DebugInfo: Fix null dereference on null DISubroutineType

[arsenm]

Summary

No idea what I'm doing or if this is the right fix, a good test, or if
this metadata should fail the verifier. I've had this crashing
testcase sitting around for months so I'm not entirely sure where it
came from, but I think I was manually reducing the metadata.

[dblaikie]

@aprantl - validator's more your wheelhouse than mine, but I'd say if we don't have a specific use case for this we should default to making it invalid/fail the verifier for now - can widen that if/when needed.

[aprantl]

It surprises me that the assembler doesn't seem to require a type either. I think I agree with David that it would be better to let the Verifier fail here and require a type. For reference, this is how clang produces void():

!9 = distinct !DISubprogram(name: "f", scope: !10, file: !10, line: 1, type: !11, scopeLine: 1, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !7)
!10 = !DIFile(filename: "/tmp/t.c", directory: "")
!11 = !DISubroutineType(types: !12)
!12 = !{null}

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

if (Scope) {
     assert(!Scope->isAbstractScope());
     // Collect lexical scope children first.
     // ObjectPointer might be a local (non-argument) local variable if it's a
     // block's synthetic this pointer.
     if (DIE *ObjectPointer = createAndAddScopeChildren(Scope, ScopeDIE))
       addDIEEntry(ScopeDIE, dwarf::DW_AT_object_pointer, *ObjectPointer);
   }
 
+  if (DISubroutineType *SRT = Sub->getType()) {
     // If this is a variadic function, add an unspecified parameter.
-  DITypeRefArray FnArgs = Sub->getType()->getTypeArray();
+    DITypeRefArray FnArgs = SRT->getTypeArray();
 
     // If we have a single element of null, it is a function that returns void.
     // If we have more than one elements and the last one is null, it is a
     // variadic function.
     if (FnArgs.size() > 1 && !FnArgs[FnArgs.size() - 1] &&
         !includeMinimalInlineScopes())
       ScopeDIE.addChild(
         DIE::get(DIEValueAllocator, dwarf::DW_TAG_unspecified_parameters));
+  }
 
   return ScopeDIE;
 }
 
 DIE *DwarfCompileUnit::createAndAddScopeChildren(LexicalScope *Scope,
                                                  DIE &ScopeDIE) {
   DIE *ObjectPointer = nullptr;
 

llvm/test/DebugInfo/X86/empty-subprogram-die-assert.ll

+; RUN: llc -mtriple=x86_64-pc-linux-gnu -filetype=obj < %s | llvm-dwarfdump -debug-info - | FileCheck %s
+
+; Check for crash when inspecting DISubroutineType
+; CHECK: DW_TAG_subprogram
+; CHECK: DW_AT_name ("foo")
+define void @foo() !dbg !3 {
+  ret void
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "foo.c", directory: "/tmp")
+!2 = !{i32 2, !"Debug Info Version", i32 3}
+!3 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0)
+!4 = !DISubroutineType(types: !5)
+!5 = !{null}