[analysis][NFC] Create a TransferFunction concept (#6033)

Factor the static assertions for transfer functions out into a new
transfer-function.h header. The concept requires the `getDependents` method to
return an input range of basic blocks, and to satisfy that requirement, fix up
_indirect_ptr_iterator in cfg-impl.h so that it is a proper iterator. Remove
part of the lattice fuzzer that was using a placeholder transfer function in a
way that does not satisfy the new type constraints; most of that code will be
overhauled in the future anyway.

1 files changed, 67 insertions, 0 deletions

diff --git a/src/analysis/transfer-function.h b/src/analysis/transfer-function.h
new file mode 100644
index 000000000..88fd8a1b1
--- /dev/null
+++ b/src/analysis/transfer-function.h
@@ -0,0 +1,67 @@
+/*
+ * Copyright 2023 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef wasm_analysis_transfer_function_h
+#define wasm_analysis_transfer_function_h
+
+#if __cplusplus >= 202002L
+#include <concepts>
+#include <iterator>
+#include <ranges>
+#endif
+
+#include <queue>
+
+#include "cfg.h"
+#include "lattice.h"
+
+namespace wasm::analysis {
+
+#if __cplusplus >= 202002L
+
+template<typename T>
+concept BasicBlockInputRange =
+  std::ranges::input_range<T> &&
+  std::is_same<std::ranges::range_value_t<T>, BasicBlock>::value;
+
+template<typename TxFn, typename L>
+concept TransferFunctionImpl = requires(TxFn& txfn,
+                                        const CFG& cfg,
+                                        BasicBlock* bb,
+                                        typename L::Element& elem,
+                                        std::queue<const BasicBlock*>& bbq) {
+  // Apply the transfer function to update a lattice element with information
+  // from a basic block.
+  { txfn.transfer(bb, elem) } noexcept -> std::same_as<void>;
+  // Initializes the worklist of basic blocks, which can affect performance
+  // depending on the direction of the analysis. TODO: Unlock performance
+  // benefits while exposing fewer implementation details.
+  { txfn.enqueueWorklist(cfg, bbq) } noexcept -> std::same_as<void>;
+  // Get a range over the basic blocks that depend on the given block.
+  { txfn.getDependents(bb) } noexcept -> BasicBlockInputRange;
+};
+
+#define TransferFunction TransferFunctionImpl<L>
+
+#else
+
+#define TransferFunction typename
+
+#endif // __cplusplus >= 202002L
+
+} // namespace wasm::analysis
+
+#endif // wasm_analysis_transfer_function_h