Implement simplistic local alias analysis, which will be used in both check elimination and load el…

src/hydrogen-alias-analysis.h

+// 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.
+
+#ifndef V8_HYDROGEN_ALIAS_ANALYSIS_H_
+#define V8_HYDROGEN_ALIAS_ANALYSIS_H_
+
+#include "hydrogen.h"
+
+namespace v8 {
+namespace internal {
+
+enum HAliasing {
+  kMustAlias,
+  kMayAlias,
+  kNoAlias
+};

[Toon Verwaest, 2013/09/11 09:05:55]

Can we move this enum into HAliasAnalyzer?

+
+
+// Defines the interface to alias analysis for the rest of the compiler.
+// A simple implementation can use only local reasoning, but a more powerful
+// analysis might employ points-to analysis.
+class HAliasAnalyzer : public ZoneObject {
+ public:
+  // Simple alias analysis distinguishes allocations, parameters,
+  // and constants using only local reasoning.
+  HAliasing Query(HValue* a, HValue* b) {
+    // The same SSA value always references the same object.
+    if (a == b) return kMustAlias;
+
+    if (a->IsAllocate() || a->IsInnerAllocatedObject()) {
+      // Two non-identical allocations can never be aliases.
+      if (b->IsAllocate()) return kNoAlias;
+      if (b->IsInnerAllocatedObject()) return kNoAlias;
+      // An allocation can never alias a parameter or a constant.
+      if (b->IsParameter()) return kNoAlias;
+      if (b->IsConstant()) return kNoAlias;
+    }
+    if (b->IsAllocate() || b->IsInnerAllocatedObject()) {
+      // An allocation can never alias a parameter or a constant.
+      if (a->IsParameter()) return kNoAlias;
+      if (a->IsConstant()) return kNoAlias;
+    }
+
+    // TODO(titzer): return MustAlias for two equivalent constants.
+    return kMayAlias;
+  }
+
+  // Checks whether the objects referred to by the given instructions may
+  // ever be aliases. Note that this is more conservative than checking
+  // {Query(a, b) == kMayAlias}, since this method considers kMustAlias
+  // objects to also be may-aliasing.
+  inline bool MayAlias(HValue* a, HValue* b) {
+    return Query(a, b) != kNoAlias;
+  }
+
+  inline bool MustAlias(HValue* a, HValue* b) {
+    return Query(a, b) == kMayAlias;
+  }
+
+  inline bool NoAlias(HValue* a, HValue* b) {
+    return Query(a, b) == kNoAlias;
+  }
+
+  // Returns the actual value of an instruction. In the case of a chain
+  // of informative definitions, return the root of the chain.
+  HValue* ActualValue(HValue* obj) {

[Michael Starzinger, 2013/09/12 20:09:08]

Can we move this onto the HValue class? Pretty please! :)

+    while (obj->IsInformativeDefinition()) {  // Walk a chain of idefs.
+      obj = obj->RedefinedOperand();
+    }
+    return obj;
+  }
+};
+
+
+} }  // namespace v8::internal
+
+#endif  // V8_HYDROGEN_ALIAS_ANALYSIS_H_