[LLVMdev] Haswell New Instructions

[David A. Greene]

Chris Lattner <clattner at apple.com> writes:

> On Jun 13, 2011, at 6:48 AM, Dan Gohman wrote:
>
>     On Jun 13, 2011, at 4:41 AM, Nicolas Capens wrote:
>
>         So I was wondering whether in LLVM a gather operation is best
>         represented with a 'load' instruction taking vector operands,
>         or whether it's better to define it as a separate 'gather'
>         instruction. What would be the pros and cons of each approach,
>         and what do you think should be the long-term goals for the
>         LLVM instruction set?
>
>     Lots of parts of LLVM "know" about loads, and would be quite broken if
>     loads could suddenly be gathers.

What are the problem areas?  It seems natural to me to extend loads to
be able to take vectors of offsets and/or pointers.

>     Also, autovectorizers have to know a fair amount about target
>     instruction sets, especially if they're going to vectorize
>     non-trivial things like gathers.

True.

>     So target-specific intrinsics seem like a reasonable start.  A
>     generic 'gather' instruction/intrinsic may make sense at some
>     point, if it can be designed cleanly enough.
>
> Right, the natural evolution of LLVM IR is that things start out as
> intrinsics, then eventually get built into the instruction set if it
> makes sense.  This is how unaligned loads were handled early on, for
> example.

Yes, intrinsics make perfect sense for the moment.

                                    -Dave