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.

llvm-svn: 316305

2 files changed, 2 insertions, 20 deletions

diff --git a/lld/ELF/InputSection.cpp b/lld/ELF/InputSection.cpp
index ac659f7256c..56b68da6116 100644
--- a/lld/ELF/InputSection.cpp
+++ b/lld/ELF/InputSection.cpp
@@ -963,27 +963,12 @@ const SectionPiece *MergeInputSection::getSectionPiece(uint64_t Offset) const {
 // 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;
 }
diff --git a/lld/ELF/InputSection.h b/lld/ELF/InputSection.h
index fc03749c845..853b6385ae2 100644
--- a/lld/ELF/InputSection.h
+++ b/lld/ELF/InputSection.h
@@ -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;
 };