[Bug middle-end/96460] New: Warn about signed module that is converted to unsigned value

[Bug middle-end/96460] New: Warn about signed module that is converted to unsigned value

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96460

            Bug ID: 96460
           Summary: Warn about signed module that is converted to unsigned
                    value
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fw at gcc dot gnu.org
  Target Milestone: ---

I do not know how many warnings it would generate to warn for code like this
(if i is not known to be non-negative):

unsigned long
f2 (int i)
{
  return i % 16;
}

The problem is that if i is negative, i % 16 is also negative, and the result
is a large unsigned value. This is rarely what is intended. Even if the
negative input never occurs, GCC still needs to generate additional code that
is valid for this case.

A fix-hit hint should suggests & for powers of two and a cast to the
appropriate unsigned type for other operands.

I'm not sure what one would write to suppress this warning in cases where it is
inappropriate, maybe (int) i % 16.

[Bug c/96460] Warn about signed module that is converted to unsigned value

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96460

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|middle-end                  |c
           Severity|normal                      |enhancement

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
It's perfectly valid code ... guess similar to -Wconversion though.

[Bug c/96460] Warn about signed module that is converted to unsigned value

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96460

--- Comment #2 from Florian Weimer <fw at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> It's perfectly valid code ... guess similar to -Wconversion though.

If the modulo result is never negative, it's not *perfectly* valid because GCC
has to add instructions to produce those negative values, and most programmers
will never realize that this is happening.

[Bug c/96460] Warn about signed modulo that is converted to unsigned value

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96460

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egallager at gcc dot gnu.org
            Summary|Warn about signed module    |Warn about signed modulo
                   |that is converted to        |that is converted to
                   |unsigned value              |unsigned value

--- Comment #3 from Eric Gallager <egallager at gcc dot gnu.org> ---
There already is a warning from -Wsign-conversion for it:

$ /usr/local/bin/gcc -c -Wall -Wextra -Wconversion  96460.c
96460.c: In function 'f2':
96460.c:4:12: warning: conversion to 'long unsigned int' from 'int' may change
the sign of the result [-Wsign-conversion]
    4 |   return i % 16;
      |          ~~^~~~
$

[Bug c/96460] Warn about signed modulo that is converted to unsigned value

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96460

--- Comment #4 from Florian Weimer <fw at gcc dot gnu.org> ---
(In reply to Eric Gallager from comment #3)
> There already is a warning from -Wsign-conversion for it:

It's for the conversion, not the modulo. My hypothesis is that for the modulo,
the warning is much more helpful and can perhaps even enabled by -Wall.