[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

void WasmObjectWriter::recordRelocation(MCAssembler &Asm,
   }
 
   // We either rejected the fixup or folded B into C at this point.
   const MCSymbolRefExpr *RefA = Target.getSymA();
   const auto *SymA = RefA ? cast<MCSymbolWasm>(&RefA->getSymbol()) : nullptr;
 
   if (SymA && SymA->isVariable()) {
     const MCExpr *Expr = SymA->getVariableValue();
-    const auto *Inner = cast<MCSymbolRefExpr>(Expr);
+    if (const auto *Inner = dyn_cast<MCSymbolRefExpr>(Expr)) {
       if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
         llvm_unreachable("weakref used in reloc not yet implemented");
     }
+  }
 
   // Put any constant offset in an addend. Offsets can be negative, and
   // LLVM expects wrapping, in contrast to wasm's immediates which can't
   // be negative and don't wrap.
   FixedValue = 0;
 
   if (SymA)
     SymA->setUsedInReloc();
 
-  assert(!IsPCRel);
   assert(SymA);
 
   unsigned Type = getRelocType(Target, Fixup);
 
   WasmRelocationEntry Rec(FixupOffset, SymA, C, Type, &FixupSection);
   DEBUG(dbgs() << "WasmReloc: " << Rec << "\n");
 
   if (FixupSection.isWasmData())
static void WriteI32(raw_pwrite_stream &Stream, uint32_t X, uint64_t Offset) {
   uint8_t Buffer[4];
   support::endian::write32le(Buffer, X);
   Stream.pwrite((char *)Buffer, sizeof(Buffer), Offset);
 }
 
 static const MCSymbolWasm* ResolveSymbol(const MCSymbolWasm& Symbol) {
   if (Symbol.isVariable()) {
     const MCExpr *Expr = Symbol.getVariableValue();
-    auto *Inner = cast<MCSymbolRefExpr>(Expr);
+    if (auto *Inner = dyn_cast<MCSymbolRefExpr>(Expr)) {
         return cast<MCSymbolWasm>(&Inner->getSymbol());
     }
+  }
   return &Symbol;
 }
 
 // Compute a value to write into the code at the location covered
 // by RelEntry. This value isn't used by the static linker; it just serves
 // to make the object format more readable and more likely to be directly
 // useable.
 uint32_t