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Correct implementation of wi::clz 

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.
This commit is contained in:
Roger Sayle 2021-09-06 22:48:53 +01:00
parent 1bc66017c1
commit 74cb45e67d
1 changed files with 4 additions and 3 deletions
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7
gcc/wide-int.cc
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@ -2050,6 +2050,10 @@ wi::arshift_large (HOST_WIDE_INT *val, const HOST_WIDE_INT *xval,
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;
@ -2058,9 +2062,6 @@ wi::clz (const wide_int_ref &x)
/* 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