[PATCH] D27152: Merge strings using sharded hash tables.

[Rui Ueyama via Phabricator via llvm-commits]

ruiu added a comment.

Resurrected the original code to use the (non-concurrent) string table table to use it when -no-thread is given, so that this patch doesn't hurt when threads are explicitly disabled.



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Comment at: ELF/OutputSections.cpp:530
 
-template <class ELFT> void MergeOutputSection<ELFT>::finalizeNoTailMerge() {
-  // Add all string pieces to the string table builder to create section
-  // contents. Because we are not tail-optimizing, offsets of strings are
-  // fixed when they are added to the builder (string table builder contains
-  // a hash table from strings to offsets).
-  for (MergeInputSection<ELFT> *Sec : Sections)
-    for (size_t I = 0, E = Sec->Pieces.size(); I != E; ++I)
-      if (Sec->Pieces[I].Live)
-        Sec->Pieces[I].OutputOff = Builder.add(Sec->getData(I));
-
-  Builder.finalizeInOrder();
-  this->Size = Builder.getSize();
+static size_t align2(size_t Val, size_t Alignment) {
+  return (Val + Alignment - 1) & ~(Alignment - 1);
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silvas wrote:
> Do we already have a helper for this in libsupport?
Done.


================
Comment at: ELF/OutputSections.cpp:562
+  const int NumShards = 8;
+  DenseMap<CachedHashStringRef, size_t> OffsetMap[NumShards];
+  size_t ShardSize[NumShards];
----------------
silvas wrote:
> Please pad this so that there isn't false sharing. DenseMap is smaller than a cacheline IIRC and so currently different threads will have false sharing.
Done. It actually reduced the latency of this function by almost 10%. Wow.


https://reviews.llvm.org/D27152