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Re: [PATCH, rtl-optimization]: Fix post-reload compare elimination pre-pass


On Sat, Feb 11, 2012 at 11:01 PM, Richard Henderson <rth@redhat.com> wrote:

>>>> ? ? ? * compare-elim.c (find_comparisons_in_bb): Eliminate only compares
>>>> ? ? ? having mode compatible with the mode of previous compare. ?Substitute
>>>> ? ? ? compare mode of previous compare with the mode, compatible
>>>> ? ? ? with eliminated and previous compare.
>>>
>>> This patch is ok for 4.8.
>>
>> Unfortunately, we need to update all uses of flag register with a new,
>> compatible mode, as well, similar to how compatible mode is handled in
>> CSE2 pass with cse_condition_code_reg in cse.c
>
> We do? ?What subsequent pass really cares?

Yes, please see an example in  [1] how CSE2 pass handles this.

I don't know if this is necessary, but when we merge arithmetic insn
with compare, we _do_ update the flags users, in the same way as CSE2
pass.

> What goes wrong leaving things as they are?

x86 tolerates wrong modes in flags users, so as far as x86 is
concerned, this is allowed. But I don't know if wrong, although
compatible modes, should be generated from a generic pass.

[1] http://gcc.gnu.org/ml/gcc-patches/2012-02/msg00547.htm

Uros.


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