gh-127937: Convert decimal to use PEP 757 import API (#127925)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Misc/NEWS.d/next/C_API/2024-12-14-03-40-15.gh-issue-127925.FF7aov.rst

@@ -0,0 +1,3 @@
+Convert the :mod:`decimal` module to use :pep:`757` C API (export-import
+integers), offering some speed-up if the integer part of the
+:class:`~decimal.Decimal` instance is small.  Patch by Sergey B Kirpichev.

Modules/_decimal/_decimal.c

@@ -2336,15 +2336,16 @@ dec_from_long(decimal_state *state, PyTypeObject *type, PyObject *v,
     }
     if (export_long.digits) {
         const PyLongLayout *layout = PyLong_GetNativeLayout();
-        uint32_t base = (uint32_t)1 << layout->bits_per_digit;
-        uint8_t sign = export_long.negative ? MPD_NEG : MPD_POS;
-        Py_ssize_t len = export_long.ndigits;
 
-        assert(layout->bits_per_digit <= 32);
+        assert(layout->bits_per_digit < 32);
         assert(layout->digits_order == -1);
         assert(layout->digit_endianness == (PY_LITTLE_ENDIAN ? -1 : 1));
         assert(layout->digit_size == 2 || layout->digit_size == 4);
 
+        uint32_t base = (uint32_t)1 << layout->bits_per_digit;
+        uint8_t sign = export_long.negative ? MPD_NEG : MPD_POS;
+        Py_ssize_t len = export_long.ndigits;
+
         if (layout->digit_size == 4) {
             mpd_qimport_u32(MPD(dec), export_long.digits, len, sign,
                             base, ctx, status);
@@ -3642,13 +3643,6 @@ dec_format(PyObject *dec, PyObject *args)
 static PyObject *
 dec_as_long(PyObject *dec, PyObject *context, int round)
 {
-    PyLongObject *pylong;
-    digit *ob_digit;
-    size_t n;
-    mpd_t *x;
-    mpd_context_t workctx;
-    uint32_t status = 0;
-
     if (mpd_isspecial(MPD(dec))) {
         if (mpd_isnan(MPD(dec))) {
             PyErr_SetString(PyExc_ValueError,
@@ -3661,12 +3655,16 @@ dec_as_long(PyObject *dec, PyObject *context, int round)
         return NULL;
     }
 
-    x = mpd_qnew();
+    mpd_t *x = mpd_qnew();
+
     if (x == NULL) {
         PyErr_NoMemory();
         return NULL;
     }
-    workctx = *CTX(context);
+
+    mpd_context_t workctx = *CTX(context);
+    uint32_t status = 0;
+
     workctx.round = round;
     mpd_qround_to_int(x, MPD(dec), &workctx, &status);
     if (dec_addstatus(context, status)) {
@@ -3675,34 +3673,56 @@ dec_as_long(PyObject *dec, PyObject *context, int round)
     }
 
     status = 0;
-    ob_digit = NULL;
-#if PYLONG_BITS_IN_DIGIT == 30
-    n = mpd_qexport_u32(&ob_digit, 0, PyLong_BASE, x, &status);
-#elif PYLONG_BITS_IN_DIGIT == 15
-    n = mpd_qexport_u16(&ob_digit, 0, PyLong_BASE, x, &status);
-#else
-    #error "PYLONG_BITS_IN_DIGIT should be 15 or 30"
-#endif
+    int64_t val = mpd_qget_i64(x, &status);
+
+    if (!status) {
+        mpd_del(x);
+        return PyLong_FromInt64(val);
+    }
+    assert(!mpd_iszero(x));
+
+    const PyLongLayout *layout = PyLong_GetNativeLayout();
+
+    assert(layout->bits_per_digit < 32);
+    assert(layout->digits_order == -1);
+    assert(layout->digit_endianness == (PY_LITTLE_ENDIAN ? -1 : 1));
+    assert(layout->digit_size == 2 || layout->digit_size == 4);
+
+    uint32_t base = (uint32_t)1 << layout->bits_per_digit;
+    /* We use a temporary buffer for digits for now, as for nonzero rdata
+       mpd_qexport_u32/u16() require either space "allocated by one of
+       libmpdec’s allocation functions" or "rlen MUST be correct" (to avoid
+       reallocation).  This can be further optimized by using rlen from
+       mpd_sizeinbase().  See gh-127925. */
+    void *tmp_digits = NULL;
+    size_t n;
+
+    status = 0;
+    if (layout->digit_size == 4) {
+        n = mpd_qexport_u32((uint32_t **)&tmp_digits, 0, base, x, &status);
+    }
+    else {
+        n = mpd_qexport_u16((uint16_t **)&tmp_digits, 0, base, x, &status);
+    }
 
     if (n == SIZE_MAX) {
         PyErr_NoMemory();
         mpd_del(x);
+        mpd_free(tmp_digits);
         return NULL;
     }
 
-    if (n == 1) {
-        sdigit val = mpd_arith_sign(x) * ob_digit[0];
-        mpd_free(ob_digit);
-        mpd_del(x);
-        return PyLong_FromLong(val);
-    }
+    void *digits;
+    PyLongWriter *writer = PyLongWriter_Create(mpd_isnegative(x), n, &digits);
 
-    assert(n > 0);
-    assert(!mpd_iszero(x));
-    pylong = _PyLong_FromDigits(mpd_isnegative(x), n, ob_digit);
-    mpd_free(ob_digit);
     mpd_del(x);
-    return (PyObject *) pylong;
+    if (writer == NULL) {
+        mpd_free(tmp_digits);
+        return NULL;
+    }
+    memcpy(digits, tmp_digits, layout->digit_size*n);
+    mpd_free(tmp_digits);
+    return PyLongWriter_Finish(writer);
 }
 
 /* Convert a Decimal to its exact integer ratio representation. */