Monomorphize dropped functions (#6734)

We now consider a drop to be part of the call context: If we see

(drop
  (call $foo)
)

(func $foo (result i32)
  (i32.const 42)
)

Then we'd monomorphize to this:

(call $foo_1)  ;; call the specialized function instead

(func $foo_1   ;; the specialized function returns nothing
  (drop        ;; the drop was moved into here
    (i32.const 42)
  )
)

With the drop now in the called function, we may be able to optimize out unused work.

Refactor a bit of code out of DAE that we can reuse here, into a new return-utils.h.

src/ir/CMakeLists.txt

@@ -17,6 +17,7 @@ set(ir_SOURCES
  LocalGraph.cpp
  LocalStructuralDominance.cpp
  ReFinalize.cpp
+ return-utils.cpp
  stack-utils.cpp
  table-utils.cpp
  type-updating.cpp

src/ir/return-utils.cpp

@@ -0,0 +1,99 @@
+/*
+ * Copyright 2024 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/return-utils.h"
+#include "ir/module-utils.h"
+#include "wasm-builder.h"
+#include "wasm-traversal.h"
+#include "wasm.h"
+
+namespace wasm::ReturnUtils {
+
+namespace {
+
+struct ReturnValueRemover : public PostWalker<ReturnValueRemover> {
+ void visitReturn(Return* curr) {
+ auto* value = curr->value;
+ assert(value);
+ curr->value = nullptr;
+ Builder builder(*getModule());
+ replaceCurrent(builder.makeSequence(builder.makeDrop(value), curr));
+ }
+
+ void visitCall(Call* curr) { handleReturnCall(curr); }
+ void visitCallIndirect(CallIndirect* curr) { handleReturnCall(curr); }
+ void visitCallRef(CallRef* curr) { handleReturnCall(curr); }
+
+ template<typename T> void handleReturnCall(T* curr) {
+ if (curr->isReturn) {
+ Fatal() << "Cannot remove return_calls in ReturnValueRemover";
+ }
+ }
+
+ void visitFunction(Function* curr) {
+ if (curr->body->type.isConcrete()) {
+ curr->body = Builder(*getModule()).makeDrop(curr->body);
+ }
+ }
+};
+
+} // anonymous namespace
+
+void removeReturns(Function* func, Module& wasm) {
+ ReturnValueRemover().walkFunctionInModule(func, &wasm);
+}
+
+std::unordered_map<Function*, bool> findReturnCallers(Module& wasm) {
+ ModuleUtils::ParallelFunctionAnalysis<bool> analysis(
+ wasm, [&](Function* func, bool& hasReturnCall) {
+ if (func->imported()) {
+ return;
+ }
+
+ struct Finder : PostWalker<Finder> {
+ bool hasReturnCall = false;
+
+ void visitCall(Call* curr) {
+ if (curr->isReturn) {
+ hasReturnCall = true;
+ }
+ }
+ void visitCallIndirect(CallIndirect* curr) {
+ if (curr->isReturn) {
+ hasReturnCall = true;
+ }
+ }
+ void visitCallRef(CallRef* curr) {
+ if (curr->isReturn) {
+ hasReturnCall = true;
+ }
+ }
+ } finder;
+
+ finder.walk(func->body);
+ hasReturnCall = finder.hasReturnCall;
+ });
+
+ // Convert to an unordered map for fast lookups. TODO: Avoid a copy here.
+ std::unordered_map<Function*, bool> ret;
+ ret.reserve(analysis.map.size());
+ for (auto& [k, v] : analysis.map) {
+ ret[k] = v;
+ }
+ return ret;
+}
+
+} // namespace wasm::ReturnUtils

src/ir/return-utils.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2024 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_ir_return_h
+#define wasm_ir_return_h
+
+#include "wasm.h"
+
+namespace wasm::ReturnUtils {
+
+// Removes values from both explicit returns and implicit ones (values that flow
+// from the body). This is useful after changing a function's type to no longer
+// return anything.
+//
+// This does *not* handle return calls, and will error on them. Removing a
+// return call may change the semantics of the program, so we do not do it
+// automatically here.
+void removeReturns(Function* func, Module& wasm);
+
+// Return a map of every function to whether it does a return call.
+using ReturnCallersMap = std::unordered_map<Function*, bool>;
+ReturnCallersMap findReturnCallers(Module& wasm);
+
+} // namespace wasm::ReturnUtils
+
+#endif // wasm_ir_return_h

