[PATCH] D124309: [SLP] Steer for the best chance in tryToVectorize() when rooting with binary ops.

[Vasileios Porpodas via Phabricator via llvm-commits]

vporpo added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:1037
 
-  /// A helper data structure to hold the operands of a vector of instructions.
-  /// This supports a fixed vector length for all operand vectors.
-  class VLOperands {
-    /// For each operand we need (i) the value, and (ii) the opcode that it
-    /// would be attached to if the expression was in a left-linearized form.
-    /// This is required to avoid illegal operand reordering.
-    /// For example:
-    /// \verbatim
-    ///                         0 Op1
-    ///                         |/
-    /// Op1 Op2   Linearized    + Op2
-    ///   \ /     ---------->   |/
-    ///    -                    -
-    ///
-    /// Op1 - Op2            (0 + Op1) - Op2
-    /// \endverbatim
-    ///
-    /// Value Op1 is attached to a '+' operation, and Op2 to a '-'.
-    ///
-    /// Another way to think of this is to track all the operations across the
-    /// path from the operand all the way to the root of the tree and to
-    /// calculate the operation that corresponds to this path. For example, the
-    /// path from Op2 to the root crosses the RHS of the '-', therefore the
-    /// corresponding operation is a '-' (which matches the one in the
-    /// linearized tree, as shown above).
-    ///
-    /// For lack of a better term, we refer to this operation as Accumulated
-    /// Path Operation (APO).
-    struct OperandData {
-      OperandData() = default;
-      OperandData(Value *V, bool APO, bool IsUsed)
-          : V(V), APO(APO), IsUsed(IsUsed) {}
-      /// The operand value.
-      Value *V = nullptr;
-      /// TreeEntries only allow a single opcode, or an alternate sequence of
-      /// them (e.g, +, -). Therefore, we can safely use a boolean value for the
-      /// APO. It is set to 'true' if 'V' is attached to an inverse operation
-      /// in the left-linearized form (e.g., Sub/Div), and 'false' otherwise
-      /// (e.g., Add/Mul)
-      bool APO = false;
-      /// Helper data for the reordering function.
-      bool IsUsed = false;
-    };
-
-    /// During operand reordering, we are trying to select the operand at lane
-    /// that matches best with the operand at the neighboring lane. Our
-    /// selection is based on the type of value we are looking for. For example,
-    /// if the neighboring lane has a load, we need to look for a load that is
-    /// accessing a consecutive address. These strategies are summarized in the
-    /// 'ReorderingMode' enumerator.
-    enum class ReorderingMode {
-      Load,     ///< Matching loads to consecutive memory addresses
-      Opcode,   ///< Matching instructions based on opcode (same or alternate)
-      Constant, ///< Matching constants
-      Splat,    ///< Matching the same instruction multiple times (broadcast)
-      Failed,   ///< We failed to create a vectorizable group
-    };
-
-    using OperandDataVec = SmallVector<OperandData, 2>;
-
-    /// A vector of operand vectors.
-    SmallVector<OperandDataVec, 4> OpsVec;
-
+  /// A helper class used for scoring candidates for two consecutive lanes.
+  class LookAheadHeuristics {
----------------
nit: It also holds the operands of a VL, so it is probably best to mention this here too.


================
Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:2007
+  /// above the LookAheadHeuristics::ScoreFail.
+  Optional<int> findBestPair(ArrayRef<std::pair<Value *, Value *>> Candidates) {
+    LookAheadHeuristics LookAhead(*DL, *SE, *this, /*NumLanes=*/2,
----------------
nit: perhaps `findBestRootPair` ?


================
Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:2009
+    LookAheadHeuristics LookAhead(*DL, *SE, *this, /*NumLanes=*/2,
+                                  LookAheadMaxDepth);
+    int BestScore = LookAheadHeuristics::ScoreFail;
----------------
We could also have a separate max-depth limit for this, because I guess it will not ran as frequently as the other one, so we could have a higher depth if required.


================
Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:9217
+  Optional<int> BestCandidate = R.findBestPair(Candidates);
+  if (!BestCandidate.hasValue())
+    return false;
----------------
nit: I think we can drop `.hasValue()`, `if (!BestCandidate)` should work fine.


Repository:
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  https://reviews.llvm.org/D124309/new/

https://reviews.llvm.org/D124309