vrp?

[Roger Sayle]

Hi Richard,
> The current cse code marks (x != 0) true on the else branch of the
> first if.  But our current gcse code has no way to make (x = NOT(0))
> available on an edge.

My thoughts on this were to extend GCSE/PRE to handle exactly that
and provide an inequality table for propagation into conditional
jumps.  This would also allow us to combine the "delete NULL pointer
checks" pass with GCSE, removing both .null and .cse passes from GCC.

For a description of how this might work, see the comment above
delete_null_pointer_checks in gcse.c.  To quote from that comment:

> This could probably be integrated with global cprop with a little work.

Roger
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