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python3.2  3.2.2
Defines | Functions
peephole.c File Reference
#include "Python.h"
#include "Python-ast.h"
#include "node.h"
#include "ast.h"
#include "code.h"
#include "symtable.h"
#include "opcode.h"

Go to the source code of this file.

Defines

#define GETARG(arr, i)   ((int)((arr[i+2]<<8) + arr[i+1]))
#define UNCONDITIONAL_JUMP(op)   (op==JUMP_ABSOLUTE || op==JUMP_FORWARD)
#define CONDITIONAL_JUMP(op)
#define ABSOLUTE_JUMP(op)
#define JUMPS_ON_TRUE(op)   (op==POP_JUMP_IF_TRUE || op==JUMP_IF_TRUE_OR_POP)
#define GETJUMPTGT(arr, i)   (GETARG(arr,i) + (ABSOLUTE_JUMP(arr[i]) ? 0 : i+3))
#define SETARG(arr, i, val)   arr[i+2] = val>>8; arr[i+1] = val & 255
#define CODESIZE(op)   (HAS_ARG(op) ? 3 : 1)
#define ISBASICBLOCK(blocks, start, bytes)   (blocks[start]==blocks[start+bytes-1])

Functions

static int tuple_of_constants (unsigned char *codestr, Py_ssize_t n, PyObject *consts)
static int fold_binops_on_constants (unsigned char *codestr, PyObject *consts)
static int fold_unaryops_on_constants (unsigned char *codestr, PyObject *consts)
static unsigned intmarkblocks (unsigned char *code, Py_ssize_t len)
static int load_global (unsigned char *codestr, Py_ssize_t i, char *name, PyObject *consts)
PyObjectPyCode_Optimize (PyObject *code, PyObject *consts, PyObject *names, PyObject *lineno_obj)

Define Documentation

#define ABSOLUTE_JUMP (   op)
#define CODESIZE (   op)    (HAS_ARG(op) ? 3 : 1)

Definition at line 22 of file peephole.c.

#define CONDITIONAL_JUMP (   op)
Value:

Definition at line 14 of file peephole.c.

#define GETARG (   arr,
  i 
)    ((int)((arr[i+2]<<8) + arr[i+1]))

Definition at line 12 of file peephole.c.

#define GETJUMPTGT (   arr,
  i 
)    (GETARG(arr,i) + (ABSOLUTE_JUMP(arr[i]) ? 0 : i+3))

Definition at line 20 of file peephole.c.

#define ISBASICBLOCK (   blocks,
  start,
  bytes 
)    (blocks[start]==blocks[start+bytes-1])

Definition at line 23 of file peephole.c.

Definition at line 19 of file peephole.c.

#define SETARG (   arr,
  i,
  val 
)    arr[i+2] = val>>8; arr[i+1] = val & 255

Definition at line 21 of file peephole.c.

#define UNCONDITIONAL_JUMP (   op)    (op==JUMP_ABSOLUTE || op==JUMP_FORWARD)

Definition at line 13 of file peephole.c.


Function Documentation

static int fold_binops_on_constants ( unsigned char *  codestr,
PyObject consts 
) [static]

Definition at line 97 of file peephole.c.

{
    PyObject *newconst, *v, *w;
    Py_ssize_t len_consts, size;
    int opcode;

    /* Pre-conditions */
    assert(PyList_CheckExact(consts));
    assert(codestr[0] == LOAD_CONST);
    assert(codestr[3] == LOAD_CONST);

