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| author | Roger Sayle <roger@nextmovesoftware.com> | 2021-09-06 22:48:53 +0100 |
|---|---|---|
| committer | Roger Sayle <roger@nextmovesoftware.com> | 2021-09-06 22:50:45 +0100 |
| commit | 74cb45e67d14624c3e2fafa99a8920d1866a5f0c (patch) | |
| tree | 977fe2d0546db1f1fffd59e03f7b3db70f09a6af /gcc | |
| parent | 1bc66017c118229a101f0a2c097a9c209f777883 (diff) | |
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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.
Diffstat (limited to 'gcc')
| -rw-r--r-- | gcc/wide-int.cc | 7 |
1 files changed, 4 insertions, 3 deletions
diff --git a/gcc/wide-int.cc b/gcc/wide-int.cc index 906f4ea..a142151 100644 --- a/gcc/wide-int.cc +++ b/gcc/wide-int.cc @@ -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 |