Simplify RelrSection<ELFT>::updateAllocSize.

[ruiu]

Summary

This patch also speeds it up by making some constants compile-time
constants. Other than that, NFC.

[pcc]

LGTM

Very clear!

lld/ELF/SyntheticSections.cpp

template <class ELFT> bool RelrSection<ELFT>::updateAllocSize() {
   // This encoding has a couple of interesting properties:
   // 1. Looking at any entry, it is clear whether it's an address or a bitmap:
   //    even means address, odd means bitmap.
   // 2. Just a simple list of addresses is a valid encoding.
 
   size_t OldSize = RelrRelocs.size();
   RelrRelocs.clear();
 
+  // Same as Config->Wordsize but faster because this is a compile-time
+  // constant.
+  const size_t Wordsize = sizeof(typename ELFT::uint);
+
   // Number of bits to use for the relocation offsets bitmap.
-  // These many relative relocations can be encoded in a single entry.
-  const size_t NBits = 8 * Config->Wordsize - 1;
+  // Must be either 63 or 31.
+  const size_t NBits = Wordsize * 8 - 1;
 
   // Get offsets for all relative relocations and sort them.
   std::vector<uint64_t> Offsets;
-  for (const RelativeReloc &Rel : Relocs) {
+  for (const RelativeReloc &Rel : Relocs)
     Offsets.push_back(Rel.getOffset());
-  }
-  std::sort(Offsets.begin(), Offsets.end());
+  llvm::sort(Offsets.begin(), Offsets.end());
 
-  uint64_t Base = 0;
-  typename std::vector<uint64_t>::iterator Curr = Offsets.begin();
-  while (Curr != Offsets.end()) {
-    uint64_t Current = *Curr;
-    assert(Current % 2 == 0);
-
-    uint64_t Bits = 0;
-    typename std::vector<uint64_t>::iterator Next = Curr;
-    if (Base > 0 && Base <= Current) {
-      while (Next != Offsets.end()) {
-        uint64_t Delta = *Next - Base;
-        // If Next is too far out, it cannot be folded into Curr.
-        if (Delta >= NBits * Config->Wordsize)
-          break;
-        // If Next is not a multiple of wordsize away, it cannot
-        // be folded into Curr.
-        if (Delta % Config->Wordsize != 0)
-          break;
-        // Next can be folded into Curr, add it to the bitmap.
-        Bits |= 1ULL << (Delta / Config->Wordsize);
-        ++Next;
-      }
+  // For each leading relocation, find following ones that can be folded
+  // as a bitmap and fold them.
+  for (size_t I = 0, E = Offsets.size(); I < E;) {
+    // Add a leading relocation.
+    RelrRelocs.push_back(Elf_Relr(Offsets[I]));
+    ++I;
+
+    // Find foldable relocations to create a bitmap.
+    uint64_t Bitmap = 0;
+    for (size_t J = I; J < E; ++J) {
+      uint64_t Delta = Offsets[J] - Offsets[I];
+
+      // If it is too far, it cannot be folded.
+      if (Delta >= NBits * Wordsize)
+
+      // If it is not a multiple of wordsize away, it cannot be folded.
+      if (Delta % Wordsize)
+
+      // Fold it.
+      Bitmap |= 1ULL << (Delta / Wordsize);
     }
 
-    if (Bits == 0) {
-      RelrRelocs.push_back(Elf_Relr(Current));
-      // This is not a continuation entry, only one offset was
-      // consumed. Set base offset for subsequent bitmap entries.
-      Base = Current + Config->Wordsize;
-      ++Curr;
-    } else {
-      RelrRelocs.push_back(Elf_Relr((Bits << 1) | 1));
-      // This is a continuation entry encoding multiple offsets
-      // in a bitmap. Advance base offset by NBits words.
-      Base += NBits * Config->Wordsize;
-      Curr = Next;
+    if (Bitmap) {
+      RelrRelocs.push_back(Elf_Relr((Bitmap << 1) | 1));
+      I += NBits;
     }
   }
 
   return RelrRelocs.size() != OldSize;
 }
 
 SymbolTableBaseSection::SymbolTableBaseSection(StringTableSection &StrTabSec)
     : SyntheticSection(StrTabSec.isDynamic() ? (uint64_t)SHF_ALLOC : 0,