    /* Create new constant */
    v = PyList_GET_ITEM(consts, GETARG(codestr, 0));
    w = PyList_GET_ITEM(consts, GETARG(codestr, 3));
    opcode = codestr[6];
    switch (opcode) {
        case BINARY_POWER:
            newconst = PyNumber_Power(v, w, Py_None);
            break;
        case BINARY_MULTIPLY:
            newconst = PyNumber_Multiply(v, w);
            break;
        case BINARY_TRUE_DIVIDE:
            newconst = PyNumber_TrueDivide(v, w);
            break;
        case BINARY_FLOOR_DIVIDE:
            newconst = PyNumber_FloorDivide(v, w);
            break;
        case BINARY_MODULO:
            newconst = PyNumber_Remainder(v, w);
            break;
        case BINARY_ADD:
            newconst = PyNumber_Add(v, w);
            break;
        case BINARY_SUBTRACT:
            newconst = PyNumber_Subtract(v, w);
            break;
        case BINARY_SUBSCR:
            newconst = PyObject_GetItem(v, w);
            /* #5057: if v is unicode, there might be differences between
               wide and narrow builds in cases like '\U00012345'[0].
               Wide builds will return a non-BMP char, whereas narrow builds
               will return a surrogate.  In both the cases skip the
               optimization in order to produce compatible pycs.
             */
            if (newconst != NULL &&
                PyUnicode_Check(v) && PyUnicode_Check(newconst)) {
                Py_UNICODE ch = PyUnicode_AS_UNICODE(newconst)[0];
#ifdef Py_UNICODE_WIDE
                if (ch > 0xFFFF) {
#else
                if (ch >= 0xD800 && ch <= 0xDFFF) {
#endif
                    Py_DECREF(newconst);
                    return 0;
                }
            }
            break;
        case BINARY_LSHIFT:
            newconst = PyNumber_Lshift(v, w);
            break;
        case BINARY_RSHIFT:
            newconst = PyNumber_Rshift(v, w);
            break;
        case BINARY_AND:
            newconst = PyNumber_And(v, w);
            break;
        case BINARY_XOR:
            newconst = PyNumber_Xor(v, w);
            break;
        case BINARY_OR:
            newconst = PyNumber_Or(v, w);
            break;
        default:
            /* Called with an unknown opcode */
            PyErr_Format(PyExc_SystemError,
                 "unexpected binary operation %d on a constant",
                     opcode);
            return 0;
    }
    if (newconst == NULL) {
        if(!PyErr_ExceptionMatches(PyExc_KeyboardInterrupt))
            PyErr_Clear();
        return 0;
    }
    size = PyObject_Size(newconst);
    if (size == -1) {
        if (PyErr_ExceptionMatches(PyExc_KeyboardInterrupt))
            return 0;
        PyErr_Clear();
    } else if (size > 20) {
        Py_DECREF(newconst);
        return 0;
    }

    /* Append folded constant into consts table */
    len_consts = PyList_GET_SIZE(consts);
    if (PyList_Append(consts, newconst)) {
        Py_DECREF(newconst);
        return 0;
    }
    Py_DECREF(newconst);

    /* Write NOP NOP NOP NOP LOAD_CONST newconst */
    memset(codestr, NOP, 4);
    codestr[4] = LOAD_CONST;
    SETARG(codestr, 4, len_consts);
    return 1;
}

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static int fold_unaryops_on_constants ( unsigned char *  codestr,
PyObject consts 
) [static]

Definition at line 208 of file peephole.c.

{
    PyObject *newconst=NULL, *v;
    Py_ssize_t len_consts;
    int opcode;

    /* Pre-conditions */
    assert(PyList_CheckExact(consts));
    assert(codestr[0] == LOAD_CONST);

    /* Create new constant */
    v = PyList_GET_ITEM(consts, GETARG(codestr, 0));
    opcode = codestr[3];
    switch (opcode) {
        case UNARY_NEGATIVE:
            /* Preserve the sign of -0.0 */
            if (PyObject_IsTrue(v) == 1)
                newconst = PyNumber_Negative(v);
            break;
        case UNARY_INVERT:
            newconst = PyNumber_Invert(v);
            break;
        case UNARY_POSITIVE:
            newconst = PyNumber_Positive(v);
            break;
        default:
            /* Called with an unknown opcode */
            PyErr_Format(PyExc_SystemError,
                 "unexpected unary operation %d on a constant",
                     opcode);
            return 0;
    }
    if (newconst == NULL) {
        if(!PyErr_ExceptionMatches(PyExc_KeyboardInterrupt))
            PyErr_Clear();
        return 0;
    }

