[LLVMdev] undefs in phis

[Evan Cheng]

On Feb 2, 2009, at 12:12 PM, David Greene wrote:

> On Monday 02 February 2009 13:14, Evan Cheng wrote:
>
>> I am sorry I don't really follow it. Is this what you are describing?
>>
>> %v1177 = undef
>> ...
>> loop:
>> ...
>> %v1176 = op ...
>>                 = %v1177
>> %v1177 = %v1176
>> jmp loop
>>
>> Why is not safe to coalesce the 2 registers?
>
> Not quite.  The original code is:
>
> %v1177 = undef
> %v1645 =  ...
> loop:
> %v1176 = %v1645
> ...
>  = %v1176
>  = %v1177
> %v1645 = op ...
> %v1177 = %v1176
> jmp loop
>
> We can't coalesce %v1177 and %v1176 legally.  But we do.

Seriously, why not? In the first iteration, it's totally legal for  
v1177 has the same value as v1176. It's defined by an undef, it's  
allowed to have contain any value.

Evan


>
>
>                                              -Dave
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