[llvm] Implements PGOBBAddrMap in Object and ObjectYAML with tests [1/5] (PR #71750)

[James Henderson via llvm-commits]

================
@@ -875,6 +882,101 @@ struct BBAddrMap {
   std::vector<BBEntry> BBEntries; // Basic block entries for this function.
 };
 
+/// A feature extension of BBAddrMap that holds information relevant to PGO.
+struct PGOAnalysisMap {
+  /// Bitmask of optional features to include in the PGO extended map.
+  enum class Features {
+    FuncEntryCnt = (1 << 0),
+    BBFreq = (1 << 1),
+    BrProb = (1 << 2),
+  };
+
+  /// Super-set of BBAddrMap::BBEntry with additional fields for block frequency
+  /// and branch probability.
+  struct PGOBBEntry {
+    using BaseMetadata = BBAddrMap::BBEntry::Metadata;
+
+    /// Enum indicating the how many successors a block has. This enum must fit
+    /// into two bits.
+    enum class SuccessorsType {
+      /// None should be present if BBAddrMap.feature has disabled branch
+      /// probability.
+      None = 0,
+      /// Single successor blocks are not present in the successor entries.
+      One = 1,
+      /// Common case for conditional branches to avoid encoding size.
+      Two = 2,
+      /// Uncommon case which needs successor size to be encoded.
+      Multiple = 3,
+    };
+
+    /// Single successor of a given basic block that contains the tag and branch
+    /// probability associated with it.
+    struct SuccessorEntry {
+      /// Unique ID of this successor basic block.
+      uint32_t ID;
+      /// Branch Probability of the edge to this successor taken from MBPI
----------------
jh7370 wrote:

```suggestion
      /// Branch Probability of the edge to this successor taken from MBPI.
```
Also missing from a few comments below.

https://github.com/llvm/llvm-project/pull/71750