    /* Append folded constant into consts table */
    len_consts = PyList_GET_SIZE(consts);
    if (PyList_Append(consts, newconst)) {
        Py_DECREF(newconst);
        return 0;
    }
    Py_DECREF(newconst);

    /* Write NOP LOAD_CONST newconst */
    codestr[0] = NOP;
    codestr[1] = LOAD_CONST;
    SETARG(codestr, 1, len_consts);
    return 1;
}

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static int load_global ( unsigned char *  codestr,
Py_ssize_t  i,
char *  name,
PyObject consts 
) [static]

Definition at line 305 of file peephole.c.

{
    Py_ssize_t j;
    PyObject *obj;
    if (name == NULL)
        return 0;
    if (strcmp(name, "None") == 0)
        obj = Py_None;
    else if (strcmp(name, "True") == 0)
        obj = Py_True;
    else if (strcmp(name, "False") == 0)
        obj = Py_False;
    else
        return 0;
    for (j = 0; j < PyList_GET_SIZE(consts); j++) {
        if (PyList_GET_ITEM(consts, j) == obj)
            break;
    }
    if (j == PyList_GET_SIZE(consts)) {
        if (PyList_Append(consts, obj) < 0)
            return -1;
    }
    assert(PyList_GET_ITEM(consts, j) == obj);
    codestr[i] = LOAD_CONST;
    SETARG(codestr, i, j);
    return 1;
}

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static unsigned int* markblocks ( unsigned char *  code,
Py_ssize_t  len 
) [static]

Definition at line 262 of file peephole.c.

{
    unsigned int *blocks = (unsigned int *)PyMem_Malloc(len*sizeof(int));
    int i,j, opcode, blockcnt = 0;

    if (blocks == NULL) {
        PyErr_NoMemory();
        return NULL;
    }
    memset(blocks, 0, len*sizeof(int));

    /* Mark labels in the first pass */
    for (i=0 ; i<len ; i+=CODESIZE(opcode)) {
        opcode = code[i];
        switch (opcode) {
            case FOR_ITER:
            case JUMP_FORWARD:
            case JUMP_IF_FALSE_OR_POP:
            case JUMP_IF_TRUE_OR_POP:
            case POP_JUMP_IF_FALSE:
            case POP_JUMP_IF_TRUE:
            case JUMP_ABSOLUTE:
            case CONTINUE_LOOP:
            case SETUP_LOOP:
            case SETUP_EXCEPT:
            case SETUP_FINALLY:
            case SETUP_WITH:
                j = GETJUMPTGT(code, i);
                blocks[j] = 1;
                break;
        }
    }
    /* Build block numbers in the second pass */
    for (i=0 ; i<len ; i++) {
        blockcnt += blocks[i];          /* increment blockcnt over labels */
        blocks[i] = blockcnt;
    }
    return blocks;
}

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PyObject* PyCode_Optimize ( PyObject code,
PyObject consts,
PyObject names,
PyObject lineno_obj 
)

Definition at line 351 of file peephole.c.

