path: root/Source/JavaScriptCore/b3/B3Kind.h

/*
 * Copyright (C) 2016 Apple Inc. All rights reserved.
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 * modification, are permitted provided that the following conditions
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 *    notice, this list of conditions and the following disclaimer in the
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#ifndef B3Kind_h
#define B3Kind_h

#if ENABLE(B3_JIT)

#include "B3Opcode.h"
#include <wtf/HashTable.h>
#include <wtf/PrintStream.h>

namespace JSC { namespace B3 {

// A Kind is a terse summary of what a Value does. There is a fixed number of possible
// Kinds. Kind is a tuple of Opcode (see B3Opcode.h) and some extra bits. Most opcodes don't
// get any extra bits, and those bits must remain zero if the Kind's opcode field is set to
// one of those opcodes. The purpose of Kind is to be like an opcode in other IRs, but to
// be multidimensional. For example, a Load has many dimensions of customization that we may
// eventually implement. A Load can have different alignments, alignment failure modes,
// temporality modes, trapping modes, ordering modes, etc. It's fine to put such flags into
// subclasses of Value, but in some cases that would be overkill, particularly since if you
// did that for a pure value then you'd also have to thread it through ValueKey. It's much
// easier to put it in Kind, and then your extra bit will get carried around by everyone who
// knows how to carry around Kinds. Most importantly, putting flags into Kind allows you to
// use them as part of B3::Value's dynamic cast facility. For example we could have a
// trapping Load that uses a Value subclass that has a stackmap while non-trapping Loads
// continue to use the normal MemoryValue.
//
// Note that any code in the compiler that transcribes IR (like a strength reduction that
// replaces an Add with a different Add, or even with a different opcode entirely) will
// probably drop unknown bits by default. This is definitely not correct for many bits (like
// isChill for Div/Mod and all of the envisioned Load/Store flags), so if you add a new bit
// you will probably have to audit the compiler to make sure that phases that transcribe
// your opcode do the right thing with your bit.

class Kind {
public:
    Kind(Opcode opcode)
        : m_opcode(opcode)
        , m_isChill(false)
        , m_traps(false)
    {
    }
    
    Kind()
        : Kind(Oops)
    {
    }
    
    Opcode opcode() const { return m_opcode; }
    void setOpcode(Opcode opcode) { m_opcode = opcode; }
    
    bool hasExtraBits() const { return m_isChill || m_traps; }
    
    // Chill bit. This applies to division-based arithmetic ops, which may trap on some
    // platforms or exhibit bizarre behavior when passed certain inputs. The non-chill
    // version will behave as unpredictably as it wants. For example, it's legal to
    // constant-fold Div(x, 0) to any value or to replace it with any effectful operation.
    // But when it's chill, that means that the semantics when it would have trapped are
    // the JS semantics. For example, Div<Chill>(@a, @b) means:
    //
    //     ((a | 0) / (b | 0)) | 0
    //
    // And Mod<Chill>(a, b) means:
    //
    //     ((a | 0) % (b | 0)) | 0
    //
    // Note that Div<Chill> matches exactly how ARM handles integer division.
    bool hasIsChill() const
    {
        switch (m_opcode) {
        case Div:
        case Mod:
            return true;
        default:
            return false;
        }
    }
    bool isChill() const
    {
        return m_isChill;
    }
    void setIsChill(bool isChill)
    {
        ASSERT(hasIsChill());
        m_isChill = isChill;
    }
    
    // Traps bit. This applies to memory access ops. It means that the instruction could
    // trap as part of some check it performs, and that we mean to make this observable. This
    // currently only applies to memory accesses (loads and stores). You don't get to find out where
    // in the Procedure the trap happened. If you try to work it out using Origin, you'll have a bad
    // time because the instruction selector is too sloppy with Origin().
    // FIXME: https://bugs.webkit.org/show_bug.cgi?id=162688
    bool hasTraps() const
    {
        switch (m_opcode) {
        case Load8Z:
        case Load8S:
        case Load16Z:
        case Load16S:
        case Load:
        case Store8:
        case Store16:
        case Store:
            return true;
        default:
            return false;
        }
    }
    bool traps() const
    {
        return m_traps;
    }
    void setTraps(bool traps)
    {
        ASSERT(hasTraps());
        m_traps = traps;
    }
    
    // Rules for adding new properties:
    // - Put the accessors here.
    // - hasBlah() should check if the opcode allows for your property.
    // - blah() returns a default value if !hasBlah()
    // - setBlah() asserts if !hasBlah()
    // - Try not to increase the size of Kind too much. But it wouldn't be the end of the
    //   world if it bloated to 64 bits.
    
    bool operator==(const Kind& other) const
    {
        return m_opcode == other.m_opcode
            && m_isChill == other.m_isChill
            && m_traps == other.m_traps;
    }
    
    bool operator!=(const Kind& other) const
    {
        return !(*this == other);
    }
    
    void dump(PrintStream&) const;
    
    unsigned hash() const
    {
        // It's almost certainly more important that this hash function is cheap to compute than
        // anything else. We can live with some kind hash collisions.
        return m_opcode + (static_cast<unsigned>(m_isChill) << 16) + (static_cast<unsigned>(m_traps) << 7);
    }
    
    Kind(WTF::HashTableDeletedValueType)
        : m_opcode(Oops)
        , m_isChill(true)
        , m_traps(false)
    {
    }
    
    bool isHashTableDeletedValue() const
    {
        return *this == Kind(WTF::HashTableDeletedValue);
    }
    
private:
    Opcode m_opcode;
    bool m_isChill : 1;
    bool m_traps : 1;
};

// For every flag 'foo' you add, it's customary to create a Kind B3::foo(Kind) function that makes
// a kind with the flag set. For example, for chill, this lets us say:
//
//     block->appendNew<Value>(m_proc, chill(Mod), Origin(), a, b);
//
// I like to make the flag name fill in the sentence "Mod _____" (like "isChill" or "traps") while
// the flag constructor fills in the phrase "_____ Mod" (like "chill" or "trapping").

inline Kind chill(Kind kind)
{
    kind.setIsChill(true);
    return kind;
}

inline Kind trapping(Kind kind)
{
    kind.setTraps(true);
    return kind;
}

struct KindHash {
    static unsigned hash(const Kind& key) { return key.hash(); }
    static bool equal(const Kind& a, const Kind& b) { return a == b; }
    static const bool safeToCompareToEmptyOrDeleted = true;
};

} } // namespace JSC::B3

namespace WTF {

template<typename T> struct DefaultHash;
template<> struct DefaultHash<JSC::B3::Kind> {
    typedef JSC::B3::KindHash Hash;
};

template<typename T> struct HashTraits;
template<> struct HashTraits<JSC::B3::Kind> : public SimpleClassHashTraits<JSC::B3::Kind> {
    static const bool emptyValueIsZero = false;
};

} // namespace WTF

#endif // ENABLE(B3_JIT)

#endif // B3Kind_h