src/passes/DeadArgumentElimination.cpp

@@ -42,6 +42,7 @@
 #include "ir/find_all.h"
 #include "ir/lubs.h"
 #include "ir/module-utils.h"
+#include "ir/return-utils.h"
 #include "ir/type-updating.h"
 #include "ir/utils.h"
 #include "param-utils.h"
@@ -358,23 +359,7 @@ struct DAE : public Pass {
  }
  }
  // Remove any return values.
- struct ReturnUpdater : public PostWalker<ReturnUpdater> {
- Module* module;
- ReturnUpdater(Function* func, Module* module) : module(module) {
- walk(func->body);
- }
- void visitReturn(Return* curr) {
- auto* value = curr->value;
- assert(value);
- curr->value = nullptr;
- Builder builder(*module);
- replaceCurrent(builder.makeSequence(builder.makeDrop(value), curr));
- }
- } returnUpdater(func, module);
- // Remove any value flowing out.
- if (func->body->type.isConcrete()) {
- func->body = Builder(*module).makeDrop(func->body);
- }
+ ReturnUtils::removeReturns(func, *module);
  }
 
  // Given a function and all the calls to it, see if we can refine the type of

src/passes/Monomorphize.cpp

@@ -92,6 +92,7 @@
 #include "ir/manipulation.h"
 #include "ir/module-utils.h"
 #include "ir/names.h"
+#include "ir/return-utils.h"
 #include "ir/type-updating.h"
 #include "ir/utils.h"
 #include "pass.h"
@@ -103,6 +104,36 @@ namespace wasm {
 
 namespace {
 
+// Core information about a call: the call itself, and if it is dropped, the
+// drop.
+struct CallInfo {
+ Call* call;
+ // Store a reference to the drop's pointer so that we can replace it, as when
+ // we optimize a dropped call we need to replace (drop (call)) with (call).
+ // Or, if the call is not dropped, this is nullptr.
+ Expression** drop;
+};
+
+// Finds the calls and whether each one of them is dropped.
+struct CallFinder : public PostWalker<CallFinder> {
+ std::vector<CallInfo> infos;
+
+ void visitCall(Call* curr) {
+ // Add the call as not having a drop, and update the drop later if we are.
+ infos.push_back(CallInfo{curr, nullptr});
+ }
+
+ void visitDrop(Drop* curr) {
+ if (curr->value->is<Call>()) {
+ // The call we just added to |infos| is dropped.
+ assert(!infos.empty());
+ auto& back = infos.back();
+ assert(back.call == curr->value);
+ back.drop = getCurrentPointer();
+ }
+ }
+};
+
 // Relevant information about a callsite for purposes of monomorphization.
 struct CallContext {
  // The operands of the call, processed to leave the parts that make sense to
@@ -181,12 +212,12 @@ struct CallContext {
  // remaining values by updating |newOperands| (for example, if all the values
  // sent are constants, then |newOperands| will end up empty, as we have
  // nothing left to send).
- void buildFromCall(Call* call,
+ void buildFromCall(CallInfo& info,
  std::vector<Expression*>& newOperands,
  Module& wasm) {
  Builder builder(wasm);
 
- for (auto* operand : call->operands) {
+ for (auto* operand : info.call->operands) {
  // Process the operand. This is a copy operation, as we are trying to move
  // (copy) code from the callsite into the called function. When we find we
  // can copy then we do so, and when we cannot that value remains as a
@@ -212,8 +243,7 @@ struct CallContext {
  }));
  }
 
- // TODO: handle drop
- dropped = false;
+ dropped = !!info.drop;
  }
 
  // Checks whether an expression can be moved into the context.
@@ -299,6 +329,11 @@ struct Monomorphize : public Pass {
  void run(Module* module) override {
  // TODO: parallelize, see comments below
 
