[llvm-dev] RFC: A change in InstCombine canonical form

[Ehsan Amiri via llvm-dev]

James,

I think (1) reduces the number of "do-not-see-through-bitcast" bugs that we
need to uncover and fix between now and the time that typeless pointer is
available. That means it is likely that we have multiple such fixes in the
code and then we have to remove each one of them. (And means each one of
those has to be done properly to be easily remove-able).

Changing canonicaliztion of memcpy, will be removing a couple of lines of
code. I am not sure about the size of backend changes to optimize
load-store patterns. But I expect it to be small.


On Tue, Mar 22, 2016 at 6:58 PM, James Y Knight <jyknight at google.com> wrote:

>
>
> On Tue, Mar 22, 2016 at 5:44 PM, Ehsan Amiri via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Thanks.
>>
>> *Phillip, *As Hal said I do not think (1) is a very large item. Please
>> let me know if I am mistaken.
>>
>> *David *I think (1) is more inline with typeless pointer work than (2).
>> Contributing to typeless pointer work will be great, but given its unknown
>> time frame we cannot stop fixing existing problems. Of course, we should
>> follow an approach consistent with the long-term solution.
>>
>
> It seems to me that the question to ask is what would be the best state of
> the code, assuming that the typeless pointers work had already been done.
> Is it the current canonical form? Or the newly proposed one?
>
> I think it'd be the current one? If so, I'd suggest that proposal #2 is
> more compatible with the typeless pointer work. That is because (if done
> properly), code which knows how to look through pointer bitcast nodes is
> something which will be much more easily deletable once pointer bitcast
> nodes cease to exist, than to change the canonicalization of memcpy back
> and remove any backend code which was added only to compensate for that
> change.
>
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