[lld] [lld][macho] Move unwind logic from equalsVariable to equalsConstant (PR #165325)

[Ellis Hoag via llvm-commits]

================
@@ -179,8 +179,30 @@ bool ICF::equalsConstant(const ConcatInputSection *ia,
       return isecA->getOffset(ra.addend) == isecB->getOffset(rb.addend);
     }
   };
-  return std::equal(ia->relocs.begin(), ia->relocs.end(), ib->relocs.begin(),
-                    f);
+  if (!std::equal(ia->relocs.begin(), ia->relocs.end(), ib->relocs.begin(), f))
+    return false;
+
+  // Check unwind info structural compatibility: if there are symbols with
+  // associated unwind info, check that both sections have compatible symbol
+  // layouts. For simplicity, we only attempt folding when all symbols are at
+  // offset zero within the section (which is typically the case with
+  // .subsections_via_symbols.)
+  auto hasUnwind = [](Defined *d) { return d->unwindEntry() != nullptr; };
+  const auto *itA = llvm::find_if(ia->symbols, hasUnwind);
+  const auto *itB = llvm::find_if(ib->symbols, hasUnwind);
+  if (itA == ia->symbols.end())
+    return itB == ib->symbols.end();
+  if (itB == ib->symbols.end())
+    return false;
+  const Defined *da = *itA;
+  const Defined *db = *itB;
+  if (da->value != 0 || db->value != 0)
+    return false;
+  auto isZero = [](Defined *d) { return d->value == 0; };
+  return std::find_if_not(std::next(itA), ia->symbols.end(), isZero) ==
+             ia->symbols.end() &&
+         std::find_if_not(std::next(itB), ib->symbols.end(), isZero) ==
+             ib->symbols.end();
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ellishg wrote:

Maybe we should add a comment here to let future folks know this isn't a bug

https://github.com/llvm/llvm-project/pull/165325