[WebAssembly] ignore non-SymbolRef MCExpr.

[patcheng]

Summary

instead of static casting a MCExpr to MCSymbolRefExpr directly, use dyn_cast and added null check.

Also removed an unnecessary (?) assertion

[sunfish]

Is this fixing a bug? It would help me to see what kind of code triggers this bug.

[sbc100]

lib/MC/WasmObjectWriter.cpp

446

Why is this unnecessary? Were you hitting this case? Perhaps we need a test that triggers this? Also, if this is unrelated to the MCSymbolRefExpr change than perhaps make it a separate change?

[patcheng]

Changes suggested by sbc100

[patcheng]

I messed up the diff. I will upload a correct one soon. hopefully with detail on the case that were hitting the code in the original diff.

[patcheng]

I rolled back the changes to ignore non-MCSymbolExpr.

Here is a dump of the MCExpr that I was encountering around line 1260 (in the for loop that's handling weak aliases.

$Ss10_ArrayBodyVN : isExternal: true isInSection: true isRegistered: true isUsed: true isVariable: true
	dumpMCExpr: 	Binary: opCode: Add
	LHS
		dumpMCExpr: 		Symbol: $Ss10_ArrayBodyVMf
	RHS
		dumpMCExpr: 		Constant: 4

[patcheng]

https://github.com/apple/swift-llvm/pull/155/files has an alternative fix.

lib/MC/WasmObjectWriter.cpp

 
   if (SymA && SymA->isVariable()) {
     const MCExpr *Expr = SymA->getVariableValue();
-    const auto *Inner = cast<MCSymbolRefExpr>(Expr);
-    if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
-      llvm_unreachable("weakref used in reloc not yet implemented");
+    if (Expr->getKind() == MCExpr::SymbolRef) {
+      const auto *Inner = cast<MCSymbolRefExpr>(Expr);
+      if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
+        llvm_unreachable("weakref used in reloc not yet implemented");      
+    } else {
+      llvm_unreachable("expression kind not supported");
+    }
   }
 
   // Put any constant offset in an addend. Offsets can be negative, and

 static const MCSymbolWasm* ResolveSymbol(const MCSymbolWasm& Symbol) {
   if (Symbol.isVariable()) {
     const MCExpr *Expr = Symbol.getVariableValue();
-    auto *Inner = cast<MCSymbolRefExpr>(Expr);
-    return cast<MCSymbolWasm>(&Inner->getSymbol());
+    if (Expr->getKind() == MCExpr::SymbolRef) {    
+      auto *Inner = cast<MCSymbolRefExpr>(Expr);
+      return cast<MCSymbolWasm>(&Inner->getSymbol());
+    } else {
+      llvm_unreachable("expression kind not supported");      
+    }
   }
   return &Symbol;
 }