[Bug tree-optimization/34244] [4.3 Regression] VRP/SCEV miscompiles Firefox

["rakdver at kam dot mff dot cuni dot cz" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #7 from rakdver at kam dot mff dot cuni dot cz  2007-11-28 16:05 -------
Subject: Re:  [4.3 Regression] VRP/SCEV miscompiles Firefox

> ------- Comment #6 from rguenth at gcc dot gnu dot org  2007-11-28 15:47 -------
> We could clear the SCEV cache for an SSA_NAME we set a new value range
> (in set_value_range and set_value_range_to_varying), but I
> see that SCEV also caches loop->nb_iterations which we probably would need to
> clear unconditionally (for all loops).

yes, the simplest solution is to call scev_reset somewhere (either in
set_value_range, or in adjust_range_with_scev).  However, I fear the
compile time impact would be too large (# of iterations analysis is
fairly time consuming, and not caching the results used to cause
us to spend 10% or more in it).

More feasible solution seems to be to determine the number of iterations
for all loops before the start of VRP, and to remember the values.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34244