[lld] r348401 - Do not use a hash table to uniquify mergeable strings.

[Rui Ueyama via llvm-commits]

Author: ruiu
Date: Wed Dec  5 11:13:31 2018
New Revision: 348401

URL: http://llvm.org/viewvc/llvm-project?rev=348401&view=rev
Log:
Do not use a hash table to uniquify mergeable strings.

Previously, we have a hash table containing strings and their offsets
to manage mergeable strings. Technically we can live without that, because
we can do binary search on a vector of mergeable strings to find a mergeable
strings.

We did have both the hash table and the binary search because we thought
that that is faster.

We recently observed that lld tend to consume more memory than gold when
building an output with debug info. A few percent of memory is consumed by
the hash table. So, we needed to reevaluate whether or not having the extra
hash table is a good CPU/memory tradeoff. I run a few benchmarks with and
without the hash table.

I got a mixed result for the benchmark. We observed a regression for some
programs by removing the hash table (that's what we expected), but we also
observed that performance imrpovements for some programs. This is perhaps
due to reduced memory usage.

Differential Revision: https://reviews.llvm.org/D55234

Modified:
    lld/trunk/ELF/InputSection.cpp
    lld/trunk/ELF/InputSection.h

Modified: lld/trunk/ELF/InputSection.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputSection.cpp?rev=348401&r1=348400&r2=348401&view=diff
==============================================================================
--- lld/trunk/ELF/InputSection.cpp (original)
+++ lld/trunk/ELF/InputSection.cpp Wed Dec  5 11:13:31 2018
@@ -1206,21 +1206,12 @@ void MergeInputSection::splitIntoPieces(
     splitStrings(data(), Entsize);
   else
     splitNonStrings(data(), Entsize);
-
-  OffsetMap.reserve(Pieces.size());
-  for (size_t I = 0, E = Pieces.size(); I != E; ++I)
-    OffsetMap[Pieces[I].InputOff] = I;
 }
 
 SectionPiece *MergeInputSection::getSectionPiece(uint64_t Offset) {
   if (this->data().size() <= Offset)
     fatal(toString(this) + ": offset is outside the section");
 
-  // Find a piece starting at a given offset.
-  auto It = OffsetMap.find(Offset);
-  if (It != OffsetMap.end())
-    return &Pieces[It->second];
-
   // If Offset is not at beginning of a section piece, it is not in the map.
   // In that case we need to  do a binary search of the original section piece vector.
   auto It2 =

Modified: lld/trunk/ELF/InputSection.h
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputSection.h?rev=348401&r1=348400&r2=348401&view=diff
==============================================================================
--- lld/trunk/ELF/InputSection.h (original)
+++ lld/trunk/ELF/InputSection.h Wed Dec  5 11:13:31 2018
@@ -253,7 +253,6 @@ public:
   // Splittable sections are handled as a sequence of data
   // rather than a single large blob of data.
   std::vector<SectionPiece> Pieces;
-  llvm::DenseMap<uint32_t, uint32_t> OffsetMap;
 
   // Returns I'th piece's data. This function is very hot when
   // string merging is enabled, so we want to inline.