Re: [patch] Fix wrong code with VCE to bit-field type at -O

[Richard Biener <richard dot guenther at gmail dot com>]

On Mon, Feb 17, 2014 at 11:27 AM, Eric Botcazou <ebotcazou@adacore.com> wrote:
>> There is nothing obvious I think, i.e. that's debatable.  I agree that a VCE
>> from a 32-bit object to a 32-bit integer with 24-bit precision should not
>> clear the upper 8 bits (so the REDUCE_BIT_FIELD part of my patch is wrong).
>> But here we have a VCE from a 24-bit object to a 32-bit integer with 24-bit
>> precision which reads *more bits* than the size of the source type; that I
>> think is plain wrong and is fixed by the bit-field extraction in the patch.
>
> Revised patch along these lines attached.  Although I agree that it's a bit of
> a kludge, it's quite localized and plausible IMO.

Woudln't it be better to do this in the series of "conversions", that is
inside the preceeding if-statement?  (the integral type case using
convert_modes looks weird enough, so adding this kind-of "less"
weird one there looks sensible)

Ok with moving it there (before the else if (!MEM_P (op0))).  You
probably want to guard with INTEGRAL_TYPE_P (type) as well,
not only GET_MODE (op0) != mode - just to prepare for weird
stuff like a vector-type where TYPE_PRECISION means sth else.

Thanks,
Richard.



>
>         * expr.c (expand_expr_real_1) <case VIEW_CONVERT_EXPR>: For a bit-field
>         destination type, extract exactly the number of valid bits if the source
>         type isn't integral or has a different precision.
>
>
> --
> Eric Botcazou