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

[Rahman Lavaee via llvm-commits]

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@@ -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 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.
+      BranchProbability Prob;
+
+      bool operator==(const SuccessorEntry &Other) const {
+        return std::tie(ID, Prob) == std::tie(Other.ID, Other.Prob);
+      }
+    };
+
+    /// Block frequency taken from MBFI
+    BlockFrequency BlockFreq;
+    /// List of successors of the current block
+    llvm::SmallVector<SuccessorEntry, 2> Successors;
+
+    /// Converts number of successors into a SuccessorsType.
+    static SuccessorsType getSuccessorsType(unsigned SuccessorsCount) {
+      return SuccessorsCount == 0   ? SuccessorsType::None
+             : SuccessorsCount == 1 ? SuccessorsType::One
+             : SuccessorsCount == 2 ? SuccessorsType::Two
+                                    : SuccessorsType::Multiple;
+    }
+
+    /// Encodes extra information in the free bits of the base metadata
+    static uint32_t encodeMD(BaseMetadata MD, SuccessorsType SuccType) {
+      return MD.encode() | static_cast<uint32_t>(SuccType)
+                               << BaseMetadata::NumberOfBits;
+    }
+
+    /// Extracts successors type then defers all errors to the base metadata
+    static Expected<std::pair<BaseMetadata, SuccessorsType>>
+    decodeMD(uint32_t V) {
+      auto SuccType = SuccessorsType((V >> BaseMetadata::NumberOfBits) & 0b11);
+      V &= ~(0b11 << BaseMetadata::NumberOfBits); // Clear extra bits
+      BaseMetadata MD;
+      if (llvm::Error E = BaseMetadata::decode(V).moveInto(MD))
+        return std::move(E);
+      return std::make_pair(MD, SuccType);
+    }
+
+    bool operator==(const PGOBBEntry &Other) const {
+      return std::tie(BlockFreq, Successors) ==
+             std::tie(Other.BlockFreq, Other.Successors);
+    }
+  };
+  // This field is duplicated from BBAddrMap since this class needs a different
+  // type for the vector of entries.
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rlavaee wrote:

which field does this refer to?

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