[LLVMdev] Back-edge taken count of loops

[Dan Gohman]

I recently added support for <= loops on trunk. ScalarEvolution now
uses the nsw flag to prove that it doesn't need to consider the
infinite-loop case.

Dan

On May 6, 2010, at 2:50 AM, ether zhhb wrote:

> hi all,
> 
> i am have a project need to compute the back-edge taken count of a loop, and the "getBackedgeTakenCount" function of ScalarEvolution could do this for me.
> 
> but later i found that ScalarEvolution could not compute loops with test expression containing a "=", such as:
> 
> void scop_func(long A[], long n) {
>   long i;
> 
>   for (i = 1; i<= n; i++)
>     A[i] = 1;
> }
> 
> after have a look at the code that computing the back-edge taken count, i found that ScalarEvolution could only compute back-edge taken count if the exit condition is ICMP_NE/ICMP_EQ/ICMP_SLT/ICMP_SGT/ICMP_ULT/ICMP_UGT (ignoring "ComputeBackedgeTakenCountExhaustively"), and the function "SimplifyICmpOperands" will try to convert other conditions to the conditions listed above, by doing something like "i <= n" to "i < n+1", but it refuse to do that unless it could prove "n + 1" will not cause a overflow.
> 
> So, could us just set "n + 1" to the Exact field of BackedgeTakenInfo, and set something like SCEVInfinite  to the Max field of BackedgeTakenInfo?
> 
> thanks
> ether
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