PowerPC has its custom scheduler heuristic. It calls parent classes'
tryCandidate in override version, but the function returns void, so this
way doesn't actually help. This patch duplicates code from base scheduler
into PPC machine scheduler class, which does what we wanted.
Reviewed By: steven.zhang
Differential Revision: https://reviews.llvm.org/D94464
tryLatency compares two sched candidates. For the top zone it prefers
the one with lesser depth, but only if that depth is greater than the
total latency of the instructions we've already scheduled -- otherwise
its latency would be hidden and there would be no stall.
Unfortunately it only tests the depth of one of the candidates. This can
lead to situations where the TopDepthReduce heuristic does not kick in,
but a lower priority heuristic chooses the other candidate, whose depth
*is* greater than the already scheduled latency, which causes a stall.
The fix is to apply the heuristic if the depth of *either* candidate is
greater than the already scheduled latency.
All this also applies to the BotHeightReduce heuristic in the bottom
zone.
Differential Revision: https://reviews.llvm.org/D72392
Qiu Chaofan