[CodeGen/AccelTable] Don't emit zero-CU name indexes

[labath]

Summary

If an input DICompileUnit is completely empty (e.g., the result of
running "clang -g" on an empty file), we don't bother emitting an empty
DWARF CU. When we do that, we must make sure we don't also emit a DWARF v5
name index, as DWARF specifies that each index must reference at least
one compilation unit.

[JDevlieghere]

Other than the inline comment this LGTM

lib/CodeGen/AsmPrinter/AccelTable.cpp

370 ↗

(On Diff #141616)

Why not put this at the top like a pre-condition? Maybe also add a message?

[labath]

lib/CodeGen/AsmPrinter/AccelTable.cpp

370 ↗

(On Diff #141616)

No particular reason. I agree it looks better at the top.

llvm/trunk/lib/CodeGen/AsmPrinter/AccelTable.cpp

void AppleAccelTableEmitter::emit() const {
   emitBuckets();
   emitHashes();
   emitOffsets(SecBegin);
   emitData();
 }
 
 void Dwarf5AccelTableEmitter::Header::emit(
     const Dwarf5AccelTableEmitter &Ctx) const {
+  assert(CompUnitCount > 0 && "Index must have at least one CU.");
+
   AsmPrinter *Asm = Ctx.Asm;
   Asm->OutStreamer->AddComment("Header: unit length");
   Asm->EmitLabelDifference(Ctx.ContributionEnd, Ctx.ContributionStart,
                            sizeof(uint32_t));
   Asm->OutStreamer->EmitLabel(Ctx.ContributionStart);
   Asm->OutStreamer->AddComment("Header: version");
   Asm->emitInt16(Version);
   Asm->OutStreamer->AddComment("Header: padding");

llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

void DwarfDebug::emitAccel(AccelTableT &Accel, MCSection *Section,
                            StringRef TableName) {
   Asm->OutStreamer->SwitchSection(Section);
 
   // Emit the full data.
   emitAppleAccelTable(Asm, Accel, TableName, Section->getBeginSymbol());
 }
 
 void DwarfDebug::emitAccelDebugNames() {
+  // Don't emit anything if we have no compilation units to index.
+  if (getUnits().empty())
+    return;
+
   Asm->OutStreamer->SwitchSection(
       Asm->getObjFileLowering().getDwarfDebugNamesSection());
   emitDWARF5AccelTable(Asm, AccelDebugNames, *this, getUnits());
 }
 
 // Emit visible names into a hashed accelerator table section.
 void DwarfDebug::emitAccelNames() {
   emitAccel(AccelNames, Asm->getObjFileLowering().getDwarfAccelNamesSection(),

llvm/trunk/test/DebugInfo/Generic/debug-names-empty-cu.ll

+; REQUIRES: object-emission
+; RUN: %llc_dwarf -accel-tables=Dwarf -filetype=obj -o %t < %s
+; RUN: llvm-dwarfdump -debug-names %t | FileCheck %s
+; RUN: llvm-dwarfdump -debug-names -verify %t | FileCheck --check-prefix=VERIFY %s
+
+; CHECK-NOT: CU count: 0
+; VERIFY: No errors.
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 7.0.0 (trunk 329543) (llvm/trunk 329551)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "-", directory: "/tmp")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 7.0.0 (trunk 329543) (llvm/trunk 329551)"}