[llvm] [DebugInfo] Add fast path for parsing DW_TAG_compile_unit abbrevs (PR #108757)

[Daniel Bertalan via llvm-commits]

================
@@ -34,36 +34,49 @@ bool DWARFDebugInfoEntry::extractFast(const DWARFUnit &U, uint64_t *OffsetPtr,
     return false;
   }
   assert(DebugInfoData.isValidOffset(UEndOffset - 1));
+  AbbrevDecl = nullptr;
+
   uint64_t AbbrCode = DebugInfoData.getULEB128(OffsetPtr);
   if (0 == AbbrCode) {
     // NULL debug tag entry.
-    AbbrevDecl = nullptr;
     return true;
   }
-  const auto *AbbrevSet = U.getAbbreviations();
-  if (!AbbrevSet) {
-    U.getContext().getWarningHandler()(
-        createStringError(errc::invalid_argument,
-                          "DWARF unit at offset 0x%8.8" PRIx64 " "
-                          "contains invalid abbreviation set offset 0x%" PRIx64,
-                          U.getOffset(), U.getAbbreviationsOffset()));
-    // Restore the original offset.
-    *OffsetPtr = Offset;
-    return false;
+
+  // Fast path: parsing the entire abbreviation table is wasteful if we only
+  // need the unit DIE (typically AbbrCode == 1).
+  if (1 == AbbrCode) {
----------------
BertalanD wrote:

(will reflect on the other bits later):

> Why is ld64 reading the CUs anyway?

For emitting `N_OSO` stabs; which indicate the source file names of symbols. We need to read `DW_AT_source_dir` for this, see `ObjFile::source_file` in LLD.

https://github.com/llvm/llvm-project/pull/108757