Remove GnuHashTableSection::getShift2().

[ruiu]

Summary

Choosing a Shift2 value based on wordsize is cargo-culted from gold.
Assuming that djb hash is a good hash function, choosing bits [4,9]
shouldn't be any worse or better than choosing bits [5,10]. We shouldn't
have copied that behavior that we can't justify in the first place.

[grimar]

LGTM.
(Though I think we could change it to 12+. We can't justify at this moment why it shows better results,
but it shows them whatever the real reason is. And that value at least would be based on observations.
I think technically setting is to 6 is at the same level of cargo-culting while we are unable to explain why
values ~12+ are faster).

[ruiu]

I don't want to change a value based on an observation that in some condition some number showed a better result. We don't know if that's an artifact of the test environment, and we don't know why these numbers are different in the first place.

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; }
+  enum { Shift2 = 6 };
 
   void writeBloomFilter(uint8_t *Buf);
   void writeHashTable(uint8_t *Buf);
 
   struct Entry {
     Symbol *Sym;
     size_t StrTabOffset;
     uint32_t Hash;

ELF/SyntheticSections.cpp

void GnuHashTableSection::writeTo(uint8_t *Buf) {
   // fill executable sections with trap instructions. This is a precaution
   // for that case, which happens only when -no-rosegment is given.
   memset(Buf, 0, Size);
 
   // Write a header.
   write32(Buf, NBuckets);
   write32(Buf + 4, InX::DynSymTab->getNumSymbols() - Symbols.size());
   write32(Buf + 8, MaskWords);
-  write32(Buf + 12, getShift2());
+  write32(Buf + 12, Shift2);
   Buf += 16;
 
   // Write a bloom filter and a hash table.
   writeBloomFilter(Buf);
   Buf += Config->Wordsize * MaskWords;
   writeHashTable(Buf);
 }
 
 // This function writes a 2-bit bloom filter. This bloom filter alone
 // usually filters out 80% or more of all symbol lookups [1].
 // The dynamic linker uses the hash table only when a symbol is not
 // filtered out by a bloom filter.
 //
 // [1] Ulrich Drepper (2011), "How To Write Shared Libraries" (Ver. 4.1.2),
 //     p.9, https://www.akkadia.org/drepper/dsohowto.pdf
 void GnuHashTableSection::writeBloomFilter(uint8_t *Buf) {
   unsigned C = Config->Is64 ? 64 : 32;
   for (const Entry &Sym : Symbols) {
     size_t I = (Sym.Hash / C) & (MaskWords - 1);
     uint64_t Val = readUint(Buf + I * Config->Wordsize);
     Val |= uint64_t(1) << (Sym.Hash % C);
-    Val |= uint64_t(1) << ((Sym.Hash >> getShift2()) % C);
+    Val |= uint64_t(1) << ((Sym.Hash >> Shift2) % C);
     writeUint(Buf + I * Config->Wordsize, Val);
   }
 }
 
 void GnuHashTableSection::writeHashTable(uint8_t *Buf) {
   uint32_t *Buckets = reinterpret_cast<uint32_t *>(Buf);
   uint32_t OldBucket = -1;
   uint32_t *Values = Buckets + NBuckets;

test/ELF/gnu-hash-table.s

 # EMPTY-NEXT:     Other: 0
 # EMPTY-NEXT:     Section: Undefined
 # EMPTY-NEXT:   }
 # EMPTY-NEXT: ]
 # EMPTY:      GnuHashTable {
 # EMPTY-NEXT:   Num Buckets: 1
 # EMPTY-NEXT:   First Hashed Symbol Index: 2
 # EMPTY-NEXT:   Num Mask Words: 1
-# EMPTY-NEXT:   Shift Count: 5
+# EMPTY-NEXT:   Shift Count: 6
 # EMPTY-NEXT:   Bloom Filter: [0x0]
 # EMPTY-NEXT:   Buckets: [0]
 # EMPTY-NEXT:   Values: []
 # EMPTY-NEXT: }
 
 # I386:      Format: ELF32-i386
 # I386:      Arch: i386
 # I386:      AddressSize: 32bit

 # I386:          Binding: Global
 # I386:          Section: .text
 # I386:        }
 # I386:      ]
 # I386:      GnuHashTable {
 # I386-NEXT:   Num Buckets: 1
 # I386-NEXT:   First Hashed Symbol Index: 4
 # I386-NEXT:   Num Mask Words: 1
-# I386-NEXT:   Shift Count: 5
-# I386-NEXT:   Bloom Filter: [0x14000220]
+# I386-NEXT:   Shift Count: 6
+# I386-NEXT:   Bloom Filter: [0x4004204]
 # I386-NEXT:   Buckets: [4]
 # I386-NEXT:   Values: [0xB8860BA, 0xB887389]
 # I386-NEXT: }
 
 # X86_64:      Format: ELF64-x86-64
 # X86_64:      Arch: x86_64
 # X86_64:      AddressSize: 64bit
 # X86_64:      Sections [