[LLVMdev] [Discussion] Portable SIMD programming using LLVM?

[Kevin Huang]

Right. LLVM supports the transformation from unsupported vector type to
supported vector type or even scalar types.
According to the LLVM's documentation, LLVM has two ways of doing the
transformation, one is promoting the small vector type to larger vector
type, the other is breaking up the large vector type into smaller ones.
But I am wondering, how good the transformation will be in LLVM's
implementation.

In my approach, I use strategies to capture all possible transformation. The
transformation are not only just promoting and breaking up, but also other
forms of transformation, like the transformation between vector types with
same size.
My goal is to make sure the simulation to be as good as possible in terms of
the number of instructions, CPU latency, etc. If LLVM has information about
the latency information of supported architectures, maybe I can use them to
optimize my approach.

Does LLVM have the latency information(such as CPU cycles) of SIMD
instructions for each supported architecture?

Thanks.

On Fri, Apr 1, 2011 at 2:30 AM, Duncan Sands <baldrick at free.fr> wrote:

> Hi Kevin,
>
> > *Background*
> > Almost modern processor families support SIMD instruction sets but the
> > instruction set designs for each platform have different combinations of
> > operations. The portable SIMD here is to make an uniform system of SIMD
> > operations at all power-of-2 field widths.
> > For example, for simd_add on SSE2, I want to have all the following
> operations
> > supported, simd<2^x>::add(a, b) for 2<=2^x<=register_size(register_size
> equals
> > 128 in SSE2). However, SSE2 only supports simd_add on field widths of 8,
> 16, 32,
> > or 64. Hence, simulation for simd_add on field widths of 2, 4 and 128 is
> needed.
> > *At mean time, I want the simulation to be as good as possible*.
>
> LLVM already does this: if you use a vector type that is not natively
> supported
> then operations on it are simulated using either vector operations in
> another
> vector type or scalar operations if no suitable vector type exists.
>
> Ciao, Duncan.
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