[lld] r314711 - Fix a data race found by tsan.

[Rui Ueyama via llvm-commits]

Author: ruiu
Date: Mon Oct  2 13:16:13 2017
New Revision: 314711

URL: http://llvm.org/viewvc/llvm-project?rev=314711&view=rev
Log:
Fix a data race found by tsan.

Reads from `Live` and writes to `OutputOff` in the following code race
even though they are logically independent because they are bitfields
sharing the same word.

  for (size_t I = 0, E = Sec->Pieces.size(); I != E; ++I) {
    if (!Sec->Pieces[I].Live)
      continue;
    CachedHashStringRef Str = Sec->getData(I);
    size_t ShardId = getShardId(Str.hash());
    if ((ShardId & (Concurrency - 1)) == ThreadId)
      Sec->Pieces[I].OutputOff = Shards[ShardId].add(Str);
  }

Modified:
    lld/trunk/ELF/InputSection.h

Modified: lld/trunk/ELF/InputSection.h
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputSection.h?rev=314711&r1=314710&r2=314711&view=diff
==============================================================================
--- lld/trunk/ELF/InputSection.h (original)
+++ lld/trunk/ELF/InputSection.h Mon Oct  2 13:16:13 2017
@@ -196,11 +196,11 @@ private:
 // be found by looking at the next one) and put the hash in a side table.
 struct SectionPiece {
   SectionPiece(size_t Off, bool Live = false)
-      : InputOff(Off), OutputOff(-1), Live(Live || !Config->GcSections) {}
+      : InputOff(Off), Live(Live || !Config->GcSections), OutputOff(-1) {}
 
-  size_t InputOff;
-  ssize_t OutputOff : 8 * sizeof(ssize_t) - 1;
+  size_t InputOff : 8 * sizeof(ssize_t) - 1;
   size_t Live : 1;
+  ssize_t OutputOff;
 };
 static_assert(sizeof(SectionPiece) == 2 * sizeof(size_t),
               "SectionPiece is too big");