gecko/js/src/jsanalyze.h

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 *
 * The Original Code is the Mozilla SpiderMonkey bytecode analysis
 *
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/* Definitions for javascript analysis. */

#ifndef jsanalyze_h___
#define jsanalyze_h___

#include "jsarena.h"
#include "jscntxt.h"
#include "jsinfer.h"
#include "jsscript.h"

struct JSScript;

/* Forward declaration of downstream register allocations computed for join points. */
namespace js { namespace mjit { struct RegisterAllocation; } }

namespace js {
namespace analyze {

class Script;

/* Information about a bytecode instruction. */
struct Bytecode
{
    friend class Script;

    /* Whether there are any incoming jumps to this instruction. */
    bool jumpTarget : 1;    

    /* Whether there is fallthrough to this instruction from a non-branching instruction. */
    bool fallthrough : 1;

    /* Whether this instruction is the fall through point of a conditional jump. */
    bool jumpFallthrough : 1;

    /* Whether this instruction can be branched to from a switch statement.  Implies jumpTarget. */
    bool switchTarget : 1;

    /* Whether this instruction has been analyzed to get its output defines and stack. */
    bool analyzed : 1;

    /* Whether this is a catch/finally entry point. */
    bool exceptionEntry : 1;

    /* Whether this is in a try block. */
    bool inTryBlock : 1;

    /* Method JIT safe point. */
    bool safePoint : 1;

    /* Stack depth before this opcode. */
    uint32 stackDepth;

    /*
     * The set of locals defined at this point.  This does not include locals which
     * were unconditionally defined at an earlier point in the script.
     */
    uint32 defineCount;
    uint32 *defineArray;

    Bytecode(Script *script, unsigned offset)
    {
        PodZero(this);
    }

  private:
    bool mergeDefines(JSContext *cx, Script *script, bool initial,
                      uint32 newDepth, uint32 *newArray, uint32 newCount);

    /* Whether a local variable is in the define set at this bytecode. */
    bool isDefined(uint32 slot)
    {
        JS_ASSERT(analyzed);
        for (unsigned ind = 0; ind < defineCount; ind++) {
            if (defineArray[ind] == slot)
                return true;
        }
        return false;
    }
};

/* Information about a script. */
class Script
{
    friend struct Bytecode;

    JSScript *script;

    Bytecode **codeArray;

    /* Maximum number of locals to consider for a script. */
    static const unsigned LOCAL_LIMIT = 50;

    /* Offsets at which each local becomes unconditionally defined, or a value below. */
    uint32 *locals;

    static const uint32 LOCAL_USE_BEFORE_DEF = uint32(-1);
    static const uint32 LOCAL_CONDITIONALLY_DEFINED = uint32(-2);

    bool outOfMemory;
    bool hadFailure;
    bool usesRval;
    bool usesScope;

  public:
    /* Pool for allocating analysis structures which will not outlive this script. */
    JSArenaPool pool;

    Script();
    ~Script();

    void analyze(JSContext *cx, JSScript *script);

    bool OOM() { return outOfMemory; }
    bool failed() { return hadFailure; }

    /* Whether there are POPV/SETRVAL bytecodes which can write to the frame's rval. */
    bool usesReturnValue() const { return usesRval; }

    /* Whether there are NAME bytecodes which can access the frame's scope chain. */
    bool usesScopeChain() const { return usesScope; }

    bool hasAnalyzed() const { return !!codeArray; }
    JSScript *getScript() const { return script; }

    /* Accessors for bytecode information. */

    Bytecode& getCode(uint32 offset) {
        JS_ASSERT(offset < script->length);
        JS_ASSERT(codeArray[offset]);
        return *codeArray[offset];
    }
    Bytecode& getCode(const jsbytecode *pc) { return getCode(pc - script->code); }

