llvm dev - Jun 2020 - How to inhibit jump table range check

I am working on a back end for an architecture whose jump via table instruction 
includes the range check.  Therefore I don't need the range check generated
(in
SelectionDAGBuilder ?).

There appears to be a struct JumpTableHeader which contains a field 
OmitRangeCheck which, when true, does what I want.  However, there doesn't 
appear to be a mechanism for a back end to set it.

Anyone have any ideas?

Thanks,
brian

Hi Brian,

I actually solved this exact problem for the upstream WebAssembly backend
in the last couple of weeks. I was originally going to use the
OmitRangeCheck field, but after prompting from my reviewers on
https://reviews.llvm.org/D80778 I ended up removing the range checks in the
backend instead. You can see the pass I added to do that here:
https://github.com/llvm/llvm-project/blob/master/llvm/lib/Target/WebAssembly/WebAssemblyFixBrTableDefaults.cpp
I
think it probably wouldn't be too hard to adapt that pass to a different
backend, but feel free to reach out if you have any questions about it!

Best,

Thomas

On Fri, Jun 19, 2020 at 12:55 PM Bagel via llvm-dev <llvm-dev at
lists.llvm.org>
wrote:
> I am working on a back end for an architecture whose jump via table
> instruction
> includes the range check.  Therefore I don't need the range check
> generated (in
> SelectionDAGBuilder ?).
>
> There appears to be a struct JumpTableHeader which contains a field
> OmitRangeCheck which, when true, does what I want.  However, there
doesn't
> appear to be a mechanism for a back end to set it.
>
> Anyone have any ideas?
>
> Thanks,
> brian
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Thank you for your prompt reply.  I will take a look at what you have done and 
hopefully it will work for me.  I would have preferred to eliminate the range 
check at the point where it was generated instead of adding a pass later to 
eliminate it.  But I guess we have to live with what ever the implementers of 
the two major architectures allow us to do.

Regards,
brian

On 6/20/20 12:52 AM, Thomas Lively wrote:
> Hi Brian,
> 
> I actually solved this exact problem for the upstream WebAssembly backend
in
> the last couple of weeks. I was originally going to use the OmitRangeCheck 
> field, but after prompting from my reviewers on 
> https://reviews.llvm.org/D80778 I ended up removing the range checks in the
> backend instead. You can see the pass I added to do that here: 
>
https://github.com/llvm/llvm-project/blob/master/llvm/lib/Target/WebAssembly/WebAssemblyFixBrTableDefaults.cpp I
> think it probably wouldn't be too hard to adapt that pass to a
different
> backend, but feel free to reach out if you have any questions about it!
> 
> Best,
> 
> Thomas
> 
> On Fri, Jun 19, 2020 at 12:55 PM Bagel via llvm-dev <llvm-dev at
lists.llvm.org
> <mailto:llvm-dev at lists.llvm.org>> wrote:
> 
>     I am working on a back end for an architecture whose jump via table
>     instruction
>     includes the range check.  Therefore I don't need the range check
>     generated (in
>     SelectionDAGBuilder ?).
> 
>     There appears to be a struct JumpTableHeader which contains a field
>     OmitRangeCheck which, when true, does what I want.  However, there
doesn't
>     appear to be a mechanism for a back end to set it.
> 
>     Anyone have any ideas?
> 
>     Thanks,
>     brian
>     _______________________________________________
>     LLVM Developers mailing list
>     llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>     https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>