#1032130 gcc-12: incorrect -Wanalyzer-shift-count-overflow warning

Package:
gcc-12
Source:
gcc-12
Description:
GNU C compiler
Submitter:
Vincent Lefevre
Date:
2026-02-23 17:39:21 UTC
Severity:
normal
Tags:
#1032130#5
Date:
2023-02-28 13:13:55 UTC
From:
To:
On the following program

void f (unsigned long *p, int r, int i)
{
  int b = 64, n = r % 64;

  while (i >= 0 && b >= 0)
    {
      if (b <= n)
        p[i--] = 1UL << b;
      b -= n;
    }
}

gcc-12 emits the following incorrect warning:

$ gcc-12 -fanalyzer -c warn-shiftcount.c
warn-shiftcount.c: In function ‘f’:
warn-shiftcount.c:8:22: warning: shift by count (‘64’) >= precision of type (‘6’) [-Wanalyzer-shift-count-overflow]
    8 |         p[i--] = 1UL << b;
      |                  ~~~~^~~~
  ‘f’: events 1-5
    |
    |    5 |   while (i >= 0 && b >= 0)
    |      |          ~~~~~~~^~~~~~~~~
    |      |                 |
    |      |                 (1) following ‘true’ branch...
    |    6 |     {
    |    7 |       if (b <= n)
    |      |          ~
    |      |          |
    |      |          (2) ...to here
    |      |          (3) following ‘true’ branch (when ‘b <= n’)...
    |    8 |         p[i--] = 1UL << b;
    |      |           ~~~    ~~~~~~~~
    |      |            |         |
    |      |            |         (5) shift by count ‘64’ here
    |      |            (4) ...to here
    |

Here, due to the "n = r % 64", one has n <= 63, so that "1UL << b"
can be executed only when b <= 63, and the shift is necessarily valid
(no overflow).

Note: gcc-11 is affected too, but not gcc-10.
#1032130#10
Date:
2026-01-10 10:08:33 UTC
From:
To:
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