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

[Jez Ng 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();
----------------
int3 wrote:

my memory is a bit hazy here, but this is preserving the logic of the existing code :)

but I think it works because `symbols` is sorted in ascending order of offset, and since we're checking that the first one with unwind info has offset zero, this subsequent check ensures all symbols in the section have offset zero too.

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