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Improve --with-wide-int mpz_t→fixnum conversion

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These tuneups and minor simplifications should affect only
platforms with EMACS_INT wider than ‘long’.
* src/alloc.c (make_number): If the number fits in long but
not in fixnum, do not attempt to convert to fixnum again.
Tighten the compile-time check for whether the second attempt
is worth trying, from sizeof (long) < sizeof (EMACS_INT) to
LONG_WIDTH < FIXNUM_BITS.  Do not bother computing the sign of
the value to tighten the bounds for whether to try the second
attempt, as it’s not worth the effort.  Do not call mpz_size,
which is unnecessary since the number of bits is already known
and the loop can iterate over a shift count instead.  Avoid
unnecessary casts.  Use + instead of | where either will do,
as + is typically better for optimization.

Improve mpz_t to fixnum when --with-wide-int
* src/alloc.c (make_number): Avoid undefined behavior
when shifting an EMACS_UINT by more than EMACS_UINT_WIDTH bits.
Check for integer overflow when shifting.
This commit is contained in:
Paul Eggert 2018-08-18 19:40:24 -07:00
parent 1d2df2fd03
commit 6eade1efde
1 changed files with 7 additions and 12 deletions
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19
src/alloc.c
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@ -3752,25 +3752,20 @@ make_number (mpz_t value)
if (!FIXNUM_OVERFLOW_P (l))
return make_fixnum (l);
}
/* Check if fixnum can be larger than long. */
if (sizeof (EMACS_INT) > sizeof (long))
else if (LONG_WIDTH < FIXNUM_BITS)
{
size_t bits = mpz_sizeinbase (value, 2);
int sign = mpz_sgn (value);
if (bits < FIXNUM_BITS + (sign < 0))
if (bits <= FIXNUM_BITS)
{
EMACS_INT v = 0;
size_t limbs = mpz_size (value);
mp_size_t i;
for (i = 0; i < limbs; i++)
int i = 0;
for (int shift = 0; shift < bits; shift += mp_bits_per_limb)
{
mp_limb_t limb = mpz_getlimbn (value, i);
v |= (EMACS_INT) ((EMACS_UINT) limb << (i * mp_bits_per_limb));
EMACS_INT limb = mpz_getlimbn (value, i++);
v += limb << shift;
}
if (sign < 0)
if (mpz_sgn (value) < 0)
v = -v;
if (!FIXNUM_OVERFLOW_P (v))