    Bytecode* maybeCode(uint32 offset) {
        JS_ASSERT(offset < script->length);
        return codeArray[offset];
    }
    Bytecode* maybeCode(const jsbytecode *pc) { return maybeCode(pc - script->code); }

    bool jumpTarget(uint32 offset) {
        JS_ASSERT(offset < script->length);
        return codeArray[offset] && codeArray[offset]->jumpTarget;
    }
    bool jumpTarget(const jsbytecode *pc) { return jumpTarget(pc - script->code); }

    /* Accessors for local variable information. */

    unsigned localCount() {
        return (script->nfixed >= LOCAL_LIMIT) ? LOCAL_LIMIT : script->nfixed;
    }

    bool localHasUseBeforeDef(uint32 local) {
        JS_ASSERT(!failed());
        return local >= localCount() || locals[local] == LOCAL_USE_BEFORE_DEF;
    }

    /* These return true for variables that may have a use before def. */
    bool localDefined(uint32 local, uint32 offset) {
        return localHasUseBeforeDef(local) || (locals[local] <= offset) ||
            getCode(offset).isDefined(local);
    }
    bool localDefined(uint32 local, jsbytecode *pc) {
        return localDefined(local, pc - script->code);
    }

    bool argEscapes(unsigned arg)
    {
        for (unsigned i = 0; i < script->nClosedArgs; i++) {
            if (arg == script->getClosedArg(i))
                return true;
        }
        return false;
    }

    bool localEscapes(unsigned local)
    {
        if (local >= localCount())
            return true;
        for (unsigned i = 0; i < script->nClosedVars; i++) {
            if (local == script->getClosedVar(i))
                return true;
        }
        return false;
    }

  private:
    void setOOM(JSContext *cx) {
        if (!outOfMemory)
            js_ReportOutOfMemory(cx);
        outOfMemory = true;
        hadFailure = true;
    }

    inline bool addJump(JSContext *cx, unsigned offset,
                        unsigned *currentOffset, unsigned *forwardJump,
                        unsigned stackDepth, uint32 *defineArray, unsigned defineCount);

    inline void setLocal(uint32 local, uint32 offset);
};

static inline unsigned
GetBytecodeLength(jsbytecode *pc)
{
    JSOp op = (JSOp)*pc;
    JS_ASSERT(op < JSOP_LIMIT);
    JS_ASSERT(op != JSOP_TRAP);

    if (js_CodeSpec[op].length != -1)
        return js_CodeSpec[op].length;
    return js_GetVariableBytecodeLength(pc);
}

static inline unsigned
GetUseCount(JSScript *script, unsigned offset)
{
    JS_ASSERT(offset < script->length);
    jsbytecode *pc = script->code + offset;
    if (js_CodeSpec[*pc].nuses == -1)
        return js_GetVariableStackUses(JSOp(*pc), pc);
    return js_CodeSpec[*pc].nuses;
}

static inline unsigned
GetDefCount(JSScript *script, unsigned offset)
{
    JS_ASSERT(offset < script->length);
    jsbytecode *pc = script->code + offset;
    if (js_CodeSpec[*pc].ndefs == -1)
        return js_GetEnterBlockStackDefs(NULL, script, pc);

    /*
     * Add an extra pushed value for OR/AND opcodes, so that they are included
     * in the pushed array of stack values for type inference.
     */
    switch (JSOp(*pc)) {
      case JSOP_OR:
      case JSOP_ORX:
      case JSOP_AND:
      case JSOP_ANDX:
        return 1;
      case JSOP_FILTER:
        return 2;
      default:
        return js_CodeSpec[*pc].ndefs;
    }
}

/* Untrap a single PC, and retrap it at scope exit. */
struct UntrapOpcode
{
    jsbytecode *pc;
    bool trap;

    UntrapOpcode(JSContext *cx, JSScript *script, jsbytecode *pc)
        : pc(pc), trap(JSOp(*pc) == JSOP_TRAP)
    {
        if (trap)
            *pc = JS_GetTrapOpcode(cx, script, pc);
    }

    ~UntrapOpcode()
    {
        if (trap)
            *pc = JSOP_TRAP;
    }
};

