[LoopUnroll] Keep UP.Count when considering complete unroll

[zzheng]

Summary

r286389 updates UP.Count unconditionally when considering complete unroll of loop with compile-time known TripCount.
This can overwrite UP.Count retrieved by getUnrollingPreferences().

[evstupac]

I agree that r286389 have changed behavior.
However to prevent this in future would you mind to add a test case?

[zzheng]

In D27911#628167, @evstupac wrote:

However to prevent this in future would you mind to add a test case?

This issue was found in our in-house branch with custom TTI::getUnrollingPreferences()... not easy to produce a test case.

[evstupac]

lib/Transforms/Scalar/LoopUnrollPass.cpp

785

Will it work if we put UP.Count change under FullUnrollTripCount condition?
If we leave the change as is it could happen that for full unroll UP.Count is less than MaxTripCount. Which is not ok.
Also could you please add context to the patch:
svn diff --diff-cmd=diff -x -U999999

[zzheng]

Moved UP.Count = FullUnrollTripCount under condition as suggested by Evgeny.

[haicheng]

lib/Transforms/Scalar/LoopUnrollPass.cpp

761

I think you need to set UP.Count in this situation, too.

[haicheng]

lib/Transforms/Scalar/LoopUnrollPass.cpp

761

Sorry, never mind.

[evstupac]

This issue was found in our in-house branch with custom TTI::getUnrollingPreferences()... not easy to produce a test case.

It looks like no one from trunk targets has UP.Count set in Unroll Preferences. So to add a test we'll need to add an option setting UP.Count default.

LGTM if all tests are passed.

lib/Transforms/Scalar/LoopUnrollPass.cpp

   assert((ExactTripCount == 0 || MaxTripCount == 0) &&
          "ExtractTripCound and MaxTripCount cannot both be non zero.");
   unsigned FullUnrollTripCount = ExactTripCount ? ExactTripCount : MaxTripCount;
-  UP.Count = FullUnrollTripCount;
+  UP.Count = UP.Count ? UP.Count : FullUnrollTripCount;
   if (FullUnrollTripCount && FullUnrollTripCount <= UP.FullUnrollMaxCount) {
     // When computing the unrolled size, note that BEInsns are not replicated
     // like the rest of the loop body.