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libgcc: Fix up bitint division [PR114397]

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The Knuth's division algorithm relies on the number of dividend limbs
to be greater ore equal to number of divisor limbs, which is why
I've added a special case for un < vn at the start of __divmodbitint4.
Unfortunately, my assumption that it then implies abs(v) > abs(u) and
so quotient must be 0 and remainder same as dividend is incorrect.
This is because this check is done before negation of the operands.
While bitint_reduce_prec reduces precision from clearly useless limbs,
the problematic case is when the dividend is unsigned or non-negative
and divisor is negative.  We can have limbs (from MS to LS):
dividend:       0       M       ?...
divisor:        -1      -N      ?...
where M has most significant bit set and M >= N (if M == N then it
also the following limbs matter) and the most significant limbs can
be even partial.  In this case, the quotient should be -1 rather than
0.  bitint_reduce_prec will reduce the precision of the dividend so
that M is the most significant limb, but can't reduce precision of the
divisor to more than having the -1 as most significant limb, because
-N doesn't have the most significant bit set.

The following patch fixes it by detecting this problematic case in the
un < vn handling, and instead of assuming q is 0 and r is u will
decrease vn by 1 because it knows the later code will negate the divisor
and it can be then expressed after negation in one fewer limbs.

2024-03-21  Jakub Jelinek  <jakub@redhat.com>

	PR libgcc/114397
	* libgcc2.c (__divmodbitint4): Don't assume un < vn always means
	abs(v) > abs(u), check for a special case of un + 1 == vn where
	u is non-negative and v negative and after v's negation vn could
	be reduced by 1.

	* gcc.dg/torture/bitint-65.c: New test.
This commit is contained in:
Jakub Jelinek 2024-03-21 13:07:50 +01:00
parent ac2f8c2a36
commit 59b6cece54
2 changed files with 99 additions and 32 deletions
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44
gcc/testsuite/gcc.dg/torture/bitint-65.c Normal file
View File

@ -0,0 +1,44 @@
/* PR libgcc/114397 */
/* { dg-do run { target bitint } } */
/* { dg-options "-std=c23" } */
/* { dg-skip-if "" { ! run_expensive_tests } { "*" } { "-O0" "-O2" } } */
/* { dg-skip-if "" { ! run_expensive_tests } { "-flto" } { "" } } */
#if __BITINT_MAXWIDTH__ >= 129
int
foo (unsigned _BitInt (128) a, _BitInt (129) b)
{
return a / b;
}
#endif
#if __BITINT_MAXWIDTH__ >= 192
int
bar (unsigned _BitInt (128) a, _BitInt (192) b)
{
return a / b;
}
#endif
int
main ()
{
#if __BITINT_MAXWIDTH__ >= 129
if (foo (336225022742818342628768636932743029911uwb,
-336225022742818342628768636932743029911wb) != -1
|| foo (336225022742818342628768636932743029912uwb,
-336225022742818342628768636932743029911wb) != -1
|| foo (336225022742818342628768636932743029911uwb,
-336225022742818342628768636932743029912wb) != 0)
__builtin_abort ();
#endif
#if __BITINT_MAXWIDTH__ >= 192
if (bar (336225022742818342628768636932743029911uwb,
-336225022742818342628768636932743029911wb) != -1
|| bar (336225022742818342628768636932743029912uwb,
-336225022742818342628768636932743029911wb) != -1
|| bar (336225022742818342628768636932743029911uwb,
-336225022742818342628768636932743029912wb) != 0)
__builtin_abort ();
#endif
}

87
libgcc/libgcc2.c
View File

@ -1707,44 +1707,67 @@ __divmodbitint4 (UBILtype *q, SItype qprec,
USItype vp = avprec % W_TYPE_SIZE;
if (__builtin_expect (un < vn, 0))
{
/* If abs(v) > abs(u), then q is 0 and r is u. */
if (q)
__builtin_memset (q, 0, qn * sizeof (UWtype));
if (r == NULL)
return;
/* If abs(v) > abs(u), then q is 0 and r is u.
Unfortunately un < vn doesn't always mean abs(v) > abs(u).
If uprec > 0 and vprec < 0 and vn == un + 1, if the
top limb of v is all ones and the second most significant
limb has most significant bit clear, then just decrease
vn/avprec/vp and continue, after negation both numbers
will have the same number of limbs. */
if (un + 1 == vn
&& uprec >= 0
&& vprec < 0
&& ((v[BITINT_END (0, vn - 1)] | (vp ? ((UWtype) -1 << vp) : 0))
== (UWtype) -1)
&& (Wtype) v[BITINT_END (1, vn - 2)] >= 0)
{
vp = 0;
--vn;
#if __LIBGCC_BITINT_ORDER__ == __ORDER_BIG_ENDIAN__
r += rn - 1;
u += un - 1;
++v;
#endif
if (up)
--un;
if (rn < un)
un = rn;
for (rn -= un; un; --un)
{
*r = *u;
r += BITINT_INC;
u += BITINT_INC;
}
if (!rn)
return;
if (up)
else
{
if (uprec > 0)
*r = *u & (((UWtype) 1 << up) - 1);
else
*r = *u | ((UWtype) -1 << up);
r += BITINT_INC;
if (!--rn)
/* q is 0 and r is u. */
if (q)
__builtin_memset (q, 0, qn * sizeof (UWtype));
if (r == NULL)
return;
#if __LIBGCC_BITINT_ORDER__ == __ORDER_BIG_ENDIAN__
r += rn - 1;
u += un - 1;
#endif
if (up)
--un;
if (rn < un)
un = rn;
for (rn -= un; un; --un)
{
*r = *u;
r += BITINT_INC;
u += BITINT_INC;
}
if (!rn)
return;
if (up)
{
if (uprec > 0)
*r = *u & (((UWtype) 1 << up) - 1);
else
*r = *u | ((UWtype) -1 << up);
r += BITINT_INC;
if (!--rn)
return;
}
UWtype c = uprec < 0 ? (UWtype) -1 : (UWtype) 0;
for (; rn; --rn)
{
*r = c;
r += BITINT_INC;
}
return;
}
UWtype c = uprec < 0 ? (UWtype) -1 : (UWtype) 0;
for (; rn; --rn)
{
*r = c;
r += BITINT_INC;
}
return;
}
USItype qn2 = un - vn + 1;
if (qn >= qn2)