mirror of
https://github.com/darlinghq/darling-JavaScriptCore.git
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90 lines
4.2 KiB
C++
90 lines
4.2 KiB
C++
/*
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* Copyright (C) 2016 Apple Inc. All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
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* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
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* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
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* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
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* THE POSSIBILITY OF SUCH DAMAGE.
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*/
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#pragma once
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#if ENABLE(B3_JIT)
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#include "B3HeapRange.h"
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#include "B3Value.h"
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namespace JSC { namespace B3 {
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class JS_EXPORT_PRIVATE FenceValue final : public Value {
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public:
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static bool accepts(Kind kind) { return kind == Fence; }
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~FenceValue() final;
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// The read/write heaps are reflected in the effects() of this value. The compiler may change
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// the lowering of a Fence based on the heaps. For example, if a fence does not write anything
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// then it is understood to be a store-store fence. On x86, this may lead us to not emit any
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// code, while on ARM we may emit a cheaper fence (dmb ishst instead of dmb ish). We will do
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// the same optimization for load-load fences, which are expressed as a Fence that writes but
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// does not read.
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//
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// This abstraction allows us to cover all of the fences on x86 and all of the standalone fences
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// on ARM. X86 really just has one fence: mfence. This fence should be used to protect stores
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// from being sunk below loads. WTF calls it the storeLoadFence. A classic example is the Steele
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// barrier:
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//
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// o.f = v => o.f = v
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// if (color(o) == black)
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// log(o)
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//
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// We are trying to ensure that if the store to o.f occurs after the collector has started
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// visiting o, then we will log o. Under sequential consistency, this would work. The collector
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// would set color(o) to black just before it started visiting. But x86's illusion of sequential
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// consistency is broken in exactly just this store->load ordering case. The store to o.f may
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// get buffered, and it may occur some time after we have loaded and checked color(o). As well,
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// the collector's store to set color(o) to black may get buffered and it may occur some time
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// after the collector has finished visiting o. Therefore, we need mfences. In B3 we model this
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// as a Fence that reads and writes some heaps. Setting writes to the empty set will cause B3 to
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// not emit any barrier on x86.
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//
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// On ARM there are many more fences. The Fence instruction is meant to model just two of them:
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// dmb ish and dmb ishst. You can emit a dmb ishst by using a Fence with an empty write heap.
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// Otherwise, you will get a dmb ish.
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// FIXME: Add fenced memory accesses. https://bugs.webkit.org/show_bug.cgi?id=162349
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// FIXME: Add a Depend operation. https://bugs.webkit.org/show_bug.cgi?id=162350
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HeapRange read { HeapRange::top() };
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HeapRange write { HeapRange::top() };
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B3_SPECIALIZE_VALUE_FOR_NO_CHILDREN
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private:
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friend class Procedure;
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friend class Value;
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static Opcode opcodeFromConstructor(Origin, HeapRange = HeapRange(), HeapRange = HeapRange()) { return Fence; }
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FenceValue(Origin origin, HeapRange read, HeapRange write);
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FenceValue(Origin origin);
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};
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} } // namespace JSC::B3
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#endif // ENABLE(B3_JIT)
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