[LLVMdev] Matching addsub

[Hal Finkel]

On Mon, 2011-10-17 at 17:33 -0700, Dan Gohman wrote:
> On Oct 17, 2011, at 3:40 PM, Hal Finkel wrote:
> 
> > How should I go about matching floating-point addsub-like vector
> > instructions? My first inclination is to write something which matches
> > build_vector 1.0, -1.0, and then use that in combination with a match on
> > fadd, but that does not seem to work. I think this is because
> > BUILD_VECTOR cannot ever be "Legal", and so it is always turned into a
> > constant load before instruction selection.
> 
> Trying to keep a vector producer this naive about the target instruction
> set is awkward. If the vector producer doesn't know whether the target
> has an addsub or subadd instruction, how is it to know the best way to
> expand complex multiplication? It's likely to get suboptimal code in many
> cases.
> 
> On the other hand, a producer that knows that the target has certain
> instructions could pretty easily just use appropriate intrinsics that can
> be mapped directly to the desired instructions. This way, there's no need
> for it to play pictionary with the backend, drawing out its desired
> semantics in terms of primitive operations and expecting codegen to
> rediscover what was meant by pattern-matching.
> 

In general, I agree with you. It is always questionable how far to
attempt to go with idiom recognition. However, some kind of combination
add/subtract is a common vector operation, and so I seem to be left with
three choices:

1. Decompose the operation and then attempt to recognize the
decomposition.
2. Add an additional LLVM instruction.
3. Add a number of target-specific special cases into the higher-level
code.

I am not sure which is better, but I'd prefer to stay away from choice
(3) as much as practical in favor of one of the first two options. Would
you support adding some kind of vector_faddsub LLVM instruction?

Also, there is a precedent, in some sense, for choices (1) and (2) in
how fneg %a is serialized (as fsub -0.0, %a). Perhaps we could recognize
fadd (fmul <-1.0, 1.0>), %b and turn it into something else for
instruction selection in a similar way.

 -Hal

> Dan
> 

-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory