[SHT_LLVM_BB_ADDR_MAP] Add assertion and clarify docstring

llvm/include/llvm/Object/ELFObjectFile.h

@@ -110,11 +110,12 @@ class ELFObjectFileBase : public ObjectFile {
   Expected<std::vector<VersionEntry>> readDynsymVersions() const;
 
   /// Returns a vector of all BB address maps in the object file. When
-  // `TextSectionIndex` is specified, only returns the BB address maps
-  // corresponding to the section with that index. When `PGOAnalyses`is
-  // specified, the vector is cleared then filled with extra PGO data.
-  // `PGOAnalyses` will always be the same length as the return value on
-  // success, otherwise it is empty.
+  /// `TextSectionIndex` is specified, only returns the BB address maps
+  /// corresponding to the section with that index. When `PGOAnalyses`is
+  /// specified (PGOAnalyses is not nullptr), the vector is cleared then filled
+  /// with extra PGO data. `PGOAnalyses` will always be the same length as the
+  /// return value when it is requested assuming no error occurs. Upon failure,
+  /// `PGOAnalyses` will be emptied.
   Expected<std::vector<BBAddrMap>>
   readBBAddrMap(std::optional<unsigned> TextSectionIndex = std::nullopt,
                 std::vector<PGOAnalysisMap> *PGOAnalyses = nullptr) const;

llvm/lib/Object/ELFObjectFile.cpp

@@ -830,6 +830,10 @@ Expected<std::vector<BBAddrMap>> static readBBAddrMapImpl(
     std::move(BBAddrMapOrErr->begin(), BBAddrMapOrErr->end(),
               std::back_inserter(BBAddrMaps));
   }
+  if (PGOAnalyses)
+    assert(PGOAnalyses->size() == BBAddrMaps.size() &&
+           "The same number of BBAddrMaps and PGOAnalysisMaps should be "
+           "returned when PGO information is requested");
   return BBAddrMaps;
 }