[LLVMdev] LoopInfo are not able to identify some natural loops?

[John Criswell]

On 4/30/11 8:52 AM, Bo Wu wrote:
> In C code, if a loop is not a natural loop, that means its loop body 
> should at least have one label, right? In that case, some BB out of 
> the loop can jump to the loop body, so the loop has more than on entry.

Off the top of my head, I would think you are right: a C label would 
have to exist within the loop in order to have a jump from outside the 
loop to the inside of the loop.

That said, have you looked at the LLVM IR generated for the function and 
determined that the loop meets all the criteria of a natural loop?  Just 
my two cents, but I think looking at the IR directly is much more 
accurate than ensuring that your conclusion about C code and natural 
loops is correct.

-- John T.

>
> Does LoopInfo guarantee to identify all natural loops? This property 
> is very important for my pass.
>
> regards,
> Bo
>
> On Fri, Apr 29, 2011 at 11:50 PM, Cameron Zwarich <zwarich at apple.com 
> <mailto:zwarich at apple.com>> wrote:
>
>     A natural loop is one whose header dominates all of the nodes in
>     the loop. There is probably some block outside of the loop jumping
>     to a block in the loop body.
>
>     Cameron
>
>     On Apr 29, 2011, at 7:43 PM, Bo Wu <bwu at cs.wm.edu
>     <mailto:bwu at cs.wm.edu>> wrote:
>
>>     Hi,
>>
>>     I found that some loops can not be identified by LoopInfo pass.
>>     For example, the loop at line 3094 of rdopt.c of benchmark
>>     464.h264ref from spec cpu2006 is not a loop or a child (pr
>>     grandchild) of any loop in the loop list generated by LoopInfo
>>     pass. The documentation of LoopInfo says that it identifies
>>     natural loops, who have exactly one entry point. But the IR of
>>     this loops shows that it's header only has one BB in preds. Does
>>     that mean LoopInfo can not identify some natural loops?
>>
>>     Thanks,
>>     Bo
>>
>>     -- 
>>     Bo Wu
>>
>>     Ph.D. student
>>     CAPS group
>>     Computer Science Department
>>     The College of William & Mary
>>     www.cs.wm.edu/~bwu <http://www.cs.wm.edu/%7Ebwu>
>>
>>
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>
>
>
>
> -- 
> Bo Wu
>
> Ph.D. student
> CAPS group
> Computer Science Department
> The College of William & Mary
> www.cs.wm.edu/~bwu <http://www.cs.wm.edu/%7Ebwu>
>
>
>
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