gh-127937: convert decimal module to use import API for ints (PEP 757)

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` API.  That offer some
+speedup, if integer part of the :class:`~decimal.Decimal` instance is small.
+Patch by Sergey B Kirpichev.

Modules/_decimal/_decimal.c

@@ -30,7 +30,6 @@
 #endif
 
 #include <Python.h>
-#include "pycore_long.h"          // _PyLong_IsZero()
 #include "pycore_pystate.h"       // _PyThreadState_GET()
 #include "pycore_typeobject.h"
 #include "complexobject.h"
@@ -2323,38 +2322,42 @@ static PyObject *
 dec_from_long(decimal_state *state, PyTypeObject *type, PyObject *v,
               const mpd_context_t *ctx, uint32_t *status)
 {
-    PyObject *dec;
-    PyLongObject *l = (PyLongObject *)v;
+    PyObject *dec = PyDecType_New(state, type);
 
-    dec = PyDecType_New(state, type);
     if (dec == NULL) {
         return NULL;
     }
 
-    if (_PyLong_IsZero(l)) {
-        _dec_settriple(dec, MPD_POS, 0, 0);
-        return dec;
-    }
-
-    uint8_t sign = _PyLong_IsNegative(l) ? MPD_NEG :  MPD_POS;
+    PyLongExport export_long;
 
-    if (_PyLong_IsCompact(l)) {
-        _dec_settriple(dec, sign, l->long_value.ob_digit[0], 0);
-        mpd_qfinalize(MPD(dec), ctx, status);
-        return dec;
+    if (PyLong_Export(v, &export_long) == -1) {
+        Py_DECREF(dec);
+        return NULL;
     }
-    size_t len = _PyLong_DigitCount(l);
-
-#if PYLONG_BITS_IN_DIGIT == 30
-    mpd_qimport_u32(MPD(dec), l->long_value.ob_digit, len, sign, PyLong_BASE,
-                    ctx, status);
-#elif PYLONG_BITS_IN_DIGIT == 15
-    mpd_qimport_u16(MPD(dec), l->long_value.ob_digit, len, sign, PyLong_BASE,
-                    ctx, status);
-#else
-  #error "PYLONG_BITS_IN_DIGIT should be 15 or 30"
-#endif
+    if (export_long.digits) {
+        const PyLongLayout *layout = PyLong_GetNativeLayout();
+        const uint8_t bpd = layout->bits_per_digit;
+        const uint8_t sign = export_long.negative ? MPD_NEG :  MPD_POS;
+        const Py_ssize_t len = export_long.ndigits;
 
+        if (bpd == 30) {
+            mpd_qimport_u32(MPD(dec), export_long.digits, len, sign,
+                            1 << bpd, ctx, status);
+        }
+        else if (bpd == 15) {
+            mpd_qimport_u16(MPD(dec), export_long.digits, len, sign,
+                            1 << bpd, ctx, status);
+        }
+        else {
+            PyLong_FreeExport(&export_long);
+            Py_DECREF(dec);
+            return NULL;
+        }
+        PyLong_FreeExport(&export_long);
+    }
+    else {
+        mpd_qset_i64(MPD(dec), export_long.value, ctx, status);
+    }
     return dec;
 }
 
@@ -3639,12 +3642,13 @@ dec_format(PyObject *dec, PyObject *args)
 static PyObject *
 dec_as_long(PyObject *dec, PyObject *context, int round)
 {
-    PyLongObject *pylong;
+    PyObject *pylong;
     digit *ob_digit;
     size_t n;
     mpd_t *x;
     mpd_context_t workctx;
     uint32_t status = 0;
+    const PyLongLayout *layout = PyLong_GetNativeLayout();
 
     if (mpd_isspecial(MPD(dec))) {
         if (mpd_isnan(MPD(dec))) {
@@ -3672,34 +3676,45 @@ dec_as_long(PyObject *dec, PyObject *context, int round)
     }
 
     status = 0;
-    ob_digit = NULL;
+
+    int64_t val = mpd_qget_i64(x, &status);
+    if (!status) {
+        mpd_del(x);
+        return PyLong_FromInt64(val);
+    }
+
+    const uint8_t bpd = layout->bits_per_digit;
+    n = (mpd_sizeinbase(x, 2) + bpd - 1) / bpd;
+    PyLongWriter *writer = PyLongWriter_Create(mpd_isnegative(x), n,
+                                               (void**)&ob_digit);
+    /* mpd_sizeinbase can overestimate size by 1 digit, set it to zero. */
+    ob_digit[n-1] = 0;
+    if (writer == NULL) {
+        PyLongWriter_Discard(writer);
+        mpd_del(x);
+        return NULL;
+    }
+
+    status = 0;
 #if PYLONG_BITS_IN_DIGIT == 30
-    n = mpd_qexport_u32(&ob_digit, 0, PyLong_BASE, x, &status);
+    n = mpd_qexport_u32(&ob_digit, n, PyLong_BASE, x, &status);
 #elif PYLONG_BITS_IN_DIGIT == 15
-    n = mpd_qexport_u16(&ob_digit, 0, PyLong_BASE, x, &status);
+    n = mpd_qexport_u16(&ob_digit, n, PyLong_BASE, x, &status);
 #else
     #error "PYLONG_BITS_IN_DIGIT should be 15 or 30"
 #endif
 
     if (n == SIZE_MAX) {
         PyErr_NoMemory();
+        PyLongWriter_Discard(writer);
         mpd_del(x);
         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);
-    }
-
     assert(n > 0);
-    assert(!mpd_iszero(x));
-    pylong = _PyLong_FromDigits(mpd_isnegative(x), n, ob_digit);
-    mpd_free(ob_digit);
+    pylong = PyLongWriter_Finish(writer);
     mpd_del(x);
-    return (PyObject *) pylong;
+    return pylong;
 }
 
 /* Convert a Decimal to its exact integer ratio representation. */