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wide-int: Fix wi::to_mpz [PR98474]

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The following testcase is miscompiled, because niter analysis miscomputes
the number of iterations to 0.
The problem is that niter analysis uses mpz_t (wonder why, wouldn't
widest_int do the same job?) and when wi::to_mpz is called e.g. on the
TYPE_MAX_VALUE of __uint128_t, it initializes the mpz_t result with wrong
value.
wi::to_mpz has code to handle negative wide_ints in signed types by
inverting all bits, importing to mpz and complementing it, which is fine,
but doesn't handle correctly the case when the wide_int's len (times
HOST_BITS_PER_WIDE_INT) is smaller than precision when wi::neg_p.
E.g. the 0xffffffffffffffffffffffffffffffff TYPE_MAX_VALUE is represented
in wide_int as 0xffffffffffffffff len 1, and wi::to_mpz would create
0xffffffffffffffff mpz_t value from that.
This patch handles it by adding the needed -1 host wide int words (and has
also code to deal with precision that aren't multiple of
HOST_BITS_PER_WIDE_INT).

2020-12-31  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/98474
	* wide-int.cc (wi::to_mpz): If wide_int has MSB set, but type
	is unsigned and excess negative, append set bits after len until
	precision.

	* gcc.c-torture/execute/pr98474.c: New test.
This commit is contained in:
Jakub Jelinek 2020-12-31 11:06:56 +01:00
parent a9ec9902d7
commit 9e603837f7
2 changed files with 44 additions and 0 deletions
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30
gcc/testsuite/gcc.c-torture/execute/pr98474.c Normal file
View File

@ -0,0 +1,30 @@
/* PR tree-optimization/98474 */
#ifdef __SIZEOF_INT128__
typedef __uint128_t T;
#define N (__SIZEOF_INT128__ * __CHAR_BIT__ / 2)
#else
typedef unsigned long long T;
#define N (__SIZEOF_LONG_LONG__ * __CHAR_BIT__ / 2)
#endif
__attribute__ ((noipa)) void
foo (T *x)
{
*x += ((T) 1) << (N + 1);
}
int
main ()
{
T a = ((T) 1) << (N + 1);
T b = a;
T n;
foo (&b);
n = b;
while (n >= a)
n -= a;
if ((int) (n >> N) != 0)
__builtin_abort ();
return 0;
}

14
gcc/wide-int.cc
View File

@ -230,6 +230,20 @@ wi::to_mpz (const wide_int_ref &x, mpz_t result, signop sgn)
t[len - 1] = (unsigned HOST_WIDE_INT) v[len - 1] << excess >> excess;
mpz_import (result, len, -1, sizeof (HOST_WIDE_INT), 0, 0, t);
}
else if (excess < 0 && wi::neg_p (x))
{
int extra
= (-excess + HOST_BITS_PER_WIDE_INT - 1) / HOST_BITS_PER_WIDE_INT;
HOST_WIDE_INT *t = XALLOCAVEC (HOST_WIDE_INT, len + extra);
for (int i = 0; i < len; i++)
t[i] = v[i];
for (int i = 0; i < extra; i++)
t[len + i] = -1;
excess = (-excess) % HOST_BITS_PER_WIDE_INT;
if (excess)
t[len + extra - 1] = (HOST_WIDE_INT_1U << excess) - 1;
mpz_import (result, len + extra, -1, sizeof (HOST_WIDE_INT), 0, 0, t);
}
else
mpz_import (result, len, -1, sizeof (HOST_WIDE_INT), 0, 0, v);
}