[llvm] r178409 - Implement XOR reassociation. It is based on following rules:

[Shuxin Yang]

No, Instcombine can only handle rule 1 (when c1 = c2). I will fire a 
enhancement ticket for instcombine
when I return back to office.

On 3/30/13 2:29 AM, Duncan Sands wrote:
> Hi Shuxin,
>
>> Implement XOR reassociation. It is based on following rules:
>>
>>    rule 1: (x | c1) ^ c2 => (x & ~c1) ^ (c1^c2),
>>       only useful when c1=c2
>>    rule 2: (x & c1) ^ (x & c2) = (x & (c1^c2))
>>    rule 3: (x | c1) ^ (x | c2) = (x & c3) ^ c3 where c3 = c1 ^ c2
>>    rule 4: (x | c1) ^ (x & c2) => (x & c3) ^ c1, where c3 = ~c1 ^ c2
>
> does instcombine know these rules too?  The disadvantage of 
> instcombine is that
> it only works locally, however it is run more often than reassociate, 
> so maybe
> implementing them in instcombine too is a win.
>
> Ciao, Duncan.
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