First implementation of store elimination.

src/hydrogen-store-elimination.cc

+// Copyright 2013 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "hydrogen-store-elimination.h"
+#include "hydrogen-instructions.h"
+
+namespace v8 {
+namespace internal {
+
+#define TRACE(x) if (FLAG_trace_store_elimination) PrintF x
+
+// Performs a block-by-block local analysis for removable stores.
+void HStoreEliminationPhase::Run() {
+  GVNFlagSet flags;  // Use GVN flags as an approximation for some instructions.
+  flags.RemoveAll();
+
+  flags.Add(kDependsOnArrayElements);
+  flags.Add(kDependsOnArrayLengths);
+  flags.Add(kDependsOnStringLengths);
+  flags.Add(kDependsOnBackingStoreFields);
+  flags.Add(kDependsOnDoubleArrayElements);
+  flags.Add(kDependsOnDoubleFields);
+  flags.Add(kDependsOnElementsPointer);
+  flags.Add(kDependsOnInobjectFields);
+  flags.Add(kDependsOnExternalMemory);
+
+  for (int i = 0; i < graph()->blocks()->length(); i++) {
+    unobserved_.Rewind(0);
+    HBasicBlock* block = graph()->blocks()->at(i);
+    for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
+      HInstruction* instr = it.Current();
+
+      // TODO(titzer): eliminate unobserved HStoreKeyed instructions too.
+      switch (instr->opcode()) {
+        case HValue::kStoreNamedField:
+          // Remove any unobserved stores overwritten by this store.
+          ProcessStore(HStoreNamedField::cast(instr));
+          break;
+        case HValue::kLoadNamedField:
+          // Observe any unobserved stores on this object + field.
+          ProcessLoad(HLoadNamedField::cast(instr));
+          break;
+        default:
+          ProcessInstr(instr, flags);
+          break;
+      }
+    }
+  }
+}
+
+
+void HStoreEliminationPhase::ProcessStore(HStoreNamedField* store) {
+  HValue* object = store->object()->ActualValue();
+  int i = 0;
+  while (i < unobserved_.length()) {
+    HStoreNamedField* prev = unobserved_.at(i);
+    if (aliasing_->MustAlias(object, prev->object()->ActualValue()) &&
+        store->access().Equals(prev->access())) {
+      // This store is guaranteed to overwrite the previous store.
+      prev->DeleteAndReplaceWith(NULL);
+      TRACE(("++ Unobserved store S%d overwritten by S%d\n",
+             prev->id(), store->id()));
+      unobserved_.Remove(i);
+    } else {
+      // TODO(titzer): remove map word clearing from folded allocations.
+      i++;
+    }
+  }
+  // Only non-transitioning stores are removable.
+  if (!store->has_transition()) {
+    TRACE(("-- Might remove store S%d\n", store->id()));
+    unobserved_.Add(store, zone());
+  }
+}
+
+
+void HStoreEliminationPhase::ProcessLoad(HLoadNamedField* load) {
+  HValue* object = load->object()->ActualValue();
+  int i = 0;
+  while (i < unobserved_.length()) {
+    HStoreNamedField* prev = unobserved_.at(i);
+    if (aliasing_->MayAlias(object, prev->object()->ActualValue()) &&
+        load->access().Equals(prev->access())) {
+      TRACE(("-- Observed store S%d by load L%d\n", prev->id(), load->id()));
+      unobserved_.Remove(i);
+    } else {
+      i++;
+    }
+  }
+}
+
+
+void HStoreEliminationPhase::ProcessInstr(HInstruction* instr,
+    GVNFlagSet flags) {
+  if (unobserved_.length() == 0) return;  // Nothing to do.
+  if (instr->CanDeoptimize()) {
+    TRACE(("-- Observed stores at I%d (might deoptimize)\n", instr->id()));
+    unobserved_.Rewind(0);
+    return;
+  }
+  if (instr->CheckGVNFlag(kChangesNewSpacePromotion)) {
+    TRACE(("-- Observed stores at I%d (might GC)\n", instr->id()));
+    unobserved_.Rewind(0);
+    return;
+  }
+  if (instr->gvn_flags().ContainsAnyOf(flags)) {
+    TRACE(("-- Observed stores at I%d (GVN flags)\n", instr->id()));
+    unobserved_.Rewind(0);
+    return;
+  }
+}
+
+} }  // namespace v8::internal