[Mlir-commits] [mlir] [mlir][ArmSME] Use liveness information in the tile allocator (PR #90448)

[Benjamin Maxwell]

================
@@ -137,172 +129,508 @@ static ArrayRef<TileMask> getMasks(ArmSMETileType type) {
   }
 }
 
-/// Allocates and returns a tile ID. Returns an error if there are no tiles
-/// left.
-static FailureOr<unsigned> allocateTileId(ArmSMETileType tileType,
-                                          TileMask &tilesInUse) {
-  auto masks = getMasks(tileType);
-  for (auto [tileId, tileMask] : llvm::enumerate(masks)) {
-    if ((tilesInUse & tileMask) == TileMask::kNone) {
-      tilesInUse |= tileMask;
-      return tileId;
+class TileAllocator {
+public:
+  /// Allocates and returns a tile ID. Fails if there are no tiles left.
+  FailureOr<unsigned> allocateTileId(ArmSMETileType tileType) {
+    auto masks = getMasks(tileType);
+    for (auto [tileId, tileMask] : llvm::enumerate(masks)) {
+      if ((tilesInUse & tileMask) == TileMask::kNone) {
+        tilesInUse |= tileMask;
+        return tileId;
+      }
     }
+    return failure();
+  }
+
+  /// Releases a previously allocated tile ID.
+  void releaseTileId(ArmSMETileType tileType, unsigned tileId) {
+    TileMask tileMask = getMasks(tileType)[tileId];
+    assert((tilesInUse & tileMask) != TileMask::kNone &&
+           "cannot release unallocated tile!");
+    tilesInUse ^= tileMask;
+  }
+
+  /// Allocates an in-memory tile ID.
+  unsigned allocateInMemoryTileId() {
+    // Note: We never release in-memory tile IDs. We could, which may allow
+    // reusing an allocation, but as we _never_ want to spill an SME tile this
+    // is not optimized.
+    return nextInMemoryTileId++;
   }
-  return failure();
-}
 
-/// Collects transitive uses of a root value through control flow. This can
-/// handle basic SCF constructs, along with control flow (br and cond_br).
-/// Simple loops work at the SCF level, while more complex control flow can be
-/// dealt with after lowering to CF. This is used to implement basic tile
-/// allocation.
-static void findDependantOps(Value rootValue,
-                             SetVector<Operation *> &dependantOps) {
-  auto traverseCorrespondingValues = [&](auto inputValues, auto exitValues) {
-    for (auto [idx, value] : llvm::enumerate(inputValues)) {
-      if (value == rootValue)
-        findDependantOps(exitValues[idx], dependantOps);
+private:
+  TileMask tilesInUse = TileMask::kNone;
+  unsigned nextInMemoryTileId = kInMemoryTileIdBase;
+};
+
+// Add new intermediate blocks for the true and false destinations of a
+// `cf.cond_br`. This prevents spurious liveness overlaps due to copies at
+// branches.
+void splitCondBranches(IRRewriter &rewriter, FunctionOpInterface function) {
+  SmallVector<cf::CondBranchOp> worklist;
+  function.walk([&](cf::CondBranchOp condBranch) {
+    if (llvm::any_of(condBranch->getOperands(), [&](Value value) {
+          return isValidSMETileVectorType(value.getType());
+        })) {
+      worklist.push_back(condBranch);
     }
+  });
+
+  auto insertJump = [&](Location loc, Block *source, Block *dest, auto args) {
+    rewriter.setInsertionPointToEnd(source);
+    rewriter.create<cf::BranchOp>(loc, dest, args);
   };
-  for (Operation *user : rootValue.getUsers()) {
-    if (dependantOps.contains(user))
+
+  for (auto condBranch : worklist) {
+    auto loc = condBranch.getLoc();
+    Block *block = condBranch->getBlock();
+    auto newTrueBranch = rewriter.splitBlock(block, block->end());
+    auto newFalseBranch = rewriter.splitBlock(block, block->end());
+    insertJump(loc, newTrueBranch, condBranch.getTrueDest(),
+               condBranch.getTrueDestOperands());
+    insertJump(loc, newFalseBranch, condBranch.getFalseDest(),
+               condBranch.getFalseDestOperands());
+    condBranch.getFalseDestOperandsMutable().clear();
+    condBranch.getTrueDestOperandsMutable().clear();
+    condBranch.setSuccessor(newTrueBranch, 0);
+    condBranch.setSuccessor(newFalseBranch, 1);
+  }
+}
+
+/// Inserts tile copies at `cf.br` operations.
+void insertCopiesAtBranches(IRRewriter &rewriter,
+                            FunctionOpInterface function) {
+  splitCondBranches(rewriter, function);
+  for (Block &block : function.getBlocks()) {
+    Operation *terminator = block.getTerminator();
+    if (!isa<cf::BranchOp>(terminator))
       continue;
-    dependantOps.insert(user);
-    TypeSwitch<Operation *>(user)
-        .Case<cf::BranchOp>([&](auto branchOp) {
-          // (CF) Follow branch.
-          traverseCorrespondingValues(branchOp.getDestOperands(),
-                                      branchOp.getDest()->getArguments());
-        })
-        .Case<cf::CondBranchOp>([&](auto condBranchOp) {
-          // (CF) Follow true branch.
-          traverseCorrespondingValues(
-              condBranchOp.getTrueOperands(),
-              condBranchOp.getTrueDest()->getArguments());
-          // (CF) Follow false branch.
-          traverseCorrespondingValues(
-              condBranchOp.getFalseOperands(),
-              condBranchOp.getFalseDest()->getArguments());
-        })
-        .Case<LoopLikeOpInterface>([&](auto loopOp) {
-          // (SCF) Follow iter_args of (basic) loops (e.g. for loops).
-          traverseCorrespondingValues(loopOp.getInits(),
-                                      loopOp.getRegionIterArgs());
-        })
-        .Case<scf::YieldOp>([&](auto yieldOp) {
-          // (SCF) Follow yields of (basic) control flow (e.g. for loops).
-          auto parent = user->getParentOp();
-          traverseCorrespondingValues(user->getOperands(),
-                                      parent->getResults());
+    rewriter.setInsertionPoint(terminator);
----------------
MacDue wrote:

There's no need to restore the insertion as every use of the rewriter sets the insertion point (it would not really make sense to assume it's at an point in particular with how it's used). 

https://github.com/llvm/llvm-project/pull/90448