{
    Py_ssize_t i, j, codelen;
    int nops, h, adj;
    int tgt, tgttgt, opcode;
    unsigned char *codestr = NULL;
    unsigned char *lineno;
    int *addrmap = NULL;
    int new_line, cum_orig_line, last_line, tabsiz;
    int cumlc=0, lastlc=0;      /* Count runs of consecutive LOAD_CONSTs */
    unsigned int *blocks = NULL;
    char *name;

    /* Bail out if an exception is set */
    if (PyErr_Occurred())
        goto exitError;

    /* Bypass optimization when the lineno table is too complex */
    assert(PyBytes_Check(lineno_obj));
    lineno = (unsigned char*)PyBytes_AS_STRING(lineno_obj);
    tabsiz = PyBytes_GET_SIZE(lineno_obj);
    if (memchr(lineno, 255, tabsiz) != NULL)
        goto exitUnchanged;

    /* Avoid situations where jump retargeting could overflow */
    assert(PyBytes_Check(code));
    codelen = PyBytes_GET_SIZE(code);
    if (codelen > 32700)
        goto exitUnchanged;

    /* Make a modifiable copy of the code string */
    codestr = (unsigned char *)PyMem_Malloc(codelen);
    if (codestr == NULL)
        goto exitError;
    codestr = (unsigned char *)memcpy(codestr,
                                      PyBytes_AS_STRING(code), codelen);

    /* Verify that RETURN_VALUE terminates the codestring.      This allows
       the various transformation patterns to look ahead several
       instructions without additional checks to make sure they are not
       looking beyond the end of the code string.
    */
    if (codestr[codelen-1] != RETURN_VALUE)
        goto exitUnchanged;

    /* Mapping to new jump targets after NOPs are removed */
    addrmap = (int *)PyMem_Malloc(codelen * sizeof(int));
    if (addrmap == NULL)
        goto exitError;

    blocks = markblocks(codestr, codelen);
    if (blocks == NULL)
        goto exitError;
    assert(PyList_Check(consts));

    for (i=0 ; i<codelen ; i += CODESIZE(codestr[i])) {
      reoptimize_current:
        opcode = codestr[i];

        lastlc = cumlc;
        cumlc = 0;

        switch (opcode) {
            /* Replace UNARY_NOT POP_JUMP_IF_FALSE
               with    POP_JUMP_IF_TRUE */
            case UNARY_NOT:
                if (codestr[i+1] != POP_JUMP_IF_FALSE
                    || !ISBASICBLOCK(blocks,i,4))
                    continue;
                j = GETARG(codestr, i+1);
                codestr[i] = POP_JUMP_IF_TRUE;
                SETARG(codestr, i, j);
                codestr[i+3] = NOP;
                goto reoptimize_current;

                /* not a is b -->  a is not b
                   not a in b -->  a not in b
                   not a is not b -->  a is b
                   not a not in b -->  a in b
                */
            case COMPARE_OP:
                j = GETARG(codestr, i);
                if (j < 6  ||  j > 9  ||
                    codestr[i+3] != UNARY_NOT  ||
                    !ISBASICBLOCK(blocks,i,4))
                    continue;
                SETARG(codestr, i, (j^1));
                codestr[i+3] = NOP;
                break;

                /* Replace LOAD_GLOBAL/LOAD_NAME None/True/False
                   with LOAD_CONST None/True/False */
            case LOAD_NAME:
            case LOAD_GLOBAL:
                j = GETARG(codestr, i);
                name = _PyUnicode_AsString(PyTuple_GET_ITEM(names, j));
                h = load_global(codestr, i, name, consts);
                if (h < 0)
                    goto exitError;
                else if (h == 0)
                    continue;
                cumlc = lastlc + 1;
                break;

                /* Skip over LOAD_CONST trueconst
                   POP_JUMP_IF_FALSE xx. This improves
                   "while 1" performance. */
            case LOAD_CONST:
                cumlc = lastlc + 1;
                j = GETARG(codestr, i);
                if (codestr[i+3] != POP_JUMP_IF_FALSE  ||
                    !ISBASICBLOCK(blocks,i,6)  ||
                    !PyObject_IsTrue(PyList_GET_ITEM(consts, j)))
                    continue;
                memset(codestr+i, NOP, 6);
                cumlc = 0;
                break;

