Re: Help with bit-field semantics in C and C++

[Roger Sayle <roger at eyesopen dot com>]

On 24 Aug 2004, Geoffrey Keating wrote:
> Roger Sayle <roger@eyesopen.com> writes:
> > extern "C" void abort(void);
> >
> > enum E { e = 0x0f };
> >
> > void test(int n)
> > {
> >   enum E x, y;
> >
> >   x = (E)0x10;
> >   y = (E)n;
> >
> >   if (x != y)
> >     abort ();
> > }
> >
> > int main()
> > {
> >   test(0x10);
> >   return 0;
> > }
> >
> >
> > Normally, when storing a narrower mode within a wider one, GCC's RTL
> > supports one of three representations: zero extended, sign extended
> > and paradoxical.  I'd just like confirmation that C/C++ bitfields
> > are intended to use a paradoxical representation.
>
> There aren't any bitfields in your example.
>
> As far as I can tell, the code is valid C or C++, at least on all
> machines that GCC runs on.  The type of 'enum E' will be compatible
> with 'char' or 'int', depending on flag settings.


My apologies.  Joseph Myers and Mark Mitchell have already pointed
out the I've used the wrong terminology.  The g++ front-end treats
C++ enumerators as "bit-sized" integers.  These are not the same
to language lawyers as bit-fields.  In the middle-end, however, they
are treated similarly.

The example code I posted will "abort()" with g++ 3.4 and mainline,
but executes to completion with gcc-3.3.3 and all earlier versions.
It also works fine with Intel icc.


> > Hence optimizations such as "(x > e)" -> false, which are valid
> > for sign and zero extended types, are invalid when TYPE_PRECISION(t)
> > != GET_MODE_BITSIZE(GET_MODE(t)).
> >
> > In the above code, should we be truncating the immediate mode constant
> > to zero, if we're not truncating the variable assignment?  Do tree
> > types need an extra bit to indicate "paradoxical" or is the test
> > that "TYPE_PRECISION(t) != GET_MODE_BITSIZE(GET_MODE(t))" sufficient.
> > One can imagine the middle-end needing to support narrow bit types
> > in other languages whose semantics may require explicit sign or zero
> > extension.
>
> I think you sent the wrong example.

No this is the one I intended to send.  In the example above, when
compiled with mainline, TYPE_PRECISION(t) != GET_MODE_BITSIZE(GET_MODE(t)).

Roger
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