[CodeView] Allow empty types in member functions

[cuviper]

Summary

CodeViewDebug::lowerTypeMemberFunction used to default to a Void
return type if the function's type array was empty. After D54667, it
started blindly indexing the 0th item for the return type, which fails
in getOperand for empty arrays if assertions are enabled.

This patch restores the Void return type for empty type arrays, and
adds a test generated by Rust in line-only debuginfo mode.

[cuviper]

FWIW, here's more context for how I arrived at this:
https://github.com/rust-lang/rust/pull/57675#issuecomment-456210064

[rnk]

lgtm, thanks!

I had a bit of deja-vu because I thought we already fixed an assertion in this new code, and we did, in rL350073.

[cuviper]

Thanks, committed!

I'll have to review that discussion of this pointers wrt Rust, since self can be different things, but it hasn't caused us problems yet AFAIK.

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

TypeIndex CodeViewDebug::lowerTypeMemberFunction(const DISubroutineType *Ty,
                                                  FunctionOptions FO) {
   // Lower the containing class type.
   TypeIndex ClassType = getTypeIndex(ClassTy);
 
   DITypeRefArray ReturnAndArgs = Ty->getTypeArray();
 
   unsigned Index = 0;
   SmallVector<TypeIndex, 8> ArgTypeIndices;
-  TypeIndex ReturnTypeIndex = getTypeIndex(ReturnAndArgs[Index++]);
+  TypeIndex ReturnTypeIndex = TypeIndex::Void();
+  if (ReturnAndArgs.size() > Index) {
+    ReturnTypeIndex = getTypeIndex(ReturnAndArgs[Index++]);
+  }
 
   // If the first argument is a pointer type and this isn't a static method,
   // treat it as the special 'this' parameter, which is encoded separately from
   // the arguments.
   TypeIndex ThisTypeIndex;
   if (!IsStaticMethod && ReturnAndArgs.size() > Index) {
     if (const DIDerivedType *PtrTy =
             dyn_cast_or_null<DIDerivedType>(ReturnAndArgs[Index].resolve())) {

llvm/test/DebugInfo/COFF/types-empty-member-fn.ll

+; RUN: llc < %s -filetype=obj | llvm-readobj - -codeview | FileCheck %s
+
+; ModuleID = 'foo.3a1fbbbh-cgu.0'
+source_filename = "foo.3a1fbbbh-cgu.0"
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+; Rust source to regenerate:
+; $ cat foo.rs
+; pub struct Foo;
+; impl Foo {
+;     pub fn foo() {}
+; }
+; $ rustc foo.rs --crate-type lib -Cdebuginfo=1 --emit=llvm-ir
+
+; CHECK:      CodeViewTypes [
+; CHECK:        MemberFunction (0x1006) {
+; CHECK-NEXT:     TypeLeafKind: LF_MFUNCTION (0x1009)
+; CHECK-NEXT:     ReturnType: void (0x3)
+; CHECK-NEXT:     ClassType: foo::Foo (0x1000)
+; CHECK-NEXT:     ThisType: 0x0
+; CHECK-NEXT:     CallingConvention: NearC (0x0)
+; CHECK-NEXT:     FunctionOptions [ (0x0)
+; CHECK-NEXT:     ]
+; CHECK-NEXT:     NumParameters: 0
+; CHECK-NEXT:     ArgListType: () (0x1005)
+; CHECK-NEXT:     ThisAdjustment: 0
+; CHECK-NEXT:   }
+; CHECK-NEXT:   MemberFuncId (0x1007) {
+; CHECK-NEXT:     TypeLeafKind: LF_MFUNC_ID (0x1602)
+; CHECK-NEXT:     ClassType: foo::Foo (0x1000)
+; CHECK-NEXT:     FunctionType: void foo::Foo::() (0x1006)
+; CHECK-NEXT:     Name: foo
+; CHECK-NEXT:   }
+; CHECK:      CodeViewDebugInfo [
+; CHECK:        FunctionLineTable [
+; CHECK-NEXT:     LinkageName: _ZN3foo3Foo3foo17hc557c2121772885bE
+; CHECK-NEXT:     Flags: 0x0
+; CHECK-NEXT:     CodeSize: 0x1
+; CHECK-NEXT:     FilenameSegment [
+; CHECK-NEXT:       Filename: D:\rust\foo.rs (0x0)
+; CHECK-NEXT:       +0x0 [
+; CHECK-NEXT:         LineNumberStart: 3
+; CHECK-NEXT:         LineNumberEndDelta: 0
+; CHECK-NEXT:         IsStatement: No
+; CHECK-NEXT:       ]
+; CHECK-NEXT:     ]
+; CHECK-NEXT:   ]
+
+; foo::Foo::foo
+; Function Attrs: uwtable
+define void @_ZN3foo3Foo3foo17hc557c2121772885bE() unnamed_addr #0 !dbg !5 {
+start:
+  ret void, !dbg !10
+}
+
+attributes #0 = { uwtable "target-cpu"="x86-64" }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_Rust, file: !1, producer: "clang LLVM (rustc version 1.33.0-nightly (8b0f0156e 2019-01-22))", isOptimized: false, runtimeVersion: 0, emissionKind: LineTablesOnly, enums: !2)
+!1 = !DIFile(filename: "foo.rs", directory: "D:\5Crust")
+!2 = !{}
+!3 = !{i32 2, !"CodeView", i32 1}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "foo", linkageName: "_ZN3foo3Foo3foo17hc557c2121772885bE", scope: !6, file: !1, line: 3, type: !9, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, templateParams: !2, retainedNodes: !2)
+!6 = !DICompositeType(tag: DW_TAG_structure_type, name: "Foo", scope: !8, file: !7, align: 8, elements: !2, templateParams: !2, identifier: "5105d9fe1a2a3c68518268151b672274")
+!7 = !DIFile(filename: "<unknown>", directory: "")
+!8 = !DINamespace(name: "foo", scope: null)
+!9 = !DISubroutineType(types: !2)
+!10 = !DILocation(line: 3, scope: !5)