[lld] r316305 - Remove a fast lookup table from MergeInputSection.

[Rui Ueyama via llvm-commits]

Author: ruiu
Date: Sun Oct 22 16:02:07 2017
New Revision: 316305

URL: http://llvm.org/viewvc/llvm-project?rev=316305&view=rev
Log:
Remove a fast lookup table from MergeInputSection.

We used to have a map from section piece offsets to section pieces
as a cache for binary search. But I found that the map took quite a
large amount of memory and didn't make linking faster. So, in this
patch, I removed the map.

This patch saves 566 MiB of RAM (2.019 GiB -> 1.453 GiB) when linking
clang with debug info, and the link time is 4% faster in that test case.

Thanks for Sean Silva for pointing this out.

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=316305&r1=316304&r2=316305&view=diff
==============================================================================
--- lld/trunk/ELF/InputSection.cpp (original)
+++ lld/trunk/ELF/InputSection.cpp Sun Oct 22 16:02:07 2017
@@ -963,27 +963,12 @@ const SectionPiece *MergeInputSection::g
 // Because contents of a mergeable section is not contiguous in output,
 // it is not just an addition to a base output offset.
 uint64_t MergeInputSection::getOffset(uint64_t Offset) const {
-  if (!this->Live)
+  if (!Live)
     return 0;
 
-  // Initialize OffsetMap lazily.
-  llvm::call_once(InitOffsetMap, [&] {
-    OffsetMap.reserve(Pieces.size());
-    for (size_t I = 0; I < Pieces.size(); ++I)
-      OffsetMap[Pieces[I].InputOff] = I;
-  });
-
-  // Find a string starting at a given offset.
-  auto It = OffsetMap.find(Offset);
-  if (It != OffsetMap.end())
-    return Pieces[It->second].OutputOff;
-
-  // If Offset is not at beginning of a section piece, it is not in the map.
-  // In that case we need to search from the original section piece vector.
-  const SectionPiece &Piece = *this->getSectionPiece(Offset);
+  const SectionPiece &Piece = *getSectionPiece(Offset);
   if (!Piece.Live)
     return 0;
-
   uint64_t Addend = Offset - Piece.InputOff;
   return Piece.OutputOff + Addend;
 }

Modified: lld/trunk/ELF/InputSection.h
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputSection.h?rev=316305&r1=316304&r2=316305&view=diff
==============================================================================
--- lld/trunk/ELF/InputSection.h (original)
+++ lld/trunk/ELF/InputSection.h Sun Oct 22 16:02:07 2017
@@ -269,9 +269,6 @@ private:
   void splitStrings(ArrayRef<uint8_t> A, size_t Size);
   void splitNonStrings(ArrayRef<uint8_t> A, size_t Size);
 
-  mutable llvm::DenseMap<uint32_t, uint32_t> OffsetMap;
-  mutable llvm::once_flag InitOffsetMap;
-
   llvm::DenseSet<uint64_t> LiveOffsets;
 };