[DebugInfo] Emit a 1-byte value as a terminator of entries list in the name index.

[ikudrin]

Summary

The termination entry of the list has the abbreviation code of 0, and one byte is enough to store that. The patch saves approximately 3 bytes per name in the name index table (the calculation is not exact because there is an alignment operator after the entry pool).

[dblaikie]

Looks good - thanks!

(for the record, the wording for this is on page 142 of the DWARFv5 spec:

The last entry for each name is followed by a zero byte that terminates the list. There may be gaps between the lists.

[ikudrin]

Thanks! I'll add the extract to the description. It sometimes so frustrating that the Standard describes the format in several places. The wording in 6.1.1.4.8, where the Entry Pool is described, is a bit vaguer.

llvm/lib/CodeGen/AsmPrinter/AccelTable.cpp

template <typename DataT> void Dwarf5AccelTableWriter<DataT>::emitData() const {
   Asm->OutStreamer->emitLabel(EntryPool);
   for (auto &Bucket : Contents.getBuckets()) {
     for (auto *Hash : Bucket) {
       // Remember to emit the label for our offset.
       Asm->OutStreamer->emitLabel(Hash->Sym);
       for (const auto *Value : Hash->Values)
         emitEntry(*static_cast<const DataT *>(Value));
       Asm->OutStreamer->AddComment("End of list: " + Hash->Name.getString());
-      Asm->emitInt32(0);
+      Asm->emitInt8(0);
     }
   }
 }
 
 template <typename DataT>
 Dwarf5AccelTableWriter<DataT>::Dwarf5AccelTableWriter(
     AsmPrinter *Asm, const AccelTableBase &Contents,
     ArrayRef<MCSymbol *> CompUnits,

llvm/test/DebugInfo/X86/debug-names-end-of-list.ll

+; This checks that the list of index entries in the name index is terminated
+; with a 1-byte value.
+
+; RUN: llc -mtriple x86_64 -accel-tables=Dwarf -dwarf-version=5 -filetype=asm %s -o - | \
+; RUN:   FileCheck %s
+
+; CHECK:   .section .debug_names,"",@progbits
+; CHECK: .Lnames_entries0:
+; CHECK:   .byte 0    # End of list: int
+; CHECK:   .byte 0    # End of list: foo
+
+@foo = common dso_local global i32 0, align 4, !dbg !5
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+!1 = !DIFile(filename: "foo.c", directory: "/tmp")
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "Manual", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !3, globals: !4)
+!3 = !{}
+!4 = !{!5}
+!5 = !DIGlobalVariableExpression(var: !6, expr: !DIExpression())
+!6 = distinct !DIGlobalVariable(name: "foo", scope: !2, file: !1, line: 1, type: !7, isLocal: false, isDefinition: true)
+!7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)