When using the LLVM flang compiler with alias analysis (AA) enabled,
SPEC2017:548.exchange2_r was running significantly slower than wihtout
the AA.
This was caused by the GVN pass replacing many of the loads in the
pre-AA code with phi-nodes that form a long chain of dependencies, which
the function specialization was unable to follow.
This adds a function to discover phi-nodes in a transitive set, with
some limitations to avoid spending ages analysing phi-nodes.
The minimum latency savings also had to be lowered - fewer load
instructions means less saving.
Adding some more prints to help debugging the isProfitable decision.
No significant change in compile time or generated code-size.
(A previous attempt to fix this was abandoned: https://github.com/llvm/llvm-project/pull/71442)
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Co-authored-by: Alexandros Lamprineas <alexandros.lamprineas@arm.com>
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