[lld] [lld-macho] Category merger: handle addends when getting symbol at offset (PR #91238)

[via llvm-commits]

https://github.com/alx32 created https://github.com/llvm/llvm-project/pull/91238

Currently the `tryFindDefinedOnIsec` takes in an `InputSection` and an `offset` and is supposed to return the target symbol that is referenced on that `InputSection` at the given offset.

However, it does not deal with the reloc `addend` and might return the incorrect symbol.
Here we add support for handling the reloc's `addend`.

>From 5d31c567b4266c7d2ddf67ebbb874f811112793c Mon Sep 17 00:00:00 2001
From: Alex Borcan <alexborcan at fb.com>
Date: Sun, 5 May 2024 19:40:10 -0700
Subject: [PATCH] [lld-macho]  Category merger: handle addends when getting
 symbol at isec offset

---
 lld/MachO/ObjC.cpp | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/lld/MachO/ObjC.cpp b/lld/MachO/ObjC.cpp
index 4760fffebe3b30..919829b36f06fd 100644
--- a/lld/MachO/ObjC.cpp
+++ b/lld/MachO/ObjC.cpp
@@ -455,6 +455,7 @@ class ObjcCategoryMerger {
   Defined *emitCategoryName(const std::string &name, ObjFile *objFile);
   void createSymbolReference(Defined *refFrom, const Symbol *refTo,
                              uint32_t offset, const Reloc &relocTemplate);
+  Defined *tryFindDefinedOnIsec(const InputSection *isec, uint32_t offset);
   Symbol *tryGetSymbolAtIsecOffset(const ConcatInputSection *isec,
                                    uint32_t offset);
   Defined *tryGetDefinedAtIsecOffset(const ConcatInputSection *isec,
@@ -518,12 +519,32 @@ void ObjcCategoryMerger::collectSectionWriteInfoFromIsec(
 Symbol *
 ObjcCategoryMerger::tryGetSymbolAtIsecOffset(const ConcatInputSection *isec,
                                              uint32_t offset) {
+  if (!isec)
+    return nullptr;
   const Reloc *reloc = isec->getRelocAt(offset);
 
   if (!reloc)
     return nullptr;
 
-  return reloc->referent.get<Symbol *>();
+  Symbol *sym = reloc->referent.get<Symbol *>();
+
+  if (reloc->addend) {
+    assert(isa<Defined>(sym) && "Expected defined for non-zero addend");
+    Defined *definedSym = cast<Defined>(sym);
+    sym = tryFindDefinedOnIsec(definedSym->isec(),
+                               definedSym->value + reloc->addend);
+  }
+
+  return sym;
+}
+
+Defined *ObjcCategoryMerger::tryFindDefinedOnIsec(const InputSection *isec,
+                                                  uint32_t offset) {
+  for (Defined *sym : isec->symbols)
+    if ((sym->value <= offset) && (sym->value + sym->size > offset))
+      return sym;
+
+  return nullptr;
 }
 
 Defined *