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wide-int: Fix up wi::shifted_mask [PR106144]

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As the following self-test testcase shows, wi::shifted_mask sometimes
doesn't create canonicalized wide_ints, which then fail to compare equal
to canonicalized wide_ints with the same value.
In particular, wi::mask (128, false, 128) gives { -1 } with len 1 and prec 128,
while wi::shifted_mask (0, 128, false, 128) gives { -1, -1 } with len 2
and prec 128.
The problem is that the code is written with the assumption that there are
3 bit blocks (or 2 if start is 0), but doesn't consider the possibility
where there are 2 bit blocks (or 1 if start is 0) where the highest block
isn't present.  In that case, there is the optional block of negate ? 0 : -1
elts, followed by just one elt (either one from the if (shift) or just
negate ? -1 : 0) and the rest is implicit sign-extension.
Only if end < prec there is 1 or more bits above it that have different bit
value and so we need to emit all the elts till end and then one more elt.

if (end == prec) would work too, because we have:
  if (width > prec - start)
    width = prec - start;
  unsigned int end = start + width;
so end is guaranteed to be end <= prec, dunno what is preferred.

2022-07-01  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/106144
	* wide-int.cc (wi::shifted_mask): If end >= prec, return right after
	emitting element for shift or if shift is 0 first element after start.
	(wide_int_cc_tests): Add tests for equivalency of wi::mask and
	wi::shifted_mask with 0 start.

(cherry picked from commit e52592073f)
This commit is contained in:
Jakub Jelinek
2022-07-01 11:17:41 +02:00
parent 99d9cdbd01
commit 6e7bf9bc3e
1 changed files with 12 additions and 1 deletions
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13
gcc/wide-int.cc
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@@ -842,6 +842,13 @@ wi::shifted_mask (HOST_WIDE_INT *val, unsigned int start, unsigned int width,
val[i++] = negate ? block : ~block;
}
if (end >= prec)
{
if (!shift)
val[i++] = negate ? 0 : -1;
return i;
}
while (i < end / HOST_BITS_PER_WIDE_INT)
/* 1111111 */
val[i++] = negate ? 0 : -1;
@@ -853,7 +860,7 @@ wi::shifted_mask (HOST_WIDE_INT *val, unsigned int start, unsigned int width,
HOST_WIDE_INT block = (HOST_WIDE_INT_1U << shift) - 1;
val[i++] = negate ? ~block : block;
}
else if (end < prec)
else
val[i++] = negate ? -1 : 0;
return i;
@@ -2549,6 +2556,10 @@ wide_int_cc_tests ()
run_all_wide_int_tests <widest_int> ();
test_overflow ();
test_round_for_mask ();
ASSERT_EQ (wi::mask (128, false, 128),
wi::shifted_mask (0, 128, false, 128));
ASSERT_EQ (wi::mask (128, true, 128),
wi::shifted_mask (0, 128, true, 128));
}
} // namespace selftest