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From: "segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug other/102440] Uinteger Opt/Param but the underlying type is
 signed
Date: Tue, 26 Oct 2021 16:45:57 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102440

--- Comment #9 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Martin Li=C5=A1ka from comment #8)
> > We could make the "UInteger" type mean it is implemented with an "unsig=
ned
> > int"
> > C type (or some other unsigned integer type).
>=20
> This would lead to the following list of -Wsign-compare warnings:
>=20
> /home/marxin/Programming/gcc/gcc/c-family/c-opts.c:934:27: warning:
> comparison of integer expressions of different signedness: =E2=80=98unsig=
ned int=E2=80=99
> and =E2=80=98int=E2=80=99 [-Wsign-compare]

That line is

  if (warn_shift_overflow =3D=3D -1)

The documentation for that warning flag says
'-Wsign-compare'
     Warn when a comparison between signed and unsigned values could
     produce an incorrect result when the signed value is converted to
     unsigned.  In C++, this warning is also enabled by '-Wall'.  In C,
     it is also enabled by '-Wextra'.

I don't see how this could produce an incorrect result.  Writing the code as

  if (warn_shift_overflow =3D=3D -1U)

means exactly the same thing, except it assumes the size of the variable so
it is a bad habit.  Plain "-1" is easier to read anyway.

It is idiom to use -1 for all-bits-set for unsigned vars.  It works correct=
ly
whatever the size of the variable is.  It is silly if

  if (warn_shift_overflow =3D=3D -1)

warns, but

  if (warn_shift_overflow + 1 =3D=3D 0)

is just dandy (and that is the current situation :-( )

> One would need to verify/adjust all these places (plus many more for other
> targets).

Or fix the bloody warning ;-)  But not something you want on your plate, I
fully
understand :-)=