llvm dev - Feb 2010 - [LLVMdev] Illegal operations generated in ShrinkDemandedOps()

The architecture that I'm targeting with an LLVM back end has SRA
instructions for all its data sizes, but is missing SRL instructions for
some data sizes. 

I'm having trouble with the SelectionDAGISel::ShrinkDemandedOps() pass.
Although it runs after the DAG has been legalized, it replaces legal
SRAs with illegal SRLs, which results in failures in instruction
selection. 

Is there a standard trick to work around this problem?

-Ken

Hi Ken,

On Tue, Feb 9, 2010 at 3:19 PM, Ken Dyck <Ken.Dyck at onsemi.com>
wrote:
> The architecture that I'm targeting with an LLVM back end has SRA
> instructions for all its data sizes, but is missing SRL instructions for
> some data sizes.
>
> I'm having trouble with the SelectionDAGISel::ShrinkDemandedOps() pass.
> Although it runs after the DAG has been legalized, it replaces legal
> SRAs with illegal SRLs, which results in failures in instruction
> selection.
>
> Is there a standard trick to work around this problem?
I ran into the same issue with my backend (which doesn't have a native
SRL instruction) and opened http://llvm.org/bugs/show_bug.cgi?id=5374
some time ago. I've been using the patch from the PR for a few months
now without any problems.

Cheers,

Tilmann