> AFAIK not one of the tree optimizers disables itself, but perhaps we
> should. The obvious candidates would be the ones that require
> recomputation of alias analysis, and the ones that don't update SSA
> info on the fly (i.e. require update_ssa, which is a horrible compile
> time hog).

Tree alias analysis can partially disable itself though:

  /* If the program has too many call-clobbered variables and/or function
     calls, create .GLOBAL_VAR and use it to model call-clobbering
     semantics at call sites.  This reduces the number of virtual operands
     considerably, improving compile times at the expense of lost
     aliasing precision.  */
  maybe_create_global_var (ai);

We have found this to be quite helpful on gigantic elaboration procedures 
generated for Ada packages instantiating gazillions of generics.  We have 
actually lowered the threshold locally.

-- 
Eric Botcazou

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