Proactive reliability fix for broken FPUs: The base conversion functions

use log10() to calculate the size of the output array. The current code
has been tested on x86/amd64 (and to a lesser extent on qemu-mips qemu-sparc)
and produces sufficiently large values for all inputs tested so far (coefficient
sizes of 10**18 - 1 are hard to test exhaustively).

The new code does not rely on the correctness of log10() and resizes
the output arrays if the allocated space is insufficient.

Modules/_decimal/_decimal.c

@@ -3203,7 +3203,8 @@ static PyObject *
 dec_as_long(PyObject *dec, PyObject *context, int round)
 {
     PyLongObject *pylong;
-    size_t maxsize, n;
+    digit *ob_digit;
+    size_t n;
     Py_ssize_t i;
     mpd_t *x;
     mpd_context_t workctx;
@@ -3234,32 +3235,33 @@ dec_as_long(PyObject *dec, PyObject *context, int round)
         return NULL;
     }
 
-    maxsize = mpd_sizeinbase(x, PyLong_BASE);
-    if (maxsize > PY_SSIZE_T_MAX) {
-        mpd_del(x);
-        PyErr_NoMemory();
-        return NULL;
-    }
-    pylong = _PyLong_New(maxsize);
-    if (pylong == NULL) {
-        mpd_del(x);
-        return NULL;
-    }
-
     status = 0;
+    ob_digit = NULL;
 #if PYLONG_BITS_IN_DIGIT == 30
-    n = mpd_qexport_u32(pylong->ob_digit, maxsize, PyLong_BASE, x, &status);
+    n = mpd_qexport_u32(&ob_digit, 0, PyLong_BASE, x, &status);
 #elif PYLONG_BITS_IN_DIGIT == 15
-    n = mpd_qexport_u16(pylong->ob_digit, maxsize, PyLong_BASE, x, &status);
+    n = mpd_qexport_u16(&ob_digit, 0, PyLong_BASE, x, &status);
 #else
-  #error "PYLONG_BITS_IN_DIGIT should be 15 or 30"
+    #error "PYLONG_BITS_IN_DIGIT should be 15 or 30"
 #endif
-    if (dec_addstatus(context, status)) {
-        Py_DECREF((PyObject *) pylong);
+
+    if (n == SIZE_MAX) {
+        PyErr_NoMemory();
+        mpd_del(x);
+        return NULL;
+    }
+
+    assert(n > 0);
+    pylong = _PyLong_New(n);
+    if (pylong == NULL) {
+        mpd_free(ob_digit);
         mpd_del(x);
         return NULL;
     }
 
+    memcpy(pylong->ob_digit, ob_digit, n * sizeof(digit));
+    mpd_free(ob_digit);
+
     i = n;
     while ((i > 0) && (pylong->ob_digit[i-1] == 0)) {
         i--;

Modules/_decimal/libmpdec/basearith.c

@@ -583,8 +583,30 @@ _mpd_shortadd_b(mpd_uint_t *w, mpd_size_t m, mpd_uint_t v, mpd_uint_t b)
     return carry;
 }
 
+/* w := product of u (len n) and v (single word). Return carry. */
+mpd_uint_t
+_mpd_shortmul_c(mpd_uint_t *w, const mpd_uint_t *u, mpd_size_t n, mpd_uint_t v)
+{
+    mpd_uint_t hi, lo;
+    mpd_uint_t carry = 0;
+    mpd_size_t i;
+
+    assert(n > 0);
+
+    for (i=0; i < n; i++) {
+
+        _mpd_mul_words(&hi, &lo, u[i], v);
+        lo = carry + lo;
+        if (lo < carry) hi++;
+
+        _mpd_div_words_r(&carry, &w[i], hi, lo);
+    }
+
+    return carry;
+}
+
 /* w := product of u (len n) and v (single word) */
-void
+mpd_uint_t
 _mpd_shortmul_b(mpd_uint_t *w, const mpd_uint_t *u, mpd_size_t n,
                 mpd_uint_t v, mpd_uint_t b)
 {
@@ -602,7 +624,8 @@ _mpd_shortmul_b(mpd_uint_t *w, const mpd_uint_t *u, mpd_size_t n,
 
