[llvm] r277749 - GlobalISel: also add G_TRUNC to IRTranslator.

[Matt Arsenault via llvm-commits]

On 08/18/2016 01:33 PM, Tim Northover via llvm-commits wrote:
>> This would be good for us. We currently have to deal with a lot of DAG combines switching between trunc and vector extracts
> I don't quite follow this. They seem pretty incompatible even in the SDag world.
>
> Tim.

We don't really have vectors or truncates, and the DAG type system is 
just an obstacle. We just have accesses of 32-bit subregisters. Various 
integer operations are lowered by bitcasting to vectors and manipulating 
the 32-bit elements.  Sometimes this breaks other combines, usually 
computeKnownBits sorts of things, when the truncate is really something 
like extract_vector_elt (v2i32 bitcast i64:x), 0


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