[NFC] Move and generalize parameter-removing logic from DeadArgumentElimination (#4544)

In preparation for removing dead arguments from all functions sharing a heap
type (which seems useful for j2wasm output), first this PR refactors that code
so it is reusable. This moves the code out of the pass into FunctionUtils, and
also generalizes it slightly by

supporting a set of functions and not just a single one, and
receiving a list of call_refs and not just calls
(no other changes to anything).

1 files changed, 177 insertions, 0 deletions

diff --git a/src/passes/param-utils.cpp b/src/passes/param-utils.cpp
new file mode 100644
index 000000000..019df6d7f
--- /dev/null
+++ b/src/passes/param-utils.cpp
@@ -0,0 +1,177 @@
+/*
+ * Copyright 2022 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.
+ */
+
+#include "ir/function-utils.h"
+#include "ir/local-graph.h"
+#include "ir/type-updating.h"
+#include "support/sorted_vector.h"
+#include "wasm.h"
+
+namespace wasm::ParamUtils {
+
+std::unordered_set<Index> getUsedParams(Function* func) {
+  LocalGraph localGraph(func);
+
+  std::unordered_set<Index> usedParams;
+
+  for (auto& [get, sets] : localGraph.getSetses) {
+    if (!func->isParam(get->index)) {
+      continue;
+    }
+
+    for (auto* set : sets) {
+      // A nullptr value indicates there is no LocalSet* that sets the value,
+      // so it must be the parameter value.
+      if (!set) {
+        usedParams.insert(get->index);
+      }
+    }
+  }
+
+  return usedParams;
+}
+
+bool removeParameter(const std::vector<Function*> funcs,
+                     Index index,
+                     const std::vector<Call*>& calls,
+                     const std::vector<CallRef*>& callRefs,
+                     Module* module,
+                     PassRunner* runner) {
+  assert(funcs.size() > 0);
+  auto* first = funcs[0];
+#ifndef NDEBUG
+  for (auto* func : funcs) {
+    assert(func->type == first->type);
+  }
+#endif
+
+  // Check if none of the calls has a param with side effects that we cannot
+  // remove (as if we can remove them, we will simply do that when we remove the
+  // parameter). Note: flattening the IR beforehand can help here.
+  bool callParamsAreValid =
+    std::none_of(calls.begin(), calls.end(), [&](Call* call) {
+      auto* operand = call->operands[index];
+      return EffectAnalyzer(runner->options, *module, operand)
+        .hasUnremovableSideEffects();
+    });
+  if (!callParamsAreValid) {
+    return false;
+  }
+
+  // The type must be valid for us to handle as a local (since we
+  // replace the parameter with a local).
+  // TODO: if there are no references at all, we can avoid creating a
+  //       local
+  bool typeIsValid = TypeUpdating::canHandleAsLocal(first->getLocalType(index));
+  if (!typeIsValid) {
+    return false;
+  }
+
+  // We can do it!
+
+  // Remove the parameter from the function. We must add a new local
+  // for uses of the parameter, but cannot make it use the same index
+  // (in general).
+  auto paramsType = first->getParams();
+  std::vector<Type> params(paramsType.begin(), paramsType.end());
+  auto type = params[index];
+  params.erase(params.begin() + index);
+  // TODO: parallelize some of these loops?
+  for (auto* func : funcs) {
+    func->setParams(Type(params));
+
+    // It's cumbersome to adjust local names - TODO don't clear them?
+    Builder::clearLocalNames(func);
+  }
+  std::vector<Index> newIndexes;
+  for (auto* func : funcs) {
+    newIndexes.push_back(Builder::addVar(func, type));
+  }
+  // Update local operations.
+  struct LocalUpdater : public PostWalker<LocalUpdater> {
+    Index removedIndex;
+    Index newIndex;
+    LocalUpdater(Function* func, Index removedIndex, Index newIndex)
+      : removedIndex(removedIndex), newIndex(newIndex) {
+      walk(func->body);
+    }
+    void visitLocalGet(LocalGet* curr) { updateIndex(curr->index); }
+    void visitLocalSet(LocalSet* curr) { updateIndex(curr->index); }
+    void updateIndex(Index& index) {
+      if (index == removedIndex) {
+        index = newIndex;
+      } else if (index > removedIndex) {
+        index--;
+      }
+    }
+  };
+  for (Index i = 0; i < funcs.size(); i++) {
+    auto* func = funcs[i];
+    if (!func->imported()) {
+      LocalUpdater(funcs[i], index, newIndexes[i]);
+      TypeUpdating::handleNonDefaultableLocals(func, *module);
+    }
+  }
+
+  // Remove the arguments from the calls.
+  for (auto* call : calls) {
+    call->operands.erase(call->operands.begin() + index);
+  }
+  for (auto* call : callRefs) {
+    call->operands.erase(call->operands.begin() + index);
+  }
+
+  return true;
+}
+
+SortedVector removeParameters(const std::vector<Function*> funcs,
+                              SortedVector indexes,
+                              const std::vector<Call*>& calls,
+                              const std::vector<CallRef*>& callRefs,
+                              Module* module,
+                              PassRunner* runner) {
+  if (indexes.empty()) {
+    return {};
+  }
+
+  assert(funcs.size() > 0);
+  auto* first = funcs[0];
+#ifndef NDEBUG
+  for (auto* func : funcs) {
+    assert(func->type == first->type);
+  }
+#endif
+
+  // Iterate downwards, as we may remove more than one, and going forwards would
+  // alter the indexes after us.
+  Index i = first->getNumParams() - 1;
+  SortedVector removed;
+  while (1) {
+    if (indexes.has(i)) {
+      if (removeParameter(funcs, i, calls, callRefs, module, runner)) {
+        // Success!
+        removed.insert(i);
+      }
+    }
+    if (i == 0) {
+      break;
+    }
+    i--;
+  }
+  return removed;
+}
+
+} // namespace wasm::ParamUtils