[ELF] - Change shift2 constant of GNU_HASH from 6->11.

[grimar]

Summary

This is for PR35536 ("Bug 35536 - lld might be creating DSOs that takes more time to
load for the dynamic loader than those created by gold").

shift2 is shift count used by the Bloom filter.

latest benchmark performed today of fresh configuration
(previous my results are available on bug page)
shows this change reduces run time of "ninja check-llvm"
for LLVM built with and -DBUILD_SHARED_LIBS=ON from ~413.45s to ~401.93s.
gold time is: ~411.1s

[ruiu]

I also wonder how these initial values (6 and 5) were decided.

[ikudrin]

In D41993#975015, @ruiu wrote:

I also wonder how these initial values (6 and 5) were decided.

As far as I can remember, these values were selected more or less arbitrary as log2(TargetBits). If other values prove to be more optimal, there are no objections from me to using the new ones.

[grimar]

Suspended basing on discussion in mail list.

ELF/SyntheticSections.h

public:
   void writeTo(uint8_t *Buf) override;
   size_t getSize() const override { return Size; }
 
   // Adds symbols to the hash table.
   // Sorts the input to satisfy GNU hash section requirements.
   void addSymbols(std::vector<SymbolTableEntry> &Symbols);
 
 private:
-  size_t getShift2() const { return Config->Is64 ? 6 : 5; }
+  size_t getShift2() const { return Config->Is64 ? 11 : 5; }
 
   void writeBloomFilter(uint8_t *Buf);
   void writeHashTable(uint8_t *Buf);
 
   struct Entry {
     Symbol *Sym;
     size_t StrTabOffset;
     uint32_t Hash;