Hi!
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.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2024-03-21 Jakub Jelinek <ja...@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.
--- libgcc/libgcc2.c.jj 2024-03-15 19:04:27.000000000 +0100
+++ libgcc/libgcc2.c 2024-03-20 18:23:51.956879521 +0100
@@ -1707,44 +1707,67 @@ __divmodbitint4 (UBILtype *q, SItype qpr
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 __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)
+ /* 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)
{
- *r = *u;
- r += BITINT_INC;
- u += BITINT_INC;
+ vp = 0;
+ --vn;
+#if __LIBGCC_BITINT_ORDER__ == __ORDER_BIG_ENDIAN__
+ ++v;
+#endif
}
- 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)
--- gcc/testsuite/gcc.dg/torture/bitint-65.c.jj 2024-03-20 18:41:38.026311007
+0100
+++ gcc/testsuite/gcc.dg/torture/bitint-65.c 2024-03-20 18:40:18.604397871
+0100
@@ -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
+}
Jakub
