From 74cb45e67d14624c3e2fafa99a8920d1866a5f0c Mon Sep 17 00:00:00 2001 From: Roger Sayle Date: Mon, 6 Sep 2021 22:48:53 +0100 Subject: [PATCH] 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 gcc/ChangeLog * wide-int.cc (wi::clz): Reorder tests to ensure the result is zero for all negative values. --- gcc/wide-int.cc | 7 ++++--- 1 file 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 -- 2.7.4