Commits on Apr 4, 2023

1. JIT: use synthesis to repair some reconstruction issues …

   In particular, run synthesis in repair mode for cases where there are profile
   counts within the method but zero counts in `fgFirstBB`.
   
   Recall that sparse profiling effectively probes return blocks to determine the
   method entry count.
   
   So the zero-entry but not zero-everywhere case can happen if we have a method
   with a very long running loop plus sparse profiling plus OSR -- we will only
   get profile counts from the instrumented Tier0 method, and it will never return
   (instead it will always escape to an OSR version which will eventually return,
   but that version won't be instrumented).
   
   I originally was a bit more ambitious and ran repair for a broader set of
   reconstruction issues, but lead to a large number of diffs, in part because
   repair doesn't cope well with irreducible loops.
   
   Leaving the entry count zero can have fairly disastrous impact on the quality
   of optimizations done in the method.
   
   Addresses quite a few of the worst-performing benchmarks in dotnet#84264.

   

   AndyAyersMS committed Apr 4, 2023
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