                /* Try to fold tuples of constants (includes a case for lists and sets
                   which are only used for "in" and "not in" tests).
                   Skip over BUILD_SEQN 1 UNPACK_SEQN 1.
                   Replace BUILD_SEQN 2 UNPACK_SEQN 2 with ROT2.
                   Replace BUILD_SEQN 3 UNPACK_SEQN 3 with ROT3 ROT2. */
            case BUILD_TUPLE:
            case BUILD_LIST:
            case BUILD_SET:
                j = GETARG(codestr, i);
                h = i - 3 * j;
                if (h >= 0  &&
                    j <= lastlc                  &&
                    ((opcode == BUILD_TUPLE &&
                      ISBASICBLOCK(blocks, h, 3*(j+1))) ||
                     ((opcode == BUILD_LIST || opcode == BUILD_SET) &&
                      codestr[i+3]==COMPARE_OP &&
                      ISBASICBLOCK(blocks, h, 3*(j+2)) &&
                      (GETARG(codestr,i+3)==6 ||
                       GETARG(codestr,i+3)==7))) &&
                    tuple_of_constants(&codestr[h], j, consts)) {
                    assert(codestr[i] == LOAD_CONST);
                    cumlc = 1;
                    break;
                }
                if (codestr[i+3] != UNPACK_SEQUENCE  ||
                    !ISBASICBLOCK(blocks,i,6) ||
                    j != GETARG(codestr, i+3) ||
                    opcode == BUILD_SET)
                    continue;
                if (j == 1) {
                    memset(codestr+i, NOP, 6);
                } else if (j == 2) {
                    codestr[i] = ROT_TWO;
                    memset(codestr+i+1, NOP, 5);
                } else if (j == 3) {
                    codestr[i] = ROT_THREE;
                    codestr[i+1] = ROT_TWO;
                    memset(codestr+i+2, NOP, 4);
                }
                break;

                /* Fold binary ops on constants.
                   LOAD_CONST c1 LOAD_CONST c2 BINOP -->  LOAD_CONST binop(c1,c2) */
            case BINARY_POWER:
            case BINARY_MULTIPLY:
            case BINARY_TRUE_DIVIDE:
            case BINARY_FLOOR_DIVIDE:
            case BINARY_MODULO:
            case BINARY_ADD:
            case BINARY_SUBTRACT:
            case BINARY_SUBSCR:
            case BINARY_LSHIFT:
            case BINARY_RSHIFT:
            case BINARY_AND:
            case BINARY_XOR:
            case BINARY_OR:
                if (lastlc >= 2                  &&
                    ISBASICBLOCK(blocks, i-6, 7)  &&
                    fold_binops_on_constants(&codestr[i-6], consts)) {
                    i -= 2;
                    assert(codestr[i] == LOAD_CONST);
                    cumlc = 1;
                }
                break;

                /* Fold unary ops on constants.
                   LOAD_CONST c1  UNARY_OP -->                  LOAD_CONST unary_op(c) */
            case UNARY_NEGATIVE:
            case UNARY_INVERT:
            case UNARY_POSITIVE:
                if (lastlc >= 1                  &&
                    ISBASICBLOCK(blocks, i-3, 4)  &&
                    fold_unaryops_on_constants(&codestr[i-3], consts))                  {
                    i -= 2;
                    assert(codestr[i] == LOAD_CONST);
                    cumlc = 1;
                }
                break;

