[llvm-objdump][WebAssembly] Fix llvm-objdump on files without symbols

If a file has no symbols, perhaps because it is a linked executable,
synthesize some symbols by walking the code section.  Otherwise the
disassembler will try to treat the whole code section as a function,
which won't parse.  Fixes https://bugs.llvm.org/show_bug.cgi?id=50957.

Differential Revision: https://reviews.llvm.org/D105539

llvm/test/tools/llvm-objdump/wasm/executable-without-symbols-debugnames.test

@@ -0,0 +1,49 @@
+# RUN: yaml2obj -o %t.wasm %s
+# RUN: llvm-objdump -d %t.wasm | FileCheck %s
+
+--- !WASM
+FileHeader:
+  Version:         0x1
+Sections:
+  - Type:            TYPE
+    Signatures:
+      - Index:           0
+        ParamTypes:      []
+        ReturnTypes:     []
+      - Index:           1
+        ParamTypes:
+          - I32
+        ReturnTypes:
+          - I32
+  - Type:            FUNCTION
+    FunctionTypes:   [ 0, 1 ]
+  - Type:            CODE
+    Functions:
+      - Index:           0
+        Locals:          []
+        Body:            0B
+      - Index:           1
+        Locals:          []
+        Body:            20000B
+  - Type:            CUSTOM
+    Name:            name
+    FunctionNames:
+      - Index:           0
+        Name:            f
+      - Index:           1
+        Name:            g
+...
+
+# CHECK:       Disassembly of section CODE:
+# CHECK-EMPTY:
+# CHECK-NEXT:  00000000 <CODE>:
+# CHECK-NEXT:          # 2 functions in section.
+# CHECK-EMPTY:
+# CHECK-NEXT:  00000001 <f>:
+# CHECK-EMPTY:
+# CHECK-NEXT:         3: 0b           	end
+# CHECK-EMPTY:
+# CHECK-NEXT:  00000004 <g>:
+# CHECK-EMPTY:
+# CHECK-NEXT:         6: 20 00        	local.get	0
+# CHECK-NEXT:         8: 0b           	end

llvm/test/tools/llvm-objdump/wasm/executable-without-symbols.test

@@ -0,0 +1,42 @@
+# RUN: yaml2obj -o %t.wasm %s
+# RUN: llvm-objdump -d %t.wasm | FileCheck %s
+
+--- !WASM
+FileHeader:
+  Version:         0x1
+Sections:
+  - Type:            TYPE
+    Signatures:
+      - Index:           0
+        ParamTypes:      []
+        ReturnTypes:     []
+      - Index:           1
+        ParamTypes:
+          - I32
+        ReturnTypes:
+          - I32
+  - Type:            FUNCTION
+    FunctionTypes:   [ 0, 1 ]
+  - Type:            CODE
+    Functions:
+      - Index:           0
+        Locals:          []
+        Body:            0B
+      - Index:           1
+        Locals:          []
+        Body:            20000B
+...
+
+# CHECK:       Disassembly of section CODE:
+# CHECK-EMPTY:
+# CHECK-NEXT:  00000000 <CODE>:
+# CHECK-NEXT:          # 2 functions in section.
+# CHECK-EMPTY:
+# CHECK-NEXT:  00000001 <>:
+# CHECK-EMPTY:
+# CHECK-NEXT:         3: 0b           	end
+# CHECK-EMPTY:
+# CHECK-NEXT:  00000004 <>:
+# CHECK-EMPTY:
+# CHECK-NEXT:         6: 20 00        	local.get	0
+# CHECK-NEXT:         8: 0b           	end

llvm/tools/llvm-objdump/llvm-objdump.cpp

@@ -736,6 +736,43 @@ addDynamicElfSymbols(const ObjectFile *Obj,
     llvm_unreachable("Unsupported binary format");
 }
 
+static Optional<SectionRef> getWasmCodeSection(const WasmObjectFile *Obj) {
+  for (auto SecI : Obj->sections()) {
+    const WasmSection &Section = Obj->getWasmSection(SecI);
+    if (Section.Type == wasm::WASM_SEC_CODE)
+      return SecI;
+  }
+  return None;
+}
+
+static void
+addMissingWasmCodeSymbols(const WasmObjectFile *Obj,
+                          std::map<SectionRef, SectionSymbolsTy> &AllSymbols) {
+  Optional<SectionRef> Section = getWasmCodeSection(Obj);
+  if (!Section)
+    return;
+  SectionSymbolsTy &Symbols = AllSymbols[*Section];
+
+  std::set<uint64_t> SymbolAddresses;
+  for (const auto &Sym : Symbols)
+    SymbolAddresses.insert(Sym.Addr);
+
+  for (const wasm::WasmFunction &Function : Obj->functions()) {
+    uint64_t Address = Function.CodeSectionOffset;
+    // Only add fallback symbols for functions not already present in the symbol
+    // table.
+    if (SymbolAddresses.count(Address))
+      continue;
+    // This function has no symbol, so it should have no SymbolName.
+    assert(Function.SymbolName.empty());
+    // We use DebugName for the name, though it may be empty if there is no
+    // "name" custom section, or that section is missing a name for this
+    // function.
+    StringRef Name = Function.DebugName;
+    Symbols.emplace_back(Address, Name, ELF::STT_NOTYPE);
+  }
+}
+
 static void addPltEntries(const ObjectFile *Obj,
                           std::map<SectionRef, SectionSymbolsTy> &AllSymbols,
                           StringSaver &Saver) {
@@ -1126,6 +1163,9 @@ static void disassembleObject(const Target *TheTarget, const ObjectFile *Obj,
   if (AllSymbols.empty() && Obj->isELF())
     addDynamicElfSymbols(Obj, AllSymbols);
 
+  if (Obj->isWasm())
+    addMissingWasmCodeSymbols(cast<WasmObjectFile>(Obj), AllSymbols);
+
   BumpPtrAllocator A;
   StringSaver Saver(A);
   addPltEntries(Obj, AllSymbols, Saver);