Cannot get Bit test RTL to cooperate with Combine.

[Andrew Hutchinson]

Thank you so much for your information!

I will investigate your patch.
(I just hacked lowpart_for_combine to allow lowering something larger 
than word and the subreg matched no problem.)

It looks like RTL generation is somewhat odd and not helping.
My test used
extern long x;
if (x & 1)....

If there is only a single reference to x then (x &1), is lowered to HI 
mode and does not included any subregs (nosplit-wide-types). So my 
patterns match.

If my test code included two bit tests -  I get HI mode subregs on the x 
&1  (which will not match) but not on x & 2  the latter is  in the wider 
SI mode and will match.
If I turn on split-wide-types, the  subregs are not removed by subreg 
lowering since there is now mixed mode usage!

Something seem backwards in expansion, regarding lowering and references.

Andy


Joern Rennecke wrote:
> On Sun, Sep 20, 2009 at 01:49:39PM -0400, Andrew Hutchinson wrote:
>> All,
>>
>> I have been debugging AVR port to see why we fail to match so many bit
>> test opportunities.
>>
>> When dealing with longer modes I have come across a problem I can not 
>> solve.
>>
>> Expansion in RTL for a bit test can produce two styles.
>>
>> STYLE 1  Bit to be tested is NOT LSB (e.g. if ( longthing & 0x10)),  the
>> expanded code contains the test as:
>>
>> (and:SI (reg:SI 45 [ lx.1 ])
>>    (const_int 16 [0x10]))
>>
>> Bit tests are matched by combine. Combine has no problems with this and
>> eventually creates a matching pattern based on the conversion of the AND
>> to a zero extraction
>>
>> (set (pc)
>>    (if_then_else (ne (zero_extract:SI (subreg:QI (reg:SI 45 [ lx.1 ]) 0)
>>                (const_int 1 [0x1])
>>                (const_int 4 [0x4]))
>>            (const_int 0 [0x0]))
>>        (label_ref:HI 133)
>>        (pc)))
>>
>> This will match Bit test patterns and produces optimal code. :-)
>
> Unfortunately, when combine knows about upper bits that are zero, it
> will generate an lshiftrt instead, which can't be legitimately matched
> by a bit test.
>
> I have a patch for this which I haven't gotten around yet to test it
> separately in trunk and formally submit to the patches list, but you can
> extract it from arc-20081210-branch:
>
> 2008-12-02  J"orn Rennecke  <joern.rennecke@arc.com>
>
>         * combine.c (undo_since): New function, broken out of:
>         (undo_all).
>         (combine_simplify_bittest): New function.
>         (combine_simplify_rtx, simplify_if_then_else): Use it.
>         * config/arc/arc.c (arc_rtx_costs): Check for bbit test.
>
> svn diff -r144651:144652 
> svn://gcc.gnu.org/svn/gcc/branches/arc-20081210-branch/gcc/combine.c
>