/*
 * Lifetime analysis. The goal of this analysis is to make a single backwards pass
 * over a script to approximate the regions where each variable is live, without
 * doing a full fixpointing live-variables pass. This is based on the algorithm
 * described in:
 *
 * "Quality and Speed in Linear-scan Register Allocation"
 * Traub et. al.
 * PLDI, 1998
 */

/*
 * Information about the lifetime of a local or argument. These form a linked list
 * describing successive intervals in the program where the variable's value
 * may be live. At points in the script not in one of these segments (points in
 * a 'lifetime hole'), the variable is dead and registers containing its type/payload
 * can be discarded without needing to be synced.
 */
struct Lifetime
{
    /*
     * Start and end offsets of this lifetime. The variable is live at the beginning
     * of every bytecode in this (inclusive) range.
     */
    uint32 start;
    uint32 end;

    /*
     * This is an artificial segment extending the lifetime of a variable to the
     * end of a loop, when it is live at the head of the loop. It will not be used
     * anymore in the loop body until the next iteration.
     */
    bool loopTail;

    /* Next lifetime. The variable is dead from this->end to next->start. */
    Lifetime *next;

    Lifetime(uint32 offset, Lifetime *next)
        : start(offset), end(offset), loopTail(false), next(next)
    {}
};

/* Lifetime and register information for a bytecode. */
struct LifetimeBytecode
{
    /* If this is a loop head, offset of the loop back edge. */
    uint32 loopBackedge;

    /* Any allocation computed downstream for this bytecode. */
    mjit::RegisterAllocation *allocation;
};

/* Current lifetime information for a variable. */
struct LifetimeVariable
{
    /* If the variable is currently live, the lifetime segment. */
    Lifetime *lifetime;

    /* If the variable is currently dead, the next live segment. */
    Lifetime *saved;

    /* Jump preceding the basic block which killed this variable. */
    uint32 savedEnd;

    Lifetime * live(uint32 offset) {
        if (lifetime && lifetime->end >= offset)
            return lifetime;
        Lifetime *segment = lifetime ? lifetime : saved;
        while (segment && segment->start <= offset) {
            if (segment->end >= offset)
                return segment;
            segment = segment->next;
        }
        return NULL;
    }
};

/*
 * Analysis approximating variable liveness information at points in a script.
 * This is separate from analyze::Script as it is computed on every compilation
 * and thrown away afterwards.
 */
class LifetimeScript
{
    analyze::Script *analysis;
    JSScript *script;
    JSFunction *fun;

    LifetimeBytecode *codeArray;
    LifetimeVariable *locals;
    LifetimeVariable *args;
    LifetimeVariable thisVar;

    LifetimeVariable **saved;
    unsigned savedCount;

  public:
    JSArenaPool pool;

    LifetimeScript();
    ~LifetimeScript();

    bool analyze(JSContext *cx, analyze::Script *analysis, JSScript *script, JSFunction *fun);

    LifetimeBytecode &getCode(uint32 offset) {
        JS_ASSERT(analysis->maybeCode(offset));
        return codeArray[offset];
    }
    LifetimeBytecode &getCode(jsbytecode *pc) { return getCode(pc - script->code); }

#ifdef DEBUG
    void dumpVariable(LifetimeVariable &var);
    void dumpLocal(unsigned i) { dumpVariable(locals[i]); }
    void dumpArg(unsigned i) { dumpVariable(args[i]); }
#endif

    Lifetime * argLive(uint32 arg, uint32 offset) { return args[arg].live(offset); }
    Lifetime * localLive(uint32 local, uint32 offset) { return locals[local].live(offset); }
    Lifetime * thisLive(uint32 offset) { return thisVar.live(offset); }

  private:

    inline bool addVariable(JSContext *cx, LifetimeVariable &var, unsigned offset);
    inline void killVariable(JSContext *cx, LifetimeVariable &var, unsigned offset);
    inline bool extendVariable(JSContext *cx, LifetimeVariable &var, unsigned start, unsigned end);
};

} /* namespace analyze */
} /* namespace js */

#endif // jsanalyze_h___