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.
This patch has been tested on x86_64-pc-linux-gnu with "make bootstrap"
and "make -k check" with no new failures.
Ok for mainline?
2021-09-05 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
* gcc/wide-int.cc (wi::clz): Reorder tests to ensure the result
is zero for all negative values.
Roger
--
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
