/* * Copyright (C) 2016 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. AND ITS 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 APPLE INC. OR ITS 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. */ #pragma once #if ENABLE(B3_JIT) #include "B3HeapRange.h" #include "B3Value.h" namespace JSC { namespace B3 { class JS_EXPORT_PRIVATE FenceValue : public Value { public: static bool accepts(Kind kind) { return kind == Fence; } ~FenceValue(); // The read/write heaps are reflected in the effects() of this value. The compiler may change // the lowering of a Fence based on the heaps. For example, if a fence does not write anything // then it is understood to be a store-store fence. On x86, this may lead us to not emit any // code, while on ARM we may emit a cheaper fence (dmb ishst instead of dmb ish). We will do // the same optimization for load-load fences, which are expressed as a Fence that writes but // does not read. // // This abstraction allows us to cover all of the fences on x86 and all of the standalone fences // on ARM. X86 really just has one fence: mfence. This fence should be used to protect stores // from being sunk below loads. WTF calls it the storeLoadFence. A classic example is the Steele // barrier: // // o.f = v => o.f = v // if (color(o) == black) // log(o) // // We are trying to ensure that if the store to o.f occurs after the collector has started // visiting o, then we will log o. Under sequential consistency, this would work. The collector // would set color(o) to black just before it started visiting. But x86's illusion of sequential // consistency is broken in exactly just this store->load ordering case. The store to o.f may // get buffered, and it may occur some time after we have loaded and checked color(o). As well, // the collector's store to set color(o) to black may get buffered and it may occur some time // after the collector has finished visiting o. Therefore, we need mfences. In B3 we model this // as a Fence that reads and writes some heaps. Setting writes to the empty set will cause B3 to // not emit any barrier on x86. // // On ARM there are many more fences. The Fence instruction is meant to model just two of them: // dmb ish and dmb ishst. You can emit a dmb ishst by using a Fence with an empty write heap. // Otherwise, you will get a dmb ish. // FIXME: Add fenced memory accesses. https://bugs.webkit.org/show_bug.cgi?id=162349 // FIXME: Add a Depend operation. https://bugs.webkit.org/show_bug.cgi?id=162350 HeapRange read { HeapRange::top() }; HeapRange write { HeapRange::top() }; protected: Value* cloneImpl() const override; private: friend class Procedure; FenceValue(Origin origin, HeapRange read, HeapRange write); FenceValue(Origin origin); }; } } // namespace JSC::B3 #endif // ENABLE(B3_JIT)