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

[Hal Finkel]

----- Original Message -----
> From: "Owen Anderson" <resistor at mac.com>
> To: "Chandler Carruth" <chandlerc at google.com>
> Cc: "Commit Messages and Patches for LLVM" <llvm-commits at cs.uiuc.edu>
> Sent: Thursday, August 28, 2014 1:26:57 PM
> Subject: Re: [llvm] r216678 - Do not introduce new shuffle patterns after	operation legalization if SHUFFLE_VECTOR
> 
> On Aug 28, 2014, at 11:08 AM, Chandler Carruth < chandlerc at google.com
> > wrote:
> 
> 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.)

I don't understand; we have a isShuffleMaskLegal callback in TLI, why can't you use that?

 -Hal

> 
> The only testcase I have for this requires a not-in-tree target.
> 
> 
> —Owen
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory