[DAGCombine] Replace std::monostate equivalent in DAGCombiner.cpp

[jloser]

Summary

Remove the UnitT type and operators in favor of using std::monostate
directly.

[craig.topper]

LGTM

[jloser]

Run git-clang-format HEAD~1. If CI passes, I'll land this.

[jloser]

Fix reference to Unit. Use std::monostate{} instead.

[jloser]

Fix another use of Unit in favor of std::monostate{}.

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

 #include <algorithm>
 #include <cassert>
 #include <cstdint>
 #include <functional>
 #include <iterator>
 #include <string>
 #include <tuple>
 #include <utility>
+#include <variant>
 
 using namespace llvm;
 
 #define DEBUG_TYPE "dagcombine"
 
 STATISTIC(NodesCombined   , "Number of dag nodes combined");
 STATISTIC(PreIndexedNodes , "Number of pre-indexed nodes created");
 STATISTIC(PostIndexedNodes, "Number of post-indexed nodes created");
SDValue DAGCombiner::FindBetterChain(SDNode *N, SDValue OldChain) {
   // If a single operand then chain to it.  We don't need to revisit it.
   if (Aliases.size() == 1)
     return Aliases[0];
 
   // Construct a custom tailored token factor.
   return DAG.getTokenFactor(SDLoc(N), Aliases);
 }
 
-namespace {
-// TODO: Replace with with std::monostate when we move to C++17.
-struct UnitT { } Unit;
-bool operator==(const UnitT &, const UnitT &) { return true; }
-bool operator!=(const UnitT &, const UnitT &) { return false; }
-} // namespace
-
 // This function tries to collect a bunch of potentially interesting
 // nodes to improve the chains of, all at once. This might seem
 // redundant, as this function gets called when visiting every store
 // node, so why not let the work be done on each store as it's visited?
 //
 // I believe this is mainly important because mergeConsecutiveStores
 // is unable to deal with merging stores of different sizes, so unless
 // we improve the chains of all the potential candidates up-front
 // before running mergeConsecutiveStores, it might only see some of
 // the nodes that will eventually be candidates, and then not be able
 // to go from a partially-merged state to the desired final
 // fully-merged state.
 
 bool DAGCombiner::parallelizeChainedStores(StoreSDNode *St) {
   SmallVector<StoreSDNode *, 8> ChainedStores;
   StoreSDNode *STChain = St;
   // Intervals records which offsets from BaseIndex have been covered. In
   // the common case, every store writes to the immediately previous address
   // space and thus merged with the previous interval at insertion time.
 
-  using IMap =
-      llvm::IntervalMap<int64_t, UnitT, 8, IntervalMapHalfOpenInfo<int64_t>>;
+  using IMap = llvm::IntervalMap<int64_t, std::monostate, 8,
+                                 IntervalMapHalfOpenInfo<int64_t>>;
   IMap::Allocator A;
   IMap Intervals(A);
 
   // This holds the base pointer, index, and the offset in bytes from the base
   // pointer.
   const BaseIndexOffset BasePtr = BaseIndexOffset::match(St, DAG);
 
   // We must have a base and an offset.
bool DAGCombiner::parallelizeChainedStores(StoreSDNode *St) {
 
   // BaseIndexOffset assumes that offsets are fixed-size, which
   // is not valid for scalable vectors where the offsets are
   // scaled by `vscale`, so bail out early.
   if (St->getMemoryVT().isScalableVector())
     return false;
 
   // Add ST's interval.
-  Intervals.insert(0, (St->getMemoryVT().getSizeInBits() + 7) / 8, Unit);
+  Intervals.insert(0, (St->getMemoryVT().getSizeInBits() + 7) / 8,
+                   std::monostate{});
 
   while (StoreSDNode *Chain = dyn_cast<StoreSDNode>(STChain->getChain())) {
     if (Chain->getMemoryVT().isScalableVector())
       return false;
 
     // If the chain has more than one use, then we can't reorder the mem ops.
     if (!SDValue(Chain, 0)->hasOneUse())
       break;