end(),
+                  [](LinkEditSection *osec) {
+                    if (osec)
+                      osec->finalizeContents();
+                  });
 
   // Now that __LINKEDIT is filled out, do a proper calculation of its
   // addresses and offsets.
@@ -1170,6 +1167,8 @@ void Writer::openFile() {
 }
 
 void Writer::writeSections() {
+  TimeTraceScope timeScope("Write output sections");
+
   uint8_t *buf = buffer->getBufferStart();
   std::vector<const OutputSection *> osecs;
   for (const OutputSegment *seg : outputSegments)
@@ -1200,18 +1199,15 @@ void Writer::writeUuid() {
 
   ArrayRef<uint8_t> data{buffer->getBufferStart(), buffer->getBufferEnd()};
   std::vector<ArrayRef<uint8_t>> chunks = split(data, 1024 * 1024);
+
   // Leave one slot for filename
   std::vector<uint64_t> hashes(chunks.size() + 1);
-  SmallVector<std::shared_future<void>> threadFutures;
-  threadFutures.reserve(chunks.size());
-  for (size_t i = 0; i < chunks.size(); ++i)
-    threadFutures.emplace_back(threadPool.async(
-        [&](size_t j) { hashes[j] = xxh3_64bits(chunks[j]); }, i));
-  for (std::shared_future<void> &future : threadFutures)
-    future.wait();
+  parallelFor(0, chunks.size(),
+              [&](size_t i) { hashes[i] = xxh3_64bits(chunks[i]); });
   // Append the output filename so that identical binaries with different names
   // don't get the same UUID.
   hashes[chunks.size()] = xxh3_64bits(sys::path::filename(config->finalOutput));
+
   uint64_t digest = xxh3_64bits({reinterpret_cast<uint8_t *>(hashes.data()),
                                  hashes.size() * sizeof(uint64_t)});
   uuidCommand->writeUuid(digest);
@@ -1330,15 +1326,18 @@ template <class LP> void Writer::run() {
   sortSegmentsAndSections();
   createLoadCommands<LP>();
   finalizeAddresses();
-  threadPool.async([&] {
+
+  llvm::thread mapFileWriter([&] {
     if (LLVM_ENABLE_THREADS && config->timeTraceEnabled)
       timeTraceProfilerInitialize(config->timeTraceGranularity, "writeMapFile");
     writeMapFile();
     if (LLVM_ENABLE_THREADS && config->timeTraceEnabled)
       timeTraceProfilerFinishThread();
   });
+
   finalizeLinkEditSegment();
   writeOutputFile();
+  mapFileWriter.join();
 }
 
 template <class LP> void macho::writeResult() { Writer().run<LP>(); }