Re: missing conditional propagation in cprop.c pass

["Amker.Cheng" <amker.cheng@gmail.com>]

>>
>> I believe, the optimization you may be referring to is value range
>> propagation which does predication of values based on predicates of
>> conditions. GCC definitely applies VRP at the tree stage, I am not
>> sure if there is an RTL pass to do the same.
> There are also RTL optimizers which perform this kind of constant
> propagation.  See cprop.c (in older versions of gcc this code was in
> gcse.c)
>
Hi Jeff,
This is exactly what I referred in the first message.
Though the cprop.c pass collected the implicit_set information, it is recorded
as local info of basic block, and cprop only does global propagation.
The result is such conditional const propagation opportunities is missed.

The whole process in cprop pass is like:

bb0 : if (x)
then
  bb1
else
  bb2
end

1, implicit_set from the preceding bb0 is tagged as local in bb1;
2, in compute_local_properties, the implicit_set is recorded in avloc[bb1];
3, in compute_cprop_available, the implicit_set is only recorded in avout[bb1],
    not in avin[bb1], which it should be;
4, in cprop_insn and find_avail_set, only info recorded in avin[bb1]
is considered
    when try to do propagation for bb1;

Well, I believe it is a small problem, since implicit_set is recorded
in avout[bb1],
The basic block bb1 is the only one get missed in propagation.

Don't know if I described the problem clearly and please comment.

Thanks very much.

-- 
Best Regards.