[llvm] [AsmPrinter][DebugNames] Implement DW_IDX_parent entries (PR #77457)

[Felipe de Azevedo Piovezan via llvm-commits]

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@@ -395,36 +401,90 @@ void Dwarf5AccelTableWriter::Header::emit(Dwarf5AccelTableWriter &Ctx) {
   Asm->OutStreamer->emitBytes({AugmentationString, AugmentationStringSize});
 }
 
-static uint32_t constexpr LowerBitSize = dwarf::DW_IDX_type_hash;
+std::optional<uint64_t>
+DWARF5AccelTableData::getDefiningParentDieOffset(const DIE &Die) {
+  if (auto *Parent = Die.getParent();
+      Parent && !Parent->findAttribute(dwarf::Attribute::DW_AT_declaration))
+    return Parent->getOffset();
+  return {};
+}
+
+enum IdxParentEncoding : uint8_t {
+  NoIndexedParent = 0, /// Parent information present but parent isn't indexed.
+  Ref4 = 1,            /// Parent information present and parent is indexed.
+  NoParent = 2,        /// Parent information missing.
+};
+
+static uint32_t constexpr NumBitsIdxParent = 2;
+
+uint8_t encodeIdxParent(const std::optional<dwarf::Form> MaybeParentForm) {
+  if (!MaybeParentForm)
+    return NoParent;
+  switch (*MaybeParentForm) {
+  case dwarf::Form::DW_FORM_flag_present:
+    return NoIndexedParent;
+  case dwarf::Form::DW_FORM_ref4:
+    return Ref4;
+  default:
+    // This is not crashing on bad input: we should only reach this if the
+    // internal compiler logic is faulty; see getFormForIdxParent.
+    llvm_unreachable("Bad form for IDX_parent");
+  }
+}
+
+static uint32_t constexpr ParentBitOffset = dwarf::DW_IDX_type_hash;
+static uint32_t constexpr TagBitOffset = ParentBitOffset + NumBitsIdxParent;
 static uint32_t getTagFromAbbreviationTag(const uint32_t AbbrvTag) {
-  return AbbrvTag >> LowerBitSize;
+  return AbbrvTag >> TagBitOffset;
 }
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felipepiovezan wrote:

>Not sure I understand quite what you're saying here - but mostly it seemed like packing these things into the "AbbrvTag" is more a shortcut to being able to use a single int in the map/lookup/hash table, rather than the most readable piece of code & it'd be good to make the code more legible/self-descrpitive rather than having this bit fiddling.

Sorry, I was mixing two things here! 

> mostly it seemed like packing these things into the "AbbrvTag" is more a shortcut to being able to use a single int in the map/lookup/hash table

Yes, this is one component that we should address for sure.

But there is a separate issue here that may not be evident: we use that same `int` as the abbreviation code itself, the value that gets AsmPrinted into the object file. And because of the encoding scheme, such `int` is very likely going to require more than a byte for its ULEB encoding. Does that make sense?

https://github.com/llvm/llvm-project/pull/77457