[cfe-dev] Which floating point to bool conversions should trigger warnings?

[Richard Trieu via cfe-dev]

On Fri, Apr 22, 2016 at 3:57 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "Richard Trieu via cfe-dev" <cfe-dev at lists.llvm.org>
> > To: "cfe-dev" <cfe-dev at lists.llvm.org>
> > Sent: Friday, April 22, 2016 5:47:59 PM
> > Subject: [cfe-dev] Which floating point to bool conversions should
> trigger    warnings?
> >
> > I recently added (r267054) and then revert (r267234) some warnings
> > for floating point to bool conversions pending more discussion on
> > the topic. From my experience, bool conversions are tricky and a
> > source of many bugs, so a warning in this place would be good.
> >
> >
> > Bool behaves a little differently than other integer types. For
> > instance, 0.5 is converted to zero for integer types, except for
> > bool which it gets converted to true. Also, bool is the type for
> > conditionals and resulting type of logical operators.
> >
> >
> > However, at least one style guide says to use floating point to bool
> > conversion (
> > https://webkit.org/code-style-guidelines/#null-false-and-zero )
> >
>
> I don't see where it says that.
>
> Regardless, I think we should always warn.
>
>  -Hal
>

It specifies that checks against 0 should be written without the == 0.  For
example:

  void check(float f) {
    if (f) { }  // This is the same as "f != 0.0"
    if (!f) { }  // This is the same as "f == 0.0"
  }

I don't intend to stop warning, just split the warning into pieces so users
can better control which warnings are active.

>
> >
> > With that, I hope to get a little more discussion on this topic
> > before implementing the floating point to bool conversion warnings
> > again. The two factors for the warning are the source of the
> > floating point value and where the conversion happens, although
> > there may be other factors we need to take into account. Only the
> > source has been taken account so far since it is the easier way to
> > implement.
> >
> >
> > Source:
> >
> > Exact floating point literals (0.0, 1.0)
> >
> > Rounded floating point literals (0.5, 4.0)
> > Exact compiler time constant (5.0 - 4.0, kZero)
> > Inexact compiler time constant (1.0 / 2.0, kHugeNumber)
> > Run time values (getFloat())
> >
> >
> > Location:
> > Function Argument
> > Assigning/Initializing bool variable
> > Return statement of bool returning function
> > Condition of if statement, for loop, while loop, or do-while loop
> > Condition of conditional operator (?:)
> > Operand of logical operators(&&, ||, !)
> >
> >
> > Currently, exact floating point literals are not warned on, rounded
> > floating point literals are under -Wliteral-conversion, and
> > everything else is under -Wfloat-conversion.
> >
> >
> > Note that exclusionary groups may also be useful, for instance
> > "-Wfloat-conversion -Wno-some-bool-warning" if that helps filter out
> > some of the more noisy warnings.
> > _______________________________________________
> > cfe-dev mailing list
> > cfe-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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