[llvm] [NFC] Add a specialization of DenseMapInfo for SmallVector (PR #140380)

[Jon Chesterfield via llvm-commits]

https://github.com/JonChesterfield created https://github.com/llvm/llvm-project/pull/140380

Equivalent to the three existing uses I found which were all pointers. Implementing the general pattern so SmallVector<int> etc will work as well.

Added to the SmallVector.h header as opposed to DenseMapInfo.h following the StringRef.h and SmallBitVector.h prior art. 

Noticed while writing an unrelated patch which currently wants a map from small vectors to other things and cleaner to generalise than add another specialisation to said patch.

>From bf044ff128e24f5fae05312c42629a4d73dcc28a Mon Sep 17 00:00:00 2001
From: Jon Chesterfield <jonathanchesterfield at gmail.com>
Date: Sat, 17 May 2025 16:24:57 +0100
Subject: [PATCH] [NFC] Add a specialization of DenseMapInfo for SmallVector

---
 llvm/include/llvm/ADT/SmallVector.h           | 21 +++++++++++++
 .../Transforms/Scalar/LoopStrengthReduce.cpp  | 30 ++-----------------
 llvm/lib/Transforms/Vectorize/VPlanSLP.h      | 24 +--------------
 3 files changed, 24 insertions(+), 51 deletions(-)

diff --git a/llvm/include/llvm/ADT/SmallVector.h b/llvm/include/llvm/ADT/SmallVector.h
index bd3e887e36bce..fb9ec9abf111b 100644
--- a/llvm/include/llvm/ADT/SmallVector.h
+++ b/llvm/include/llvm/ADT/SmallVector.h
@@ -14,6 +14,7 @@
 #ifndef LLVM_ADT_SMALLVECTOR_H
 #define LLVM_ADT_SMALLVECTOR_H
 
+#include "llvm/ADT/DenseMapInfo.h"
 #include "llvm/Support/Compiler.h"
 #include <algorithm>
 #include <cassert>
@@ -1319,6 +1320,26 @@ extern template class llvm::SmallVectorBase<uint32_t>;
 extern template class llvm::SmallVectorBase<uint64_t>;
 #endif
 
+// Provide DenseMapInfo for SmallVector of a type which has info.
+template <typename T, unsigned N> struct DenseMapInfo<llvm::SmallVector<T, N>> {
+  static SmallVector<T, N> getEmptyKey() {
+    return {DenseMapInfo<T>::getEmptyKey()};
+  }
+
+  static SmallVector<T, N> getTombstoneKey() {
+    return {DenseMapInfo<T>::getTombstoneKey()};
+  }
+
+  static unsigned getHashValue(const SmallVector<T, N> &V) {
+    return static_cast<unsigned>(hash_combine_range(V));
+  }
+
+  static bool isEqual(const SmallVector<T, N> &LHS,
+                      const SmallVector<T, N> &RHS) {
+    return LHS == RHS;
+  }
+};
+
 } // end namespace llvm
 
 namespace std {
diff --git a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
index 464e6e3b2ab97..33d6c77f61cfd 100644
--- a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -1275,38 +1275,13 @@ struct LSRFixup {
   void dump() const;
 };
 
-/// A DenseMapInfo implementation for holding DenseMaps and DenseSets of sorted
-/// SmallVectors of const SCEV*.
-struct UniquifierDenseMapInfo {
-  static SmallVector<const SCEV *, 4> getEmptyKey() {
-    SmallVector<const SCEV *, 4>  V;
-    V.push_back(reinterpret_cast<const SCEV *>(-1));
-    return V;
-  }
-
-  static SmallVector<const SCEV *, 4> getTombstoneKey() {
-    SmallVector<const SCEV *, 4> V;
-    V.push_back(reinterpret_cast<const SCEV *>(-2));
-    return V;
-  }
-
-  static unsigned getHashValue(const SmallVector<const SCEV *, 4> &V) {
-    return static_cast<unsigned>(hash_combine_range(V));
-  }
-
-  static bool isEqual(const SmallVector<const SCEV *, 4> &LHS,
-                      const SmallVector<const SCEV *, 4> &RHS) {
-    return LHS == RHS;
-  }
-};
-
 /// This class holds the state that LSR keeps for each use in IVUsers, as well
 /// as uses invented by LSR itself. It includes information about what kinds of
 /// things can be folded into the user, information about the user itself, and
 /// information about how the use may be satisfied.  TODO: Represent multiple
 /// users of the same expression in common?
 class LSRUse {
-  DenseSet<SmallVector<const SCEV *, 4>, UniquifierDenseMapInfo> Uniquifier;
+  DenseSet<SmallVector<const SCEV *, 4>> Uniquifier;
 
 public:
   /// An enum for a kind of use, indicating what types of scaled and immediate
@@ -4754,8 +4729,7 @@ void LSRInstance::FilterOutUndesirableDedicatedRegisters() {
 
   // Collect the best formula for each unique set of shared registers. This
   // is reset for each use.
-  using BestFormulaeTy =
-      DenseMap<SmallVector<const SCEV *, 4>, size_t, UniquifierDenseMapInfo>;
+  using BestFormulaeTy = DenseMap<SmallVector<const SCEV *, 4>, size_t>;
 
   BestFormulaeTy BestFormulae;
 
diff --git a/llvm/lib/Transforms/Vectorize/VPlanSLP.h b/llvm/lib/Transforms/Vectorize/VPlanSLP.h
index 2b927b93e24cf..157e78363e55f 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanSLP.h
+++ b/llvm/lib/Transforms/Vectorize/VPlanSLP.h
@@ -73,30 +73,8 @@ class VPInterleavedAccessInfo {
 class VPlanSlp {
   enum class OpMode { Failed, Load, Opcode };
 
-  /// A DenseMapInfo implementation for using SmallVector<VPValue *, 4> as
-  /// DenseMap keys.
-  struct BundleDenseMapInfo {
-    static SmallVector<VPValue *, 4> getEmptyKey() {
-      return {reinterpret_cast<VPValue *>(-1)};
-    }
-
-    static SmallVector<VPValue *, 4> getTombstoneKey() {
-      return {reinterpret_cast<VPValue *>(-2)};
-    }
-
-    static unsigned getHashValue(const SmallVector<VPValue *, 4> &V) {
-      return static_cast<unsigned>(hash_combine_range(V));
-    }
-
-    static bool isEqual(const SmallVector<VPValue *, 4> &LHS,
-                        const SmallVector<VPValue *, 4> &RHS) {
-      return LHS == RHS;
-    }
-  };
-
   /// Mapping of values in the original VPlan to a combined VPInstruction.
-  DenseMap<SmallVector<VPValue *, 4>, VPInstruction *, BundleDenseMapInfo>
-      BundleToCombined;
+  DenseMap<SmallVector<VPValue *, 4>, VPInstruction *> BundleToCombined;
 
   VPInterleavedAccessInfo &IAI;