For some specific cases with bitcast A->B->A with intervening PHI nodes InstCombiner::optimizeBitCastFromPhi transformation creates extra PHI nodes, which are actually a copy of already created PHI or in another words, they are redundant. These extra PHI nodes could lead to extra move instructions generated after DeSSA transformation. This happens when several conditions are met
- SROA kicks in and creates new alloca;
- there is a simple assignment L = R, which falls under 'canonicalize loads' done by combineLoadToOperationType (this transformation is by default). Exactly this transformation is the reason of bitcasts generated;
- the alloca is then used in A->B->A + PHI chain;
- there is a loop unrolling.
As a result optimizeBitCastFromPhi creates as many of PHI nodes for each new SROA alloca as loop unrolling factor is. These new extra PHI nodes are redundant actually except of one and should not be created. Moreover the idea of optimizeBitCastFromPhi is to get rid of the cast (when possible) but that doesn't happen in these conditions.
The proposed fix is to do the cast replacement for the whole calculated/accumulated PHI closure not for one cast only, which is an argument to the optimizeBitCastFromPhi. These will help to accomplish several things: 1) avoid extra PHI nodes generated as all casts which may trigger optimizeBitCastFromPhi transformation will be replaced, 3) bitcasts will be replaced, and 3) create more opportunities to remove dead code, which appears after the replacement.
A new test case shows that it's possible to get rid of all bitcasts completely and get quite good code reduction.