[LLVMdev] LLVMdev Digest, Vol 50, Issue 33

[Sheng Zhou]

>> 7 for(i=0; i<j && i+j+1<N; i++) {
>>
>> 8 for(i=0; i<j && i<N-j-1; i++) {
>>
>>
>> Line 7 and Line 8 actually have the same expression,
>>     
>
> The expressions are logically similar, but they aren't the same to the  
> compiler unless some pass that recognizes these types of polynomial  
> equations and can prove that they are the same is run (I'm not sure if  
> such a pass exists in LLVM). For instance, because you use "j - 1" in  
> 8, you now have a situation where you have a negative number when j ==  
> 0, etc.
>   
ok, i see.
>   
>> Line 8 just move the "j+1" to the right hand of the inequation.
>>
>> I just wonder why the two equal inequation result in different cfg,  
>> the
>> additional basicblock, backedge...
>> Surely the two cfg are all correct, but just the additional backedge
>> makes it hard for loop pipeline.
>>
>> Can we improve it?
>>
>>     
>
> For what it's worth, I got the exact same code for both forms when  
> compiling at -O3.
>   
I also tried -O3, but got different ll code, what llvm revision are you 
using ?

> -bw
>
>
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> End of LLVMdev Digest, Vol 50, Issue 33
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