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

[Tim Northover via llvm-commits]

On 18 Aug 2016, at 12:10, Quentin Colombet <qcolombet at apple.com> wrote:
> We were discussing this offline and came to the conclusion that G_TRUNC is a special case of G_EXTRACT.
> In other words, we wouldn’t need that instruction.

I think it's different for vectors (I considered reusing it at the time).

    %trunc = G_EXTRACT { <4 x i16>, <4 x i32> } %src

would extract the low 64-bits of the %src reg.

It could probably be represented as a G_EXTRACT followed by a G_SEQUENCE though:

   %e0, %e1, %e2, %e3 = G_EXTRACT {s16, s16, s16, s16, <4 x s32> } %src, 0, 32, 48, 64
   %vec = G_SEQUENCE { <4 x s16>, s16, s16, s16, s16 } %e0, %e1, %e2, %e3

I'm not entirely convinced it would be an improvement though. Depends on how orthogonal we want these things to be.

> 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.