+ // Find all the return-calling functions. We cannot remove their returns
+ // (because turning a return call into a normal call may break the program
+ // by using more stack).
+ auto returnCallersMap = ReturnUtils::findReturnCallers(*module);
+
  // Note the list of all functions. We'll be adding more, and do not want to
  // operate on those.
  std::vector<Name> funcNames;
@@ -309,26 +344,38 @@ struct Monomorphize : public Pass {
  // to call the monomorphized targets.
  for (auto name : funcNames) {
  auto* func = module->getFunction(name);
- for (auto* call : FindAll<Call>(func->body).list) {
- if (call->type == Type::unreachable) {
+
+ CallFinder callFinder;
+ callFinder.walk(func->body);
+ for (auto& info : callFinder.infos) {
+ if (info.call->type == Type::unreachable) {
  // Ignore unreachable code.
  // TODO: return_call?
  continue;
  }
 
- if (call->target == name) {
+ if (info.call->target == name) {
  // Avoid recursion, which adds some complexity (as we'd be modifying
  // ourselves if we apply optimizations).
  continue;
  }
 
- processCall(call, *module);
+ // If the target function does a return call, then as noted earlier we
+ // cannot remove its returns, so do not consider the drop as part of the
+ // context in such cases (as if we reverse-inlined the drop into the
+ // target then we'd be removing the returns).
+ if (returnCallersMap[module->getFunction(info.call->target)]) {
+ info.drop = nullptr;
+ }
+
+ processCall(info, *module);
  }
  }
  }
 
  // Try to optimize a call.
- void processCall(Call* call, Module& wasm) {
+ void processCall(CallInfo& info, Module& wasm) {
+ auto* call = info.call;
  auto target = call->target;
  auto* func = wasm.getFunction(target);
  if (func->imported()) {
@@ -342,19 +389,16 @@ struct Monomorphize : public Pass {
  // if we use that context.
  CallContext context;
  std::vector<Expression*> newOperands;
- context.buildFromCall(call, newOperands, wasm);
+ context.buildFromCall(info, newOperands, wasm);
 
  // See if we've already evaluated this function + call context. If so, then
  // we've memoized the result.
  auto iter = funcContextMap.find({target, context});
  if (iter != funcContextMap.end()) {
  auto newTarget = iter->second;
  if (newTarget != target) {
- // When we computed this before we found a benefit to optimizing, and
- // created a new monomorphized function to call. Use it by simply
- // applying the new operands we computed, and adjusting the call target.
- call->operands.set(newOperands);
- call->target = newTarget;
+ // We saw benefit to optimizing this case. Apply that.
+ updateCall(info, newTarget, newOperands, wasm);
  }
  return;
  }
@@ -419,8 +463,7 @@ struct Monomorphize : public Pass {
  if (worthwhile) {
  // We are using the monomorphized function, so update the call and add it
  // to the module.
- call->operands.set(newOperands);
- call->target = monoFunc->name;
+ updateCall(info, monoFunc->name, newOperands, wasm);
 
  wasm.addFunction(std::move(monoFunc));
  }
@@ -453,8 +496,9 @@ struct Monomorphize : public Pass {
  newParams.push_back(operand->type);
  }
  }
- // TODO: support changes to results.
- auto newResults = func->getResults();
+ // If we were dropped then we are pulling the drop into the monomorphized
+ // function, which means we return nothing.
+ auto newResults = context.dropped ? Type::none : func->getResults();
  newFunc->type = Signature(Type(newParams), newResults);
 
  // We must update local indexes: the new function has a potentially
@@ -549,9 +593,32 @@ struct Monomorphize : public Pass {
  newFunc->body = builder.makeBlock(pre);
  }
 
+ if (context.dropped) {
+ ReturnUtils::removeReturns(newFunc.get(), wasm);
+ }
+
  return newFunc;
  }
 
+ // Given a call and a new target it should be calling, apply that new target,
+ // including updating the operands and handling dropping.
+ void updateCall(const CallInfo& info,
+ Name newTarget,
+ const std::vector<Expression*>& newOperands,
+ Module& wasm) {
+ info.call->target = newTarget;
+ info.call->operands.set(newOperands);
+
+ if (info.drop) {
+ // Replace (drop (call)) with (call), that is, replace the drop with the
+ // (updated) call which now has type none. Note we should have handled
+ // unreachability before getting here.
+ assert(info.call->type != Type::unreachable);
+ info.call->type = Type::none;
+ *info.drop = info.call;
+ }
+ }
+
  // Run some function-level optimizations on a function. Ideally we would run a
  // minimal amount of optimizations here, but we do want to give the optimizer
  // as much of a chance to work as possible, so for now do all of -O3 (in