https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98447
Bug ID: 98447
Summary: incorrect -Wanalyzer-shift-count-overflow warning
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: analyzer
Assignee: dmalcolm at gcc dot gnu.org
Reporter: vincent-gcc at vinc17 dot net
Target Milestone: ---
Consider the following code on a platform with a 64-bit unsigned long:
void f (unsigned long *p, int r, int i)
{
int b = 63, n = r % 16;
while (i >= 0)
{
if (n > b)
{
p[i--] = b + 1 >= 64 ? 0UL : 1UL << (b + 1);
b += 64;
}
b -= n;
}
}
The "1UL << (b + 1)" can be executed only if b + 1 < 64, so that the shift
count is always strictly less than 64.
But I get the following incorrect warning.
zira:~> gcc-snapshot -O2 -fanalyzer -c tst.c
tst.c: In function 'f':
tst.c:9:38: warning: shift by count ('64') >= precision of type ('64')
[-Wanalyzer-shift-count-overflow]
9 | p[i--] = b + 1 >= 64 ? 0UL : 1UL << (b + 1);
| ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
'f': events 1-5
|
| 5 | while (i >= 0)
| | ~~^~~~
| | |
| | (1) following 'true' branch (when 'i >= 0')...
| 6 | {
| 7 | if (n > b)
| | ~
| | |
| | (2) ...to here
| | (3) following 'true' branch (when 'b < n')...
| 8 | {
| 9 | p[i--] = b + 1 >= 64 ? 0UL : 1UL << (b + 1);
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| | | |
| | | (4) ...to here
| | (5) shift by count '64' here
|
with
gcc (Debian 20201224-1) 11.0.0 20201224 (experimental) [master revision
085fd2a46e5:21536a75ed4:168be2b3afddd41d4575ed55439231812bc3e7c9]
Note: This is a simplified testcase obtained from GNU MPFR's tests/random2.c
file, on which I got this issue.
