[lld-macho] Don't allocate memory in parallelForEach

[int3]

Summary

... since BumpPtrAllocator isn't thread-safe.

[int3]

format

lld/MachO/ICF.cpp

bool isHashable = (isCodeSection(isec) || isCfStringSection(isec) ||
                        isClassRefsSection(isec)) &&
                       !isec->shouldOmitFromOutput() &&
                       sectionType(isec->getFlags()) == MachO::S_REGULAR;
     if (isHashable) {
       hashable.push_back(isec);
       for (Defined *d : isec->symbols)
         if (d->unwindEntry)
           hashable.push_back(d->unwindEntry);
-    } else {
-      isec->icfEqClass[0] = ++icfUniqueID;
-    }
-  }
-  parallelForEach(hashable, [](ConcatInputSection *isec) {
+
       // __cfstring has embedded addends that foil ICF's hashing / equality
       // checks. (We can ignore embedded addends when doing ICF because the same
       // information gets recorded in our Reloc structs.) We therefore create a
       // mutable copy of the CFString and zero out the embedded addends before
       // performing any hashing / equality checks.
       if (isCfStringSection(isec) || isClassRefsSection(isec)) {
+        // We have to do this copying serially as the BumpPtrAllocator is not
+        // thread-safe. FIXME: Make a thread-safe allocator.
         MutableArrayRef<uint8_t> copy = isec->data.copy(bAlloc());
         for (const Reloc &r : isec->relocs)
-        target->relocateOne(copy.data() + r.offset, r, /*va=*/0, /*relocVA=*/0);
+          target->relocateOne(copy.data() + r.offset, r, /*va=*/0,
+                              /*relocVA=*/0);
         isec->data = copy;
       }
+    } else {
+      isec->icfEqClass[0] = ++icfUniqueID;
+    }
+  }
+  parallelForEach(hashable, [](ConcatInputSection *isec) {
     assert(isec->icfEqClass[0] == 0); // don't overwrite a unique ID!
     // Turn-on the top bit to guarantee that valid hashes have no collisions
     // with the small-integer unique IDs for ICF-ineligible sections
     isec->icfEqClass[0] = xxHash64(isec->data) | (1ull << 31);
   });
   // Now that every input section is either hashed or marked as unique, run the
   // segregation algorithm to detect foldable subsections.
   ICF(hashable).run();
 }