Merge pull request #84688 from rjmccall/stack-nesting-2

Rewrite `StackNesting` to use a single-pass algorithm

Author: rjmccall

docs/SIL/SIL.md

@@ -866,7 +866,8 @@ scope-ending instructions if:
   through a scope-ending instruction of `I` in between.
 
 - All initial paths that pass through a scope-ending instruction
-  of `I` twice must also pass through `I` in between.
+  of `I` twice (not necessarily the same instruction) must also
+  pass through `I` in between.
 
 - All terminating initial paths that pass through `I` must also
   pass through a scope-ending instruction of `I`.

docs/StackNestingProof.txt

@@ -45,8 +45,11 @@ namespace swift {
 ///   dealloc_stack %1
 /// \endcode
 ///
+/// Each allocation must still be properly jointly post-dominated by
+/// its deallocations. StackNesting only fixes the nesting of allocations
+/// deallocations; it does not insert required deallocations that are
+/// missing entirely.
 class StackNesting {
-
 public:
 
   /// The possible return values of fixNesting().
@@ -61,122 +64,6 @@ class StackNesting {
     CFG
   };
 
-private:
-  typedef SmallBitVector BitVector;
-
-  /// Data stored for each block (actually for each block which is not dead).
-  struct BlockInfo {
-    /// The list of stack allocating/deallocating instructions in the block.
-    llvm::SmallVector<SILInstruction *, 8> StackInsts;
-
-    /// The bit-set of alive stack locations at the block entry.
-    BitVector AliveStackLocsAtEntry;
-
-    /// The bit-set of alive stack locations at the block exit.
-    BitVector AliveStackLocsAtExit;
-
-    /// Used in the setup function to walk over the CFG.
-    bool visited = false;
-
-    /// True for dead-end blocks, i.e. blocks from which there is no path to
-    /// a function exit, e.g. blocks which end with `unreachable` or an
-    /// infinite loop.
-    bool isDeadEnd = false;
-  };
-
-  /// Data stored for each stack location (= allocation).
-  ///
-  /// Each stack location is allocated by a single allocation instruction.
-  struct StackLoc {
-    StackLoc(SILInstruction *Alloc) : Alloc(Alloc) {}
-
-    /// Back-link to the allocation instruction.
-    SILInstruction *Alloc;
-
-    /// Bit-set which represents all alive locations at this allocation.
-    /// It obviously includes this location itself. And it includes all "outer"
-    /// locations which surround this location.
-    BitVector AliveLocs;
-  };
-
-  /// Mapping from stack allocations (= locations) to bit numbers.
-  llvm::DenseMap<SILInstruction *, unsigned> StackLoc2BitNumbers;
-
-  /// The list of stack locations. The index into this array is also the bit
-  /// number in the bit-sets.
-  llvm::SmallVector<StackLoc, 8> StackLocs;
-
-  BasicBlockData<BlockInfo> BlockInfos;
-
-  StackNesting(SILFunction *F) : BlockInfos(F) { }
-
-  /// Performs correction of stack nesting by moving stack-deallocation
-  /// instructions down the control flow.
-  ///
-  /// Returns the status of what changes were made.
-  Changes run();
-  
-  /// For debug dumping.
-  void dump() const;
-
-  static void dumpBits(const BitVector &Bits);
-
-  /// Initializes the data structures.
-  void setup();
-
-  /// Solves the dataflow problem.
-  ///
-  /// Returns true if there is a nesting of locations in any way, which can
-  /// potentially in the wrong order.
-  bool solve();
-
-  bool analyze() {
-    setup();
-    return solve();
-  }
-
-  /// Insert deallocation instructions for all locations which are alive before
-  /// the InsertionPoint (AliveBefore) but not alive after the InsertionPoint
-  /// (AliveAfter).
-  ///
-  /// Returns true if any deallocations were inserted.
-  bool insertDeallocs(const BitVector &AliveBefore, const BitVector &AliveAfter,
-                      SILInstruction *InsertionPoint,
-                      std::optional<SILLocation> Location);
-
-  /// Returns the location bit number for a stack allocation instruction.
-  int bitNumberForAlloc(SILInstruction *AllocInst) {
-    assert(AllocInst->isAllocatingStack());
-    return StackLoc2BitNumbers[AllocInst];
-  }
-
-  /// Returns the location bit number for a stack deallocation instruction.
-  int bitNumberForDealloc(SILInstruction *DeallocInst) {
-    assert(DeallocInst->isDeallocatingStack());
-    auto *AllocInst = getAllocForDealloc(DeallocInst);
-    return bitNumberForAlloc(AllocInst);
-  }
-
-  /// Returns the stack allocation instruction for a stack deallocation
-  /// instruction.
-  SILInstruction *getAllocForDealloc(SILInstruction *Dealloc) const {
-    SILValue op = Dealloc->getOperand(0);
-    while (auto *mvi = dyn_cast<MoveValueInst>(op)) {
-      op = mvi->getOperand();
-    }
-    return op->getDefiningInstruction();
-  }
-
-  /// Insert deallocations at block boundaries.
-  Changes insertDeallocsAtBlockBoundaries();
-
-  /// Modifies the SIL to end up with a correct stack nesting.
-  ///
-  /// Returns the status of what changes were made.
-  Changes adaptDeallocs();
-
-public:
-
   /// Performs correction of stack nesting by moving stack-deallocation
   /// instructions down the control flow.
   ///