         _mpd_div_words(&carry, &w[i], hi, lo, b);
     }
-    w[i] = carry;
+
+    return carry;
 }
 
 /*

Modules/_decimal/libmpdec/basearith.h

@@ -49,8 +49,10 @@ void _mpd_basemul(mpd_uint_t *w, const mpd_uint_t *u, const mpd_uint_t *v,
                   mpd_size_t m, mpd_size_t n);
 void _mpd_shortmul(mpd_uint_t *w, const mpd_uint_t *u, mpd_size_t n,
                    mpd_uint_t v);
-void _mpd_shortmul_b(mpd_uint_t *w, const mpd_uint_t *u, mpd_size_t n,
-                     mpd_uint_t v, mpd_uint_t b);
+mpd_uint_t _mpd_shortmul_c(mpd_uint_t *w, const mpd_uint_t *u, mpd_size_t n,
+                           mpd_uint_t v);
+mpd_uint_t _mpd_shortmul_b(mpd_uint_t *w, const mpd_uint_t *u, mpd_size_t n,
+                           mpd_uint_t v, mpd_uint_t b);
 mpd_uint_t _mpd_shortdiv(mpd_uint_t *w, const mpd_uint_t *u, mpd_size_t n,
                          mpd_uint_t v);
 mpd_uint_t _mpd_shortdiv_b(mpd_uint_t *w, const mpd_uint_t *u, mpd_size_t n,

Modules/_decimal/libmpdec/mpdecimal.h

@@ -511,16 +511,16 @@ void mpd_qsqrt(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx, uint32_t
 void mpd_qinvroot(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx, uint32_t *status);
 
 
-size_t mpd_sizeinbase(mpd_t *a, uint32_t base);
+size_t mpd_sizeinbase(const mpd_t *a, uint32_t base);
 void mpd_qimport_u16(mpd_t *result, const uint16_t *srcdata, size_t srclen,
                      uint8_t srcsign, uint32_t srcbase,
                      const mpd_context_t *ctx, uint32_t *status);
 void mpd_qimport_u32(mpd_t *result, const uint32_t *srcdata, size_t srclen,
                      uint8_t srcsign, uint32_t srcbase,
                      const mpd_context_t *ctx, uint32_t *status);
-size_t mpd_qexport_u16(uint16_t *rdata, size_t rlen, uint32_t base,
+size_t mpd_qexport_u16(uint16_t **rdata, size_t rlen, uint32_t base,
                        const mpd_t *src, uint32_t *status);
-size_t mpd_qexport_u32(uint32_t *rdata, size_t rlen, uint32_t base,
+size_t mpd_qexport_u32(uint32_t **rdata, size_t rlen, uint32_t base,
                        const mpd_t *src, uint32_t *status);
 
 
@@ -531,8 +531,8 @@ size_t mpd_qexport_u32(uint32_t *rdata, size_t rlen, uint32_t base,
 char * mpd_format(const mpd_t *dec, const char *fmt, mpd_context_t *ctx);
 void mpd_import_u16(mpd_t *result, const uint16_t *srcdata, size_t srclen, uint8_t srcsign, uint32_t base, mpd_context_t *ctx);
 void mpd_import_u32(mpd_t *result, const uint32_t *srcdata, size_t srclen, uint8_t srcsign, uint32_t base, mpd_context_t *ctx);
-size_t mpd_export_u16(uint16_t *rdata, size_t rlen, uint32_t base, const mpd_t *src, mpd_context_t *ctx);
-size_t mpd_export_u32(uint32_t *rdata, size_t rlen, uint32_t base, const mpd_t *src, mpd_context_t *ctx);
+size_t mpd_export_u16(uint16_t **rdata, size_t rlen, uint32_t base, const mpd_t *src, mpd_context_t *ctx);
+size_t mpd_export_u32(uint32_t **rdata, size_t rlen, uint32_t base, const mpd_t *src, mpd_context_t *ctx);
 void mpd_finalize(mpd_t *result, mpd_context_t *ctx);
 int mpd_check_nan(mpd_t *result, const mpd_t *a, mpd_context_t *ctx);
 int mpd_check_nans(mpd_t *result, const mpd_t *a, const mpd_t *b, mpd_context_t *ctx);