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

[Alexander Richardson via llvm-commits]

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@@ -133,21 +133,56 @@ void InputSectionBase::decompress() const {
   compressed = false;
 }
 
-template <class ELFT> RelsOrRelas<ELFT> InputSectionBase::relsOrRelas() const {
+template <class ELFT>
+RelsOrRelas<ELFT> InputSectionBase::relsOrRelas(bool supportsCrel) const {
   if (relSecIdx == 0)
     return {};
   RelsOrRelas<ELFT> ret;
-  typename ELFT::Shdr shdr =
-      cast<ELFFileBase>(file)->getELFShdrs<ELFT>()[relSecIdx];
+  auto *f = cast<ObjFile<ELFT>>(file);
+  typename ELFT::Shdr shdr = f->template getELFShdrs<ELFT>()[relSecIdx];
+  if (shdr.sh_type == SHT_CREL) {
+    // Return an iterator if supported by caller.
+    if (supportsCrel) {
+      ret.crels = Relocs<typename ELFT::Crel>(
+          (const uint8_t *)f->mb.getBufferStart() + shdr.sh_offset);
+      return ret;
+    }
+    InputSectionBase *const &relSec = f->getSections()[relSecIdx];
+    // Otherwise, allocate a buffer to hold the decoded RELA relocations. When
+    // called for the first time, relSec is null (without --emit-relocs) or an
+    // InputSection with zero eqClass[0].
+    if (!relSec || !cast<InputSection>(relSec)->eqClass[0]) {
+      auto *sec = makeThreadLocal<InputSection>(*f, shdr, name);
+      f->setSection(relSecIdx, sec);
+      cast<InputSection>(sec)->eqClass[0] = SHT_RELA;
+
+      RelocsCrel<ELFT::Is64Bits> entries(sec->content_);
+      sec->size = entries.size();
+      auto *relas = makeThreadLocalN<typename ELFT::Rela>(entries.size());
+      sec->content_ = reinterpret_cast<uint8_t *>(relas);
+      size_t i = 0;
+      for (Elf_Crel_Impl<ELFT::Is64Bits> r : entries) {
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arichardson wrote:

Could use llvm::enumerate() here?

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