[PATCH] D42204: Make the bloom filter a bit larger.

[Rui Ueyama via Phabricator via llvm-commits]

ruiu created this revision.
ruiu added reviewers: grimar, rafael.
Herald added a subscriber: emaste.

I created https://reviews.llvm.org/D42202 to see how large the bloom
filter should be. With that patch, I tested various bloom filter sizes
with the following commands:

  $ cmake -GNinja -DCMAKE_BUILD_TYPE=Debug -DLLVM_ENABLE_LLD=true \
    -DLLVM_ENABLE_PROJECTS='clang;lld' -DBUILD_SHARED_LIBS=ON \
    -DCMAKE_SHARED_LINKER_FLAGS=-Wl,-bloom-filter-bits=<some integer> \
    ../llvm-project/llvm
  $ rm -f $(find . -name \*.so.7.0.0svn)
  $ ninja lld
  $ LD_BIND_NOW=1 perf stat bin/ld.lld

Here is the result:

  -bloom-filter-bits=8   0.220351609 seconds
  -bloom-filter-bits=10  0.217146597 seconds
  -bloom-filter-bits=12  0.206870826 seconds
  -bloom-filter-bits=16  0.209456312 seconds
  -bloom-filter-bits=32  0.195092075 seconds

Currently we allocate 8 bits for a symbol, but according to the above
result, that number is not optimal. Even though the numbers follow the
diminishing return rule, the point where a marginal improvement becomes
too small is not -bloom-filter-bits=8 but 12. So this patch sets it to 12.


https://reviews.llvm.org/D42204

Files:
  lld/ELF/SyntheticSections.cpp


Index: lld/ELF/SyntheticSections.cpp
===================================================================
--- lld/ELF/SyntheticSections.cpp
+++ lld/ELF/SyntheticSections.cpp
@@ -1701,12 +1701,14 @@
 void GnuHashTableSection::finalizeContents() {
   getParent()->Link = InX::DynSymTab->getParent()->SectionIndex;
 
-  // Computes bloom filter size in word size. We want to allocate 8
+  // Computes bloom filter size in word size. We want to allocate 12
   // bits for each symbol. It must be a power of two.
-  if (Symbols.empty())
+  if (Symbols.empty()) {
     MaskWords = 1;
-  else
-    MaskWords = NextPowerOf2((Symbols.size() - 1) / Config->Wordsize);
+  } else {
+    uint64_t NumBits = Symbols.size() * 12;
+    MaskWords = NextPowerOf2(NumBits / (Config->Wordsize * 8));
+  }
 
   Size = 16;                            // Header
   Size += Config->Wordsize * MaskWords; // Bloom filter


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