[lld] r323010 - Make the bloom filter a bit larger.

[Rui Ueyama via llvm-commits]

Author: ruiu
Date: Fri Jan 19 15:54:31 2018
New Revision: 323010

URL: http://llvm.org/viewvc/llvm-project?rev=323010&view=rev
Log:
Make the bloom filter a bit larger.

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.

Differential Revision: https://reviews.llvm.org/D42204

Modified:
    lld/trunk/ELF/SyntheticSections.cpp

Modified: lld/trunk/ELF/SyntheticSections.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/SyntheticSections.cpp?rev=323010&r1=323009&r2=323010&view=diff
==============================================================================
--- lld/trunk/ELF/SyntheticSections.cpp (original)
+++ lld/trunk/ELF/SyntheticSections.cpp Fri Jan 19 15:54:31 2018
@@ -1701,12 +1701,14 @@ GnuHashTableSection::GnuHashTableSection
 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