                /* Simplify conditional jump to conditional jump where the
                   result of the first test implies the success of a similar
                   test or the failure of the opposite test.
                   Arises in code like:
                   "if a and b:"
                   "if a or b:"
                   "a and b or c"
                   "(a and b) and c"
                   x:JUMP_IF_FALSE_OR_POP y   y:JUMP_IF_FALSE_OR_POP z
                      -->  x:JUMP_IF_FALSE_OR_POP z
                   x:JUMP_IF_FALSE_OR_POP y   y:JUMP_IF_TRUE_OR_POP z
                      -->  x:POP_JUMP_IF_FALSE y+3
                   where y+3 is the instruction following the second test.
                */
            case JUMP_IF_FALSE_OR_POP:
            case JUMP_IF_TRUE_OR_POP:
                tgt = GETJUMPTGT(codestr, i);
                j = codestr[tgt];
                if (CONDITIONAL_JUMP(j)) {
                    /* NOTE: all possible jumps here are
                       absolute! */
                    if (JUMPS_ON_TRUE(j) == JUMPS_ON_TRUE(opcode)) {
                        /* The second jump will be
                           taken iff the first is. */
                        tgttgt = GETJUMPTGT(codestr, tgt);
                        /* The current opcode inherits
                           its target's stack behaviour */
                        codestr[i] = j;
                        SETARG(codestr, i, tgttgt);
                        goto reoptimize_current;
                    } else {
                        /* The second jump is not taken
                           if the first is (so jump past
                           it), and all conditional
                           jumps pop their argument when
                           they're not taken (so change
                           the first jump to pop its
                           argument when it's taken). */
                        if (JUMPS_ON_TRUE(opcode))
                            codestr[i] = POP_JUMP_IF_TRUE;
                        else
                            codestr[i] = POP_JUMP_IF_FALSE;
                        SETARG(codestr, i, (tgt + 3));
                        goto reoptimize_current;
                    }
                }
                /* Intentional fallthrough */

                /* Replace jumps to unconditional jumps */
            case POP_JUMP_IF_FALSE:
            case POP_JUMP_IF_TRUE:
            case FOR_ITER:
            case JUMP_FORWARD:
            case JUMP_ABSOLUTE:
            case CONTINUE_LOOP:
            case SETUP_LOOP:
            case SETUP_EXCEPT:
            case SETUP_FINALLY:
            case SETUP_WITH:
                tgt = GETJUMPTGT(codestr, i);
                /* Replace JUMP_* to a RETURN into just a RETURN */
                if (UNCONDITIONAL_JUMP(opcode) &&
                    codestr[tgt] == RETURN_VALUE) {
                    codestr[i] = RETURN_VALUE;
                    memset(codestr+i+1, NOP, 2);
                    continue;
                }
                if (!UNCONDITIONAL_JUMP(codestr[tgt]))
                    continue;
                tgttgt = GETJUMPTGT(codestr, tgt);
                if (opcode == JUMP_FORWARD) /* JMP_ABS can go backwards */
                    opcode = JUMP_ABSOLUTE;
                if (!ABSOLUTE_JUMP(opcode))
                    tgttgt -= i + 3;     /* Calc relative jump addr */
                if (tgttgt < 0)                           /* No backward relative jumps */
                    continue;
                codestr[i] = opcode;
                SETARG(codestr, i, tgttgt);
                break;

            case EXTENDED_ARG:
                if (codestr[i+3] != MAKE_FUNCTION)
                    goto exitUnchanged;
                /* don't visit MAKE_FUNCTION as GETARG will be wrong */
                i += 3;
                break;

                /* Replace RETURN LOAD_CONST None RETURN with just RETURN */
                /* Remove unreachable JUMPs after RETURN */
            case RETURN_VALUE:
                if (i+4 >= codelen)
                    continue;
                if (codestr[i+4] == RETURN_VALUE &&
                    ISBASICBLOCK(blocks,i,5))
                    memset(codestr+i+1, NOP, 4);
                else if (UNCONDITIONAL_JUMP(codestr[i+1]) &&
                         ISBASICBLOCK(blocks,i,4))
                    memset(codestr+i+1, NOP, 3);
                break;
        }
    }

