[Bug tree-optimization/35518] [4.4 Regression] FAIL: gcc.c-torture/execute/20040709-1.c execution at -O2 and above

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #23 from rguenther at suse dot de  2008-06-25 21:48 -------
Subject: Re:  [4.4 Regression] FAIL:
 gcc.c-torture/execute/20040709-1.c execution at -O2 and above

On Wed, 25 Jun 2008, aoliva at gcc dot gnu dot org wrote:

> ------- Comment #22 from aoliva at gcc dot gnu dot org  2008-06-25 20:20 -------
> Sorry that it took me so long to look at this.
>
> Richi, I have a feeling that your patch will just paper over the problem.

I know.

> See, if we take a bit-range that's not the entire bit-field, it will emit the
> same shifts, and it will break in the same way.
>
> The problem is that there's a disconnect between the width of the underlying
> mode/type and the width of the variable in which it is held.
>
> When I introduced bit-fields in SRA, one of my goals was to introduce scalars
> for "remaining" fields (i.e., those that didn't get scalar versions of their
> own) in such a way that no extraneous shifting was needed: we'd just extract
> the bits without shifting them around, for this would be just wasted
> computation.  It sufficed to apply masks to recombine these remaining fields
> into their proper place.
>
> Now, when you changed type from a mode-sized type to a nonstandard
> bitfield-sized type and replaced some bitfield references with plain
> conversions, you broke this property and removed the fix-ups that would have
> taken care of ensuring the shifts (if any) matched.

The _result_ type is what I changed.  The result type of a BIT_FIELD_REF
needs to match the bitfield width.  If you can point out where I changed
the field type that is referenced then you have found the error.

Richard.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35518