[LLVMdev] LowerPacked pass

[Morten Ofstad]

Chris Lattner wrote:
> Note that packed support in LLVM is not complete yet.  In
> particular, here are some of the big missing pieces:
> 
> 1. No code generators can generate vector instructions yet (SSE or
>    altivec, for example).  This should be fairly easy to add though.
> 2. The lowerpacked pass, which currently converts packed ops into their
>    scalar counterparts, has a few limitations:
>      A. It does not handle packed arguments to functions
>      B. It always lowers all of the way to scalar ops, even if the target
>         supports SOME packed types.  For example, it would be nice for it
>         to eventually lower <16 x float> into 4 <4 x float>'s if the
>         target supports them.

>      C. It has never been thoroughly tested, primarily because we don't
>         have a producer of packed operations yet.  I believe it should
>         work reasonably well though.

It works reasonably well, quite impressive really considering it's not 
been tested ;-) B is not much of a problem for my use, but A is a bit 
annoying even though I mostly pass pointers to packed types anyway. Can 
you elaborate a bit on what is the problem with this? I have calls going 
back into our code by adding mappings to the JIT, but I'm not sure if I 
can get it to call functions with R32x4 (<float x 4>) args without 
making a wrapper that takes a pointer.

> For your work, it might be most expedient to just ignore the lower packed
> pass and add SSE support to the X86 backend: that will get you up and
> running quickly and get you the performance you are obviously after.  If
> backwards compatibility with old hardware is an issue, revisiting the
> lower packed pass would make sense.

Is it easy to add intrinsics to do things like dot product of packed 
types using SSE instructions? That's probably all I need...

> Let me know what you think.  In the very short term, the hook exposed to
> create the lower packed pass can be plunked into the X86TargetMachine and
> get intra function packed types working for you.

The patch you did was missing the actual implementation of 
createLowerPackedPass, so I'm including my own differences -- I guess 
you don't want to apply the changes to X86TargetMachine as I'm the only 
one actually generating packed types, but I include it for completeness..

m.

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