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/*
* Copyright (C) 2016 Apple Inc. All rights reserved.
*
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* modification, are permitted provided that the following conditions
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* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
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*
* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
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#pragma once
#if ENABLE(DFG_JIT)
#include "DFGBackwardsDominators.h"
#include "DFGDominators.h"
namespace JSC { namespace DFG {
class ControlEquivalenceAnalysis {
WTF_MAKE_NONCOPYABLE(ControlEquivalenceAnalysis);
WTF_MAKE_FAST_ALLOCATED;
public:
ControlEquivalenceAnalysis(Graph& graph)
: m_dominators(graph.ensureSSADominators())
, m_backwardsDominators(graph.ensureBackwardsDominators())
{
}
// This returns true iff:
//
// - If b executes then a must have executed before it (a dominates b).
// - If a executes then b will execute after it (b backwards-dominates a).
//
// Note that like Dominators and BackwardsDominators, this analysis ignores OSR:
//
// - This may return true even if we OSR enter in beteen a and b. OSR entry would mean that b
// could execute even if a had not executed. This is impossible in DFG SSA but it's possible
// in DFG CPS.
// - This may return true even if we OSR exit in between a and b. OSR exit would mean that a
// could execute even though b will not execute. This is possible in all forms of DFG IR.
//
// In DFG SSA you only have to worry about the definition being weaked by exits. This is usually
// OK, since we use this analysis to determine the cost of moving exits from one block to
// another. If we move an exit from b to a and a equivalently dominates b then at worst we have
// made the exit happen sooner. If we move an exit from b to a and a dominates b but not
// equivalently then we've done something much worse: the program may now exit even if it would
// not have ever exited before.
bool dominatesEquivalently(BasicBlock* a, BasicBlock* b)
{
return m_dominators.dominates(a, b)
&& m_backwardsDominators.dominates(b, a);
}
// This returns true iff the execution of a implies that b also executes and vice-versa.
bool areEquivalent(BasicBlock* a, BasicBlock* b)
{
return dominatesEquivalently(a, b)
|| dominatesEquivalently(b, a);
}
private:
SSADominators& m_dominators;
BackwardsDominators& m_backwardsDominators;
};
} } // namespace JSC::DFG
#endif // ENABLE(DFG_JIT)