No warning for comparison of integers of different signs

[chengniansun]

Bugzilla Link	18504
Version	trunk
OS	Linux
CC	@zygoloid,@zhendongsu

Extended Description

Clang does not emit a warning for the comparison of integers of different signs.

I have the following program:

int unreported(unsigned a, int *b) {
        return a > (~(95 != *b));
}

And compile it with the following command, but Clang emits no warning for the expression "a > (~(95 != *b))"

$clang -Wall -Wextra -std=c11 -pedantic -c s.c
$clang --version
clang version 3.5 (trunk 198918)
Target: x86_64-unknown-linux-gnu
Thread model: posix

Differently, GCC emits a warning with the following command

gcc -Wall -Wextra -std=c11 -pedantic -c s.c

s.c: In function ‘unreported’:
s.c:7:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  return a > (~(95 != *b));
           ^

[zygoloid]

From GetExprRange:

  if (UnaryOperator *UO = dyn_cast<UnaryOperator>(E)) {
    switch (UO->getOpcode()) {
    // Boolean-valued operations are white-listed.
    case UO_LNot:
      return IntRange::forBoolType();

    // Operations with opaque sources are black-listed.
    case UO_Deref:
    case UO_AddrOf: // should be impossible
      return IntRange::forValueOfType(C, GetExprType(E));

    default:
      return GetExprRange(C, UO->getSubExpr(), MaxWidth);
    }
  }

That's wrong for at least UO_PreInc, UO_PreDec, UO_Minus, and UO_Not.

[AshleyRoll]

I have made an initial attempt at fixing this issue as a first time contributor.

https://reviews.llvm.org/D126960

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Assign the issue to you.
2. Fix the issue locally.
3. Run the test suite locally.
   3.1) Remember that the subdirectories under test/ create fine-grained testing targets, so you can
   e.g. use make check-clang-ast to only run Clang's AST tests.
4. Create a git commit
5. Run git clang-format HEAD~1 to format your changes.
6. Submit the patch to Phabricator.
   6.1) Detailed instructions can be found here

For more instructions on how to submit a patch to LLVM, see our documentation.

If you have any further questions about this issue, don't hesitate to ask via a comment on this Github issue.

@llvm/issue-subscribers-good-first-issue

[AvoidMe]

Hi!

I see that there was no progress on this one for some time.
Can I take @AshleyRoll patch rebase & fix comments?