[WebAssembly] Add Object and ObjectWriter support for wasm COMDAT sections

Allow sections to be placed into COMDAT groups, in addtion to functions and data
segments.

Also make section symbols unnamed, which allows sections with identical names
(section names are independent of their section symbols, but previously we
gave the symbols the same name as their sections, which results in collisions
when sections are identically-named).

Differential Revision: https://reviews.llvm.org/D92691

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);