Cannot get Bit test RTL to cooperate with Combine.

[Andrew Hutchinson]

Why doesn't combine try matching "unsimplified" expressions when it fails?

This would at least permit creating patterns based on explicit format 
of  input RTL without the added vagaries of simplification

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
>