[LLVMdev] Is this a bug with loop unrolling and TargetTransformInfo ?

[Hal Finkel]

----- Original Message -----
> From: "Tom Stellard" <tom at stellard.net>
> To: llvmdev at cs.uiuc.edu
> Sent: Wednesday, February 4, 2015 1:45:26 PM
> Subject: [LLVMdev] Is this a bug with loop unrolling and TargetTransformInfo	?
> 
> Hi,
> 
> I ran into this issue recently and wanted to know if it was a bug or
> expected behavior.
> 
> In the R600 backend's TargetTransformInfo implementation, we were
> setting
> UnrollingPreferences::Count = UINT_MAX.  This was a mistake as we
> should have been
> setting UnrollingPreferences::MaxCount instead.  However, as a result
> of setting
> Count to UINT_MAX, this loop would be unrolled 15 times:

A bug regardless? Where did 15 come from?

 -Hal

> 
> if (b + 4 > a) {
>   for (int i = 0; i < 4; i++, b++) {
>     if (b + 1 <= a)
>       *(dst + c + b) = 0;
>      else
>        break;
>   }
> }
> 
> Is this the expected behavior?  Is the loop unroll pass supposed to
> use the
> value of UnrollingPreferecnes::Count even if it is clearly wrong?
> 
> I have an LLVM IR test case for this issue attached to this patch:
> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150202/257276.html
> 
> -Tom
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory