As diagnosed with Jakub and Richard in the analysis of PR 102134, the
current implementation of wi::clz has incorrect/inconsistent behaviour.
As mentioned by Richard in comment #7, clz should (always) return zero
for negative values, but the current implementation can only return 0
when precision is a multiple of HOST_BITS_PER_WIDE_INT. The fix is
simply to reorder/shuffle the existing tests.
2021-09-06 Roger Sayle <roger@nextmovesoftware.com>
gcc/ChangeLog
* wide-int.cc (wi::clz): Reorder tests to ensure the result
is zero for all negative values.
int
wi::clz (const wide_int_ref &x)
{
+ if (x.sign_mask () < 0)
+ /* The upper bit is set, so there are no leading zeros. */
+ return 0;
+
/* Calculate how many bits there above the highest represented block. */
int count = x.precision - x.len * HOST_BITS_PER_WIDE_INT;
/* The upper -COUNT bits of HIGH are not part of the value.
Clear them out. */
high = (high << -count) >> -count;
- else if (x.sign_mask () < 0)
- /* The upper bit is set, so there are no leading zeros. */
- return 0;
/* We don't need to look below HIGH. Either HIGH is nonzero,
or the top bit of the block below is nonzero; clz_hwi is