1 files changed, 20 insertions, 2 deletions
diff --git a/llvm/lib/MC/WasmObjectWriter.cpp b/llvm/lib/MC/WasmObjectWriter.cpp
index 211f0de..77df4ac 100644
--- a/llvm/lib/MC/WasmObjectWriter.cpp
+++ b/llvm/lib/MC/WasmObjectWriter.cpp
@@ -1421,6 +1421,16 @@ uint64_t WasmObjectWriter::writeOneObject(MCAssembler &Asm,
         continue;
       }
 
+      // Custom sections can also belong to COMDAT groups. In this case the
+      // decriptor's "index" field is the section index (in the final object
+      // file), but that is not known until after layout, so it must be fixed up
+      // later
+      if (const MCSymbolWasm *C = Section.getGroup()) {
+        Comdats[C->getName()].emplace_back(
+            WasmComdatEntry{wasm::WASM_COMDAT_SECTION,
+                            static_cast<uint32_t>(CustomSections.size())});
+      }
+
       CustomSections.emplace_back(Name, &Section);
     }
   }
@@ -1799,9 +1809,17 @@ uint64_t WasmObjectWriter::writeOneObject(MCAssembler &Asm,
     DataSectionIndex = writeDataSection(Layout);
   }
 
-  for (auto &CustomSection : CustomSections) {
-    writeCustomSection(CustomSection, Asm, Layout);
+  // The Sections in the COMDAT list have placeholder indices (their index among
+  // custom sections, rather than among all sections). Fix them up here.
+  for (auto &Group : Comdats) {
+    for (auto &Entry : Group.second) {
+      if (Entry.Kind == wasm::WASM_COMDAT_SECTION) {
+        Entry.Index += SectionCount;
+      }
+    }
   }
+  for (auto &CustomSection : CustomSections)
+    writeCustomSection(CustomSection, Asm, Layout);
 
   if (Mode != DwoMode::DwoOnly) {
     writeLinkingMetaDataSection(SymbolInfos, InitFuncs, Comdats);