[Support][Parallel] Add sequential mode to TaskGroup::spawn().

This patch allows to specify that some part of tasks should be
done in sequential order. It makes it possible to not use
condition operator for separating sequential tasks:

TaskGroup tg;
for () {
  if(condition)      ==>   tg.spawn([](){fn();}, condition)
    fn();
  else
    tg.spawn([](){fn();});
}

It also prevents execution on main thread. Which allows adding
checks for getThreadIndex() function discussed in D142318.

The patch also replaces std::stack with std::deque in the
ThreadPoolExecutor to have natural execution order in case
(parallel::strategy.ThreadsRequested == 1).

Differential Revision: https://reviews.llvm.org/D148728

2 files changed, 3 insertions, 6 deletions

diff --git a/lld/ELF/OutputSections.cpp b/lld/ELF/OutputSections.cpp
index 2251ecc83cde..a1aec83f4ac8 100644
--- a/lld/ELF/OutputSections.cpp
+++ b/lld/ELF/OutputSections.cpp
@@ -534,7 +534,7 @@ void OutputSection::writeTo(uint8_t *buf, parallel::TaskGroup &tg) {
     taskSize += sections[i]->getSize();
     bool done = ++i == numSections;
     if (done || taskSize >= taskSizeLimit) {
-      tg.execute([=] { fn(begin, i); });
+      tg.spawn([=] { fn(begin, i); });
       if (done)
         break;
       begin = i;
diff --git a/lld/ELF/Relocations.cpp b/lld/ELF/Relocations.cpp
index 6f2280b678b4..bda979c3066a 100644
--- a/lld/ELF/Relocations.cpp
+++ b/lld/ELF/Relocations.cpp
@@ -1534,16 +1534,13 @@ template <class ELFT> void elf::scanRelocations() {
           scanner.template scanSection<ELFT>(*s);
       }
     };
-    if (serial)
-      fn();
-    else
-      tg.execute(fn);
+    tg.spawn(fn, serial);
   }
 
   // Both the main thread and thread pool index 0 use getThreadIndex()==0. Be
   // careful that they don't concurrently run scanSections. When serial is
   // true, fn() has finished at this point, so running execute is safe.
-  tg.execute([] {
+  tg.spawn([] {
     RelocationScanner scanner;
     for (Partition &part : partitions) {
       for (EhInputSection *sec : part.ehFrame->sections)