Local CSE (#930)

Simple local common subexpression elimination. Useful mostly to reduce code size (as VMs do GVN etc.). Enabled by default in -Oz.

5 files changed, 222 insertions, 0 deletions

diff --git a/src/ast/hashed.h b/src/ast/hashed.h
new file mode 100644
index 000000000..04ee7f401
--- /dev/null
+++ b/src/ast/hashed.h
@@ -0,0 +1,59 @@
+/*
+ * Copyright 2017 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_ast_hashed_h
+
+#include "support/hash.h"
+#include "wasm.h"
+#include "ast_utils.h"
+
+namespace wasm {
+
+// An expression with a cached hash value
+struct HashedExpression {
+  Expression* expr;
+  size_t hash;
+
+  HashedExpression(Expression* expr) : expr(expr) {
+    if (expr) {
+      hash = ExpressionAnalyzer::hash(expr);
+    }
+  }
+
+  HashedExpression(const HashedExpression& other) : expr(other.expr), hash(other.hash) {}
+};
+
+struct ExpressionHasher {
+  size_t operator()(const HashedExpression value) const {
+    return value.hash;
+  }
+};
+
+struct ExpressionComparer {
+  bool operator()(const HashedExpression a, const HashedExpression b) const {
+    if (a.hash != b.hash) return false;
+    return ExpressionAnalyzer::equal(a.expr, b.expr);
+  }
+};
+
+template<typename T>
+class HashedExpressionMap : public std::unordered_map<HashedExpression, T, ExpressionHasher, ExpressionComparer> {
+};
+
+} // namespace wasm
+
+#endif // _wasm_ast_hashed_h
+
diff --git a/src/passes/CMakeLists.txt b/src/passes/CMakeLists.txt
index 74f120e85..876a71d74 100644
--- a/src/passes/CMakeLists.txt
+++ b/src/passes/CMakeLists.txt
@@ -7,6 +7,7 @@ SET(passes_SOURCES
   ExtractFunction.cpp
   Inlining.cpp
   LegalizeJSInterface.cpp
+  LocalCSE.cpp
   MemoryPacking.cpp
   MergeBlocks.cpp
   Metrics.cpp
diff --git a/src/passes/LocalCSE.cpp b/src/passes/LocalCSE.cpp
new file mode 100644
index 000000000..6d48a1107
--- /dev/null
+++ b/src/passes/LocalCSE.cpp
@@ -0,0 +1,156 @@
+/*
+ * Copyright 2017 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.
+ */
+
+//
+// Local CSE
+//
+// In each linear area of execution,
+//  * track each relevant (big enough) expression
+//  * if already seen, write to a local if not already, and reuse
+//  * invalidate the list as we see effects
+//
+// TODO: global, inter-block gvn etc.
+//
+
+#include <wasm.h>
+#include <wasm-builder.h>
+#include <wasm-traversal.h>
+#include <pass.h>
+#include <ast_utils.h>
+#include <ast/hashed.h>
+
+namespace wasm {
+
+const Index UNUSED = -1;
+
+struct LocalCSE : public WalkerPass<LinearExecutionWalker<LocalCSE>> {
+  bool isFunctionParallel() override { return true; }
+
+  Pass* create() override { return new LocalCSE(); }
+
+  // information for an expression we can reuse
+  struct UsableInfo {
+    Expression** item;
+    Index index; // if not UNUSED, then the local we are assigned to, use that to reuse us
+    EffectAnalyzer effects;
+
+    UsableInfo(Expression** item, PassOptions& passOptions) : item(item), index(UNUSED), effects(passOptions, *item) {}
+  };
+
+  // a list of usables in a linear execution trace
+  typedef HashedExpressionMap<UsableInfo> Usables;
+
+  // locals in current linear execution trace, which we try to sink
+  Usables usables;
+
+  static void doNoteNonLinear(LocalCSE* self, Expression** currp) {
+    self->usables.clear();
+  }
+
+  void checkInvalidations(EffectAnalyzer& effects) {
+    // TODO: this is O(bad)
+    std::vector<HashedExpression> invalidated;
+    for (auto& sinkable : usables) {
+      if (effects.invalidates(sinkable.second.effects)) {
+        invalidated.push_back(sinkable.first);
+      }
+    }
+    for (auto index : invalidated) {
+      usables.erase(index);
+    }
+  }
+
+  std::vector<Expression*> expressionStack;
+
+  static void visitPre(LocalCSE* self, Expression** currp) {
+    // pre operations
+    Expression* curr = *currp;
+
+    EffectAnalyzer effects(self->getPassOptions());
+    if (effects.checkPre(curr)) {
+      self->checkInvalidations(effects);
+    }
+
