Hi!
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.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2022-07-01 Jakub Jelinek <ja...@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.
--- gcc/wide-int.cc.jj 2022-01-11 23:11:23.592273263 +0100
+++ gcc/wide-int.cc 2022-06-30 20:41:25.506292687 +0200
@@ -842,6 +842,13 @@ wi::shifted_mask (HOST_WIDE_INT *val, un
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;
@@ -2583,6 +2590,10 @@ 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
Jakub
