[llvm] [Parallel] Revert sequential task changes (PR #109084)

[Fangrui Song via llvm-commits]

================
@@ -1647,30 +1647,44 @@ template <class ELFT> void elf::scanRelocations() {
   bool serial = !config->zCombreloc || config->emachine == EM_MIPS ||
                 config->emachine == EM_PPC64;
   parallel::TaskGroup tg;
-  for (ELFFileBase *f : ctx.objectFiles) {
-    auto fn = [f]() {
+  auto outerFn = [&]() {
+    for (ELFFileBase *f : ctx.objectFiles) {
+      auto fn = [f]() {
+        RelocationScanner scanner;
+        for (InputSectionBase *s : f->getSections()) {
+          if (s && s->kind() == SectionBase::Regular && s->isLive() &&
+              (s->flags & SHF_ALLOC) &&
+              !(s->type == SHT_ARM_EXIDX && config->emachine == EM_ARM))
+            scanner.template scanSection<ELFT>(*s);
+        }
+      };
+      if (serial)
+        fn();
+      else
+        tg.spawn(fn);
+    }
+    auto scanEH = [] {
       RelocationScanner scanner;
-      for (InputSectionBase *s : f->getSections()) {
-        if (s && s->kind() == SectionBase::Regular && s->isLive() &&
-            (s->flags & SHF_ALLOC) &&
-            !(s->type == SHT_ARM_EXIDX && config->emachine == EM_ARM))
-          scanner.template scanSection<ELFT>(*s);
+      for (Partition &part : ctx.partitions) {
+        for (EhInputSection *sec : part.ehFrame->sections)
+          scanner.template scanSection<ELFT>(*sec, /*isEH=*/true);
+        if (part.armExidx && part.armExidx->isLive())
+          for (InputSection *sec : part.armExidx->exidxSections)
+            if (sec->isLive())
+              scanner.template scanSection<ELFT>(*sec);
       }
     };
-    tg.spawn(fn, serial);
-  }
-
-  tg.spawn([] {
-    RelocationScanner scanner;
-    for (Partition &part : ctx.partitions) {
-      for (EhInputSection *sec : part.ehFrame->sections)
-        scanner.template scanSection<ELFT>(*sec, /*isEH=*/true);
-      if (part.armExidx && part.armExidx->isLive())
-        for (InputSection *sec : part.armExidx->exidxSections)
-          if (sec->isLive())
-            scanner.template scanSection<ELFT>(*sec);
-    }
-  });
+    if (serial)
+      scanEH();
+    else
+      tg.spawn(scanEH);
----------------
MaskRay wrote:

We need to run `scanEH` in a worker thread to avoid `UINT_MAX` `threadIndex`.

The `if (serial) scanEH(); else tg.spawn(scanEH);` pattern is similar to `if (serial) fn(); else tg.spawn(fn);` .

`scanEH` scans `.eh_frame` in all input files. `.eh_frame` relocations are smaller than other sections, so the previous code scans all `.eh_frame` in one task.


https://github.com/llvm/llvm-project/pull/109084