[PATCH] D22696: [CodeGenPrep] Skip merging empty case blocks

[Jun Bum Lim via llvm-commits]

junbuml added a comment.

> On second thought, considering the cost of a direct branch, it is probably better to set the default frequency ratio to be >=2 . Also the check of number of phis should >probably a 'OR' instead of 'AND'. The default value of MinNumPhiInDestToSkipMerge should be 2:
> if (Freq(Pred) >= FreqRatio*Freq(BB) || NumCopyInsertionPHIs > MinNumPhis)
> 
>   return true;

Thanks David for the review. I ran spec2000/2006 with the change you suggested. Most of the score changes seems to be noise, but I can see -2% reproducible regression in spec2006/povray when we are aggressive in this (i.e., the default frequency ratio to be >=2). I can see more I-cache miss with "FreqRatio=2" in spec2006/povray. It seems that it skips merging empty cases even for a switch with small number of cases  and the cost of branch is non-trivial especially when it mess up the I-cache.

I think we need to be more conservative for the frequency ratio so that we skip merging only when we are sure that the empty case is less frequently executed.


https://reviews.llvm.org/D22696