Fix thread race between SectionPiece's OutputOff and Live members

[dim]

Summary

As reported in bug 35788, rL316280 reintroduces a race between two
members of SectionPiece, which share the same 64 bit memory location.

Reduce the Hash member by 1 bit, and move the Live member next to it,
instead, while making OuputOff use the full 64 bit again.

Adjust getShardId to cope with Hashes being 31 bits wide (I hope I
interpreted that function correctly... :)

[ruiu]

LGTM

[smeenai]

Should this be considered for picking to the 6.0 release branch?

ELF/InputSection.h

 };
 
 // SectionPiece represents a piece of splittable section contents.
 // We allocate a lot of these and binary search on them. This means that they
 // have to be as compact as possible, which is why we don't store the size (can
 // be found by looking at the next one).
 struct SectionPiece {
   SectionPiece(size_t Off, uint32_t Hash, bool Live)
-      : InputOff(Off), Hash(Hash), OutputOff(-1),
-        Live(Live || !Config->GcSections) {}
+      : InputOff(Off), Hash(Hash), Live(Live || !Config->GcSections),
+        OutputOff(-1) {}
 
   uint32_t InputOff;
-  uint32_t Hash;
-  int64_t OutputOff : 63;
-  uint64_t Live : 1;
+  uint32_t Hash : 31;
+  uint32_t Live : 1;
+  int64_t OutputOff;
 };
 
 static_assert(sizeof(SectionPiece) == 16, "SectionPiece is too big");
 
 // This corresponds to a SHF_MERGE section of an input file.
 class MergeInputSection : public InputSectionBase {
 public:
   template <class ELFT>

ELF/SyntheticSections.h

 
 private:
   // We use the most significant bits of a hash as a shard ID.
   // The reason why we don't want to use the least significant bits is
   // because DenseMap also uses lower bits to determine a bucket ID.
   // If we use lower bits, it significantly increases the probability of
   // hash collisons.
   size_t getShardId(uint32_t Hash) {
-    return Hash >> (32 - llvm::countTrailingZeros(NumShards));
+    return Hash >> (31 - llvm::countTrailingZeros(NumShards));
   }
 
   // Section size
   size_t Size;
 
   // String table contents
   constexpr static size_t NumShards = 32;
   std::vector<llvm::StringTableBuilder> Shards;