+    self->expressionStack.push_back(curr);
+  }
+
+  static void visitPost(LocalCSE* self, Expression** currp) {
+    auto* curr = *currp;
+
+    // main operations
+    if (self->isRelevant(curr)) {
+      self->handle(currp, curr);
+    }
+
+    // post operations
+
+    EffectAnalyzer effects(self->getPassOptions());
+    if (effects.checkPost(curr)) {
+      self->checkInvalidations(effects);
+    }
+
+    self->expressionStack.pop_back();
+  }
+
+  // override scan to add a pre and a post check task to all nodes
+  static void scan(LocalCSE* self, Expression** currp) {
+    self->pushTask(visitPost, currp);
+
+    WalkerPass<LinearExecutionWalker<LocalCSE>>::scan(self, currp);
+
+    self->pushTask(visitPre, currp);
+  }
+
+  bool isRelevant(Expression* curr) {
+    if (curr->is<GetLocal>()) {
+      return false; // trivial, this is what we optimize to!
+    }
+    if (!isConcreteWasmType(curr->type)) {
+      return false; // don't bother with unreachable etc.
+    }
+    if (EffectAnalyzer(getPassOptions(), curr).hasSideEffects()) {
+      return false; // we can't combine things with side effects
+    }
+    // check what we care about TODO: use optimize/shrink levels?
+    return Measurer::measure(curr) > 1;
+  }
+
+  void handle(Expression** currp, Expression* curr) {
+    HashedExpression hashed(curr);
+    auto iter = usables.find(hashed);
+    if (iter != usables.end()) {
+      // already exists in the table, this is good to reuse
+      auto& info = iter->second;
+      if (info.index == UNUSED) {
+        // we need to assign to a local. create a new one
+        auto index = info.index = Builder::addVar(getFunction(), curr->type);
+        (*info.item) = Builder(*getModule()).makeTeeLocal(index, *info.item);
+      }
+      replaceCurrent(
+        Builder(*getModule()).makeGetLocal(info.index, curr->type)
+      );
+    } else {
+      // not in table, add this, maybe we can help others later
+      usables.emplace(std::make_pair(hashed, UsableInfo(currp, getPassOptions())));
+    }
+  }
+};
+
+Pass *createLocalCSEPass() {
+  return new LocalCSE();
+}
+
+} // namespace wasm
diff --git a/src/passes/pass.cpp b/src/passes/pass.cpp
index 081164d90..a82b2fa58 100644
--- a/src/passes/pass.cpp
+++ b/src/passes/pass.cpp
@@ -71,6 +71,7 @@ void PassRegistry::registerPasses() {
   registerPass("extract-function", "leaves just one function (useful for debugging)", createExtractFunctionPass);
   registerPass("inlining", "inlines functions (currently only ones with a single use)", createInliningPass);
   registerPass("legalize-js-interface", "legalizes i64 types on the import/export boundary", createLegalizeJSInterfacePass);
+  registerPass("local-cse", "common subexpression elimination inside basic blocks", createLocalCSEPass);
   registerPass("memory-packing", "packs memory into separate segments, skipping zeros", createMemoryPackingPass);
   registerPass("merge-blocks", "merges blocks to their parents", createMergeBlocksPass);
   registerPass("metrics", "reports metrics", createMetricsPass);
@@ -132,6 +133,10 @@ void PassRunner::addDefaultFunctionOptimizationPasses() {
   add("merge-blocks");
   add("optimize-instructions");
   add("precompute");
+  if (options.shrinkLevel >= 2) {
+    add("local-cse"); // TODO: run this early, before first coalesce-locals. right now doing so uncovers some deficiencies we need to fix first
+    add("coalesce-locals"); // just for localCSE
+  }
   add("vacuum"); // should not be needed, last few passes do not create garbage, but just to be safe
 }
 
diff --git a/src/passes/passes.h b/src/passes/passes.h
index 83bf556d8..c3660b048 100644
--- a/src/passes/passes.h
+++ b/src/passes/passes.h
@@ -31,6 +31,7 @@ Pass *createExtractFunctionPass();
 Pass *createFullPrinterPass();
 Pass *createInliningPass();
 Pass *createLegalizeJSInterfacePass();
+Pass *createLocalCSEPass();
 Pass *createLowerIfElsePass();
 Pass *createMemoryPackingPass();
 Pass *createMergeBlocksPass();