[lld] [lld][MachO] Tail merge strings (PR #161262)

[Kyungwoo Lee via llvm-commits]

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@@ -1754,17 +1755,57 @@ void DeduplicatedCStringSection::finalizeContents() {
       assert(isec->align != 0);
       auto align = getStringPieceAlignment(isec, piece);
       auto [it, wasInserted] = strToAlignment.try_emplace(s, align);
+      if (wasInserted)
+        deduplicatedStrs.push_back(s);
       if (!wasInserted && it->second < align)
         it->second = align;
     }
   }
 
+  // Like lexigraphical sort, except we read strings in reverse and take the
+  // longest string first
+  // TODO: We could improve performance by implementing our own sort that avoids
+  // comparing characters we know to be the same. See
+  // StringTableBuilder::multikeySort() for details
+  llvm::sort(deduplicatedStrs, [](const auto &left, const auto &right) {
+    for (const auto &[leftChar, rightChar] :
+         llvm::zip(llvm::reverse(left.val()), llvm::reverse(right.val()))) {
+      if (leftChar == rightChar)
+        continue;
+      return leftChar < rightChar;
+    }
+    return left.size() > right.size();
+  });
+  std::optional<CachedHashStringRef> mergeCandidate;
+  DenseMap<CachedHashStringRef, std::pair<CachedHashStringRef, uint64_t>>
+      tailMergeMap;
+  for (auto &s : deduplicatedStrs) {
+    if (!mergeCandidate || !mergeCandidate->val().ends_with(s.val())) {
+      mergeCandidate = s;
+      continue;
+    }
+    uint64_t tailOffset = mergeCandidate->size() - s.size();
+    // TODO: If the tail offset is incompatible with this string's alignment, we
+    // might be able to find another superstring with a compatible tail offset.
+    // The difficulty is how to do this efficiently
+    const auto &align = strToAlignment.at(s);
+    if (!isAligned(align, tailOffset))
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kyulee-com wrote:

I was a bit confused initially. Because the alignment of superstring will be promoted by the following code, here we can just check if the tailOffset is aligned and bail-out the failing case early.

https://github.com/llvm/llvm-project/pull/161262