    /* Fixup linenotab */
    for (i=0, nops=0 ; i<codelen ; i += CODESIZE(codestr[i])) {
        addrmap[i] = i - nops;
        if (codestr[i] == NOP)
            nops++;
    }
    cum_orig_line = 0;
    last_line = 0;
    for (i=0 ; i < tabsiz ; i+=2) {
        cum_orig_line += lineno[i];
        new_line = addrmap[cum_orig_line];
        assert (new_line - last_line < 255);
        lineno[i] =((unsigned char)(new_line - last_line));
        last_line = new_line;
    }

    /* Remove NOPs and fixup jump targets */
    for (i=0, h=0 ; i<codelen ; ) {
        opcode = codestr[i];
        switch (opcode) {
            case NOP:
                i++;
                continue;

            case JUMP_ABSOLUTE:
            case CONTINUE_LOOP:
            case POP_JUMP_IF_FALSE:
            case POP_JUMP_IF_TRUE:
            case JUMP_IF_FALSE_OR_POP:
            case JUMP_IF_TRUE_OR_POP:
                j = addrmap[GETARG(codestr, i)];
                SETARG(codestr, i, j);
                break;

            case FOR_ITER:
            case JUMP_FORWARD:
            case SETUP_LOOP:
            case SETUP_EXCEPT:
            case SETUP_FINALLY:
            case SETUP_WITH:
                j = addrmap[GETARG(codestr, i) + i + 3] - addrmap[i] - 3;
                SETARG(codestr, i, j);
                break;
        }
        adj = CODESIZE(opcode);
        while (adj--)
            codestr[h++] = codestr[i++];
    }
    assert(h + nops == codelen);

    code = PyBytes_FromStringAndSize((char *)codestr, h);
    PyMem_Free(addrmap);
    PyMem_Free(codestr);
    PyMem_Free(blocks);
    return code;

 exitError:
    code = NULL;

 exitUnchanged:
    if (blocks != NULL)
        PyMem_Free(blocks);
    if (addrmap != NULL)
        PyMem_Free(addrmap);
    if (codestr != NULL)
        PyMem_Free(codestr);
    Py_XINCREF(code);
    return code;
}

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static int tuple_of_constants ( unsigned char *  codestr,
Py_ssize_t  n,
PyObject consts 
) [static]

Definition at line 36 of file peephole.c.

{
    PyObject *newconst, *constant;
    Py_ssize_t i, arg, len_consts;

    /* Pre-conditions */
    assert(PyList_CheckExact(consts));
    assert(codestr[n*3] == BUILD_TUPLE || codestr[n*3] == BUILD_LIST || codestr[n*3] == BUILD_SET);
    assert(GETARG(codestr, (n*3)) == n);
    for (i=0 ; i<n ; i++)
        assert(codestr[i*3] == LOAD_CONST);

    /* Buildup new tuple of constants */
    newconst = PyTuple_New(n);
    if (newconst == NULL)
        return 0;
    len_consts = PyList_GET_SIZE(consts);
    for (i=0 ; i<n ; i++) {
        arg = GETARG(codestr, (i*3));
        assert(arg < len_consts);
        constant = PyList_GET_ITEM(consts, arg);
        Py_INCREF(constant);
        PyTuple_SET_ITEM(newconst, i, constant);
    }

    /* If it's a BUILD_SET, use the PyTuple we just built to create a
      PyFrozenSet, and use that as the constant instead: */
    if (codestr[n*3] == BUILD_SET) {
        PyObject *tuple = newconst;
        newconst = PyFrozenSet_New(tuple);
        Py_DECREF(tuple);
        if (newconst == NULL)
            return 0;
    }

    /* Append folded constant onto consts */
    if (PyList_Append(consts, newconst)) {
        Py_DECREF(newconst);
        return 0;
    }
    Py_DECREF(newconst);

    /* Write NOPs over old LOAD_CONSTS and
       add a new LOAD_CONST newconst on top of the BUILD_TUPLE n */
    memset(codestr, NOP, n*3);
    codestr[n*3] = LOAD_CONST;
    SETARG(codestr, (n*3), len_consts);
    return 1;
}

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