[CodeGen/AccelTable]: Don't emit accelerator entries for functions with no names

[labath]

Summary

We were emitting accelerator entries for functions with no name, which
is contrary to the DWARF v5 spec: "All other (i.e., *not*
DW_TAG_namespace) debugging information entries without a DW_AT_name
attribute are excluded." Besides that, a name table entry with an empty
string as a key is fairly useless.

We can sometimes end up with functions which have a DW_AT_linkage_name but no
DW_AT_name. One such example is the global-constructor-initialization functions,
which C++ compilers synthesize for each compilation unit with global
constructors.
A very strict reading of the DWARF v5 spec would suggest that we should not even
emit the accelerator entry for the linkage name in this case, but I don't think
we should go that far.

I found this when running the dwarf verifier over llvm codebase compiled
with DWARF v5 accelerator tables.

[labath]

lib/CodeGen/AsmPrinter/DwarfDebug.cpp

389–391

I'm not sure what this TODO here is trying to say, but it could be that this patch resolves it...

[labath]

Fix a CHECK stanza in the test.

[JDevlieghere]

I agree, let's keep the linkage name in there. LGTM.

lib/CodeGen/AsmPrinter/DwarfDebug.cpp

389–391

Sounds like it :-)

[labath]

lib/CodeGen/AsmPrinter/DwarfDebug.cpp

389–391

OK, I'll remove it.

lib/CodeGen/AsmPrinter/DwarfDebug.cpp

static void getObjCClassCategory(StringRef In, StringRef &Class,
   Category = In.slice(In.find('[') + 1, In.find(' '));
 }
 
 static StringRef getObjCMethodName(StringRef In) {
   return In.slice(In.find(' ') + 1, In.find(']'));
 }
 
 // Add the various names to the Dwarf accelerator table names.
 // TODO: Determine whether or not we should add names for programs
 // that do not have a DW_AT_name or DW_AT_linkage_name field - this
 // is only slightly different than the lookup of non-standard ObjC names.
 void DwarfDebug::addSubprogramNames(const DISubprogram *SP, DIE &Die) {
   if (!SP->isDefinition())
     return;
+
+  if (SP->getName() != "")
     addAccelName(SP->getName(), Die);
 
   // If the linkage name is different than the name, go ahead and output
   // that as well into the name table.
   if (SP->getLinkageName() != "" && SP->getName() != SP->getLinkageName())
     addAccelName(SP->getLinkageName(), Die);
 
   // If this is an Objective-C selector name add it to the ObjC accelerator
   // too.

test/DebugInfo/Generic/debug-names-empty-name.ll

+; REQUIRES: object-emission
+; RUN: %llc_dwarf -accel-tables=Dwarf -filetype=obj -o %t < %s
+; RUN: llvm-dwarfdump -find=_GLOBAL__sub_I__ %t | FileCheck --check-prefix=INFO %s
+; RUN: llvm-dwarfdump -debug-names %t | FileCheck --check-prefix=NAMES %s
+; RUN: llvm-dwarfdump -debug-names -verify %t | FileCheck --check-prefix=VERIFY %s
+
+; The debug info entry should not have a DW_AT_name, only a DW_AT_linkage_name.
+; INFO: DW_TAG_subprogram
+; INFO-NOT: DW_AT_name
+; INFO:     DW_AT_linkage_name  ("_GLOBAL__sub_I__")
+; INFO-NOT: DW_AT_name
+
+; The accelerator table should contain only one entry.
+; NAMES: Name count: 1
+; And it should be the linkage name.
+; NAMES: String: 0x{{[0-9a-f]*}} "_GLOBAL__sub_I__"
+
+; Verification should succeed.
+; VERIFY: No errors.
+
+define internal void @_GLOBAL__sub_I__() section ".text.startup" !dbg !7 {
+entry:
+  ret void
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 7.0.0 (trunk 329378) (llvm/trunk 329379)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, globals: !2)
+!1 = !DIFile(filename: "-", directory: "/usr/local/google/home/labath/ll/build/opt")
+!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 329378) (llvm/trunk 329379)"}
+!7 = distinct !DISubprogram(linkageName: "_GLOBAL__sub_I__", scope: !1, file: !1, type: !8, isLocal: true, isDefinition: true, flags: DIFlagArtificial, isOptimized: false, unit: !0, variables: !2)
+!8 = !DISubroutineType(types: !2)