[llvm] r216678 - Do not introduce new shuffle patterns after operation legalization if SHUFFLE_VECTOR

[Chandler Carruth]

On Thu, Aug 28, 2014 at 11:04 AM, Chandler Carruth <chandlerc at google.com>
wrote:

> On Thu, Aug 28, 2014 at 10:49 AM, Owen Anderson <resistor at mac.com> wrote:
>
>> Do not introduce new shuffle patterns after operation legalization if
>> SHUFFLE_VECTOR
>> was marked custom.  The target independent DAG combine has no way to know
>> if
>> the shuffles it is introducing are ones that the target could support or
>> not.
>>
>
> Test case? This seems somewhat arbitrary otherwise...


(To be completely clear, in case this email wasn't, I understand that
currently not all backends support a custom lowering of all shuffles....
but the x86 backend actually does support it (for legal types) and so it
seems quite likely that someone could accidentally change this back unless
we have more checking in the backend to back up the assertion that we can't
form an arbitrary shuffle here. Essentially, I'm fine with the patch, but
I'm worried that I or someone else could easily re-break this in the future
unless we have some test coverage.)
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