[lld] [ELF] Support relocatable files using CREL (PR #98115)

[Fangrui Song via llvm-commits]

================
@@ -205,6 +206,94 @@ class ThunkCreator {
   uint32_t pass = 0;
 };
 
+// Decode LEB128 without error checking. Only used by performance critical code
+// like RelocsCrel.
+inline uint64_t readLEB128(const uint8_t *&p, uint64_t leb) {
+  uint64_t acc = 0, shift = 0, byte;
+  do {
+    byte = *p++;
+    acc |= (byte - 128 * (byte >= leb)) << shift;
+    shift += 7;
+  } while (byte >= 128);
+  return acc;
+}
+inline uint64_t readULEB128(const uint8_t *&p) { return readLEB128(p, 128); }
+inline int64_t readSLEB128(const uint8_t *&p) { return readLEB128(p, 64); }
+
+// This class implements a CREL iterator that does not allocate extra memory.
+template <bool is64> struct RelocsCrel {
+  using uint = std::conditional_t<is64, uint64_t, uint32_t>;
+  struct const_iterator {
+    using iterator_category = std::forward_iterator_tag;
+    using value_type = llvm::object::Elf_Crel_Impl<is64>;
+    using difference_type = ptrdiff_t;
+    using pointer = value_type *;
+    using reference = const value_type &;
+    uint32_t count;
+    uint8_t flagBits, shift;
+    const uint8_t *p;
+    llvm::object::Elf_Crel_Impl<is64> crel{};
+    const_iterator(size_t hdr, const uint8_t *p)
+        : count(hdr / 8), flagBits(hdr & 4 ? 3 : 2), shift(hdr % 4), p(p) {
+      if (count)
+        step();
+    }
+    void step() {
+      // See object::decodeCrel.
+      const uint8_t b = *p++;
+      crel.r_offset += b >> flagBits << shift;
+      if (b >= 0x80)
+        crel.r_offset +=
+            ((readULEB128(p) << (7 - flagBits)) - (0x80 >> flagBits)) << shift;
+      if (b & 1)
+        crel.r_symidx += readSLEB128(p);
+      if (b & 2)
+        crel.r_type += readSLEB128(p);
+      if (b & 4 && flagBits == 3)
+        crel.r_addend += static_cast<uint>(readSLEB128(p));
+    }
+    llvm::object::Elf_Crel_Impl<is64> operator*() const { return crel; };
+    const llvm::object::Elf_Crel_Impl<is64> *operator->() const {
+      return &crel;
+    }
+    bool operator==(const const_iterator &r) const { return count == r.count; }
+    bool operator!=(const const_iterator &r) const { return count != r.count; }
+    const_iterator &operator++() {
+      if (--count)
+        step();
+      return *this;
+    }
+    void operator+=(size_t n) {
+      for (; n; --n)
+        operator++();
+    }
+
+    // R_PPC64_REL24_NOTOC in Relocations.cpp
----------------
MaskRay wrote:

The commend `// R_PPC64_REL24_NOTOC in Relocations.cpp` can be expanded. The `operator*()` is used by 
` end = static_cast<const void *>(rels.end()); ` in Relocations.cpp

https://github.com/llvm/llvm-project/pull/98115