[Bug c/95157] New: Missing -Wtautological-compare warning

[Bug c/95157] New: Missing -Wtautological-compare warning

[simon.marchi at polymtl dot ca]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95157

            Bug ID: 95157
           Summary: Missing -Wtautological-compare warning
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: simon.marchi at polymtl dot ca
  Target Milestone: ---

Using this code:

volatile short insn;

int main()
{
  if (insn < 0 && insn > 3) {
      return 1;
  } else {
      return 0;
  }
}


clang 11 warns, but gcc doesn't:

$ gcc-10 test.c -c -Wall
$ g++-10 test.c -c -Wall
$ clang-11 test.c -c -Wall 
test.c:5:16: warning: overlapping comparisons always evaluate to false
[-Wtautological-overlap-compare]
  if (insn < 0 && insn > 3) {
      ~~~~~~~~~^~~~~~~~~~~
1 warning generated.
$ clang++-11 test.c -c -Wall
clang: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is
deprecated [-Wdeprecated]
test.c:5:16: warning: overlapping comparisons always evaluate to false
[-Wtautological-overlap-compare]
  if (insn < 0 && insn > 3) {
      ~~~~~~~~~^~~~~~~~~~~
1 warning generated.


I think -Wtautological-compare should trigger here.

[Bug c/95157] Missing -Wtautological-compare warning

[harald at gigawatt dot nl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95157

Harald van Dijk <harald at gigawatt dot nl> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |harald at gigawatt dot nl

--- Comment #1 from Harald van Dijk <harald at gigawatt dot nl> ---
For your test case, this seems like a bug in clang to me. The idea of volatile
is that the variable might change in ways the compiler cannot tell. The
compiler must read insn twice (assuming the first insn < 0 is true) and must
not assume the second read has no side effects or produces the same value as
the first read. If the two reads of insn can produce different values, it is
possible for insn < 0 && insn > 3 to be true.

However, if volatile is removed, the warning is correct, and GCC does not warn
in that case either.

[Bug c/95157] Missing -Wtautological-compare warning

[simon.marchi at polymtl dot ca]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95157

--- Comment #2 from Simon Marchi <simon.marchi at polymtl dot ca> ---
Ah, good point!

So yeah, please consider the test case without `volatile`.

[Bug c/95157] Missing -Wtautological-compare warning

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95157

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2020-05-26
     Ever confirmed|0                           |1
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.