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From: "kristerw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/111280] New: CLZ(0) generated when CLZ_DEFINED_VALUE_AT_ZERO is false
Date: Sun, 03 Sep 2023 23:26:59 +0000 [thread overview]
Message-ID: <bug-111280-4@http.gcc.gnu.org/bugzilla/> (raw)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111280
Bug ID: 111280
Summary: CLZ(0) generated when CLZ_DEFINED_VALUE_AT_ZERO is
false
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: kristerw at gcc dot gnu.org
Target Milestone: ---
GCC may generate an internal call to CLZ with 0 when CLZ_DEFINED_VALUE_AT_ZERO
is false, which can be seen with gcc.c-torture/execute/920501-6.c where sccp
changes a loop to
_36 = t_10(D) != 0;
_35 = .CLZ (t_10(D));
_34 = 63 - _35;
_33 = (unsigned int) _34;
_32 = (long long unsigned int) _33;
_31 = _32 + 1;
b_38 = _36 ? _31 : 1;
The value _35 is not used when t_10(D) is 0, so it may be reasonable to allow
this. But the value _35 may then be any value, so _34 may overflow. I.e., the
calculation
_34 = 63 - _35;
must be changed to be done unsigned.
And the ranges calculated during the dom3 pass claims that _35 has a range
_35 : [irange] int [0, 63]
which also is wrong.
next reply other threads:[~2023-09-03 23:26 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-03 23:26 kristerw at gcc dot gnu.org [this message]
2023-09-04 1:10 ` [Bug tree-optimization/111280] " pinskia at gcc dot gnu.org
2023-09-04 1:11 ` pinskia at gcc dot gnu.org
2023-09-12 11:02 ` rguenth at gcc dot gnu.org
2023-09-12 11:07 ` jakub at gcc dot gnu.org
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