llvm dev - Feb 2007 - [LLVMdev] variant generation question

I've got an instruction that has the following pattern (R32C is the  
32-bit register class):

(set R32C:$rT, (or (and R32C:$rA, R32C:$rC),
                    (and R32C:$rB, (not R32C:$rC))))

tblgen generates the following variants (I've dropped the R32C for  
brevity):

(or (and $rA, $rC), (and $rB, (not $rC)))  # original
(or (and $rA, $rC), (and (not $rC), $rB))
(or (and $rB, (not $rC)), (and $rA, $rC))
(or (and (not $rC), $rB), (and $rA, $rC))

I would have expected four additional patterns, each with the (and  
$rC, $rA) variant in it. But I only get the above four.

Is this a bug or a feature? :-)


-scooter
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On Fri, 9 Feb 2007, Scott Michel wrote:
> I would have expected four additional patterns, each with the (and $rC,
$rA)
> variant in it. But I only get the above four.
>
> Is this a bug or a feature? :-)
This is a feature.  They would match the same pattern, so it would just be 
generating dead code.  For the same reason, if you write something like:

(set $rd, (and $rs, 123))

you will only  get matching code for that pattern, and not for:

(set $rd, (and 123, $rs))

which is always canonicalized to the former.

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/

On Feb 9, 2007, at 7:42 PM, Chris Lattner wrote:
> On Fri, 9 Feb 2007, Scott Michel wrote:
>> I would have expected four additional patterns, each with the (and  
>> $rC, $rA)
>> variant in it. But I only get the above four.
>>
>> Is this a bug or a feature? :-)
>
> This is a feature.  They would match the same pattern, so it would  
> just be
> generating dead code.  For the same reason, if you write something  
> like:
I'm not sure how much of a feature that is -- evidently, in my  
particular case, enumerating all eight variants ends up generating  
the desired code. Leaving llvm to its own devices doesn't generate  
the desired code. I would have expected that the "(and $rA, $rC)" to  
match "(and $rC, $rA)", were they inverted, but it doesn't get  
generated in my test code.

Generated patterns:

(or (and $rA, $rC), (and $rB, (not $rC)))  # original
(or (and $rA, $rC), (and (not $rC), $rB))
(or (and $rB, (not $rC)), (and $rA, $rC))
(or (and (not $rC), $rB), (and $rA, $rC))

Missing patterns:

(or (and $rC, $rA), (and $rB, (not $rC)))
(or (and $rC, $rA), (and (not $rC), $rB))
(or (and $rB, (not $rC)), (and $rC, $rA))
(or (and (not $rC), $rB), (and $rC, $rA))


-scooter

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