Source/JavaScriptCore/b3/B3Effects.h

/*
 * Copyright (C) 2015-2017 Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. 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.
 *
 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``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 APPLE INC. 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
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 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 
 */

#pragma once

#if ENABLE(B3_JIT)

#include "B3HeapRange.h"
#include <wtf/PrintStream.h>

namespace JSC { namespace B3 {

struct Effects {
    // True if this cannot continue execution in the current block.
    bool terminal { false };

    // True if this value can cause execution to terminate abruptly, and that this abrupt termination is
    // observable. An example of how this gets used is to limit the hoisting of controlDependent values.
    // Note that if exitsSideways is set to true but reads is bottom, then B3 is free to assume that
    // after abrupt termination of this procedure, none of the heap will be read. That's usually false,
    // so make sure that reads corresponds to the set of things that are readable after this function
    // terminates abruptly.
    bool exitsSideways { false };

    // True if the instruction may change semantics if hoisted above some control flow. For example,
    // loads are usually control-dependent because we must assume that any control construct (either
    // a terminal like Branch or anything that exits sideways, like Check) validates whether the
    // pointer is valid. Hoisting the load above control may cause the load to trap even though it
    // would not have otherwise trapped.
    bool controlDependent { false };

    // True if this writes to the local state. Operations that write local state don't write to anything
    // in "memory" but they have a side-effect anyway. This is for modeling Upsilons, Sets, and Fences.
    // This is a way of saying: even though this operation is not a terminal, does not exit sideways,
    // and does not write to the heap, you still cannot kill this operation.
    bool writesLocalState { false };

    // True if this reads from the local state. This is only used for Phi and Get.
    bool readsLocalState { false };

    // B3 understands things about pinned registers. Therefore, it needs to know who reads them and
    // who writes them. We don't track this on a per-register basis because that would be harder and
    // we don't need it. Note that if you want to construct an immutable pinned register while also
    // having other pinned registers that are mutable, then you can use ArgumentReg. Also note that
    // nobody will stop you from making this get out-of-sync with your clobbered register sets in
    // Patchpoint. It's recommended that you err on the side of being conservative.
    // FIXME: Explore making these be RegisterSets. That's mainly hard because it would be awkward to
    // reconcile with StackmapValue's support for clobbered regs.
    // https://bugs.webkit.org/show_bug.cgi?id=163173
    bool readsPinned { false };
    bool writesPinned { false };
    
    // Memory fences cannot be reordered around each other regardless of their effects. This is flagged
    // if the operation is a memory fence.
    bool fence { false };
    
    // WARNING: The B3::hoistLoopInvariantValues() phase thinks that it understands this exhaustively. If you
    // add any new kinds of things that can be read or written, you should check that phase.

    HeapRange writes;
    HeapRange reads;
    
    static Effects none()
    {
        return Effects();
    }

    static Effects forCall()
    {
        Effects result;
        result.exitsSideways = true;
        result.controlDependent = true;
        result.writes = HeapRange::top();
        result.reads = HeapRange::top();
        result.readsPinned = true;
        result.writesPinned = true;
        result.fence = true;
        return result;
    }
    
    static Effects forCheck()
    {
        Effects result;
        result.exitsSideways = true;
        // The program could read anything after exiting, and it's on us to declare this.
        result.reads = HeapRange::top();
        return result;
    }

    bool mustExecute() const
    {
        return terminal || exitsSideways || writesLocalState || writes || writesPinned || fence;
    }

    // Returns true if reordering instructions with these respective effects would change program
    // behavior in an observable way.
    bool interferes(const Effects&) const;
    
    JS_EXPORT_PRIVATE bool operator==(const Effects&) const;
    JS_EXPORT_PRIVATE bool operator!=(const Effects&) const;

    JS_EXPORT_PRIVATE void dump(PrintStream& out) const;
};

} } // namespace JSC::B3

#endif // ENABLE(B3_JIT)