WebAssembly: attempt to fix non-SymbolExpr relocations

Also try hard to extract a symbol from a MCExpr when writing relocations

I doubt this is the right way, but hey it emits an object now

Author: zhuowei

lib/MC/WasmObjectWriter.cpp

@@ -477,8 +477,8 @@ void WasmObjectWriter::recordRelocation(MCAssembler &Asm,
 
   if (SymA && SymA->isVariable()) {
     const MCExpr *Expr = SymA->getVariableValue();
-    const auto *Inner = cast<MCSymbolRefExpr>(Expr);
-    if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
+    const auto *Inner = dyn_cast<MCSymbolRefExpr>(Expr);
+    if (Inner && Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
       llvm_unreachable("weakref used in reloc not yet implemented");
   }
 
@@ -564,9 +564,32 @@ static void WriteI32(raw_pwrite_stream &Stream, uint32_t X, uint64_t Offset) {
   Stream.pwrite((char *)Buffer, sizeof(Buffer), Offset);
 }
 
+static const MCSymbolRefExpr* pullSymbol(const MCExpr *TheExpr) {
+  if (!TheExpr) return nullptr;
+  const MCSymbolRefExpr* S = dyn_cast<MCSymbolRefExpr>(TheExpr);
+  if (S) return S;
+  const MCBinaryExpr* Expr = dyn_cast<MCBinaryExpr>(TheExpr);
+  if (!Expr) return nullptr;
+  S = dyn_cast_or_null<MCSymbolRefExpr>(Expr->getLHS());
+  if (S) return S;
+  S = dyn_cast_or_null<MCSymbolRefExpr>(Expr->getRHS());
+  if (S) return S;
+  S = pullSymbol(Expr->getLHS());
+  if (S) return S;
+  S = pullSymbol(Expr->getRHS());
+  if (S) return S;
+  return nullptr;
+}
+
 static const MCSymbolWasm *ResolveSymbol(const MCSymbolWasm &Symbol) {
   if (Symbol.isVariable()) {
     const MCExpr *Expr = Symbol.getVariableValue();
+    if (Expr->getKind() == MCExpr::Binary) {
+      Expr = pullSymbol(Expr);
+      if (!Expr) {
+        llvm_unreachable("can't find a symbol in binary expression\n");
+      }
+    }
     auto *Inner = cast<MCSymbolRefExpr>(Expr);
     return cast<MCSymbolWasm>(&Inner->getSymbol());
   }