[PATCH] D116736: [Clang] Add __builtin_reduce_or and __builtin_reduce_and

[Jun Zhang via Phabricator via cfe-commits]

junaire added a comment.

In D116736#3230040 <https://reviews.llvm.org/D116736#3230040>, @fhahn wrote:

> LGTM, thanks!
>
>> The last __builtin_reduce_add will be separated into another one.
>
> Are you planning on putting up a patch for this one as well? What makes add a bit different is that `‘llvm.vector.reduce.fadd.*’` can only perform reductions either in the original order or in an unspecified order. For the extension, we need a particular evaluation order (reduction tree adding adjacent element pairs). Technically this order is required for all reduction builtins, but for integers the order doesn't matter, same for min/max.

Sorry about the late response. Yeah, I'm trying to work on this builtin too, but actually, I don't know if I can do this as all my previous work is like kind of boilerplate or something? I have read the whole discussion in the mailing list and the related LLVM IR reference, but I still get confused a little bit.

So the difference of this builtin is because LLVM intrinsic declare it like:

  declare float @llvm.vector.reduce.fadd.v4f32(float %start_value, <4 x float> %a)
  declare double @llvm.vector.reduce.fadd.v2f64(double %start_value, <2 x double> %a)

And it performs sequential reduction which is not what we want right? We need it to reduce like:

  [e3, e2, e1, e0] => (e3, e2) + (e1, e0)

Does it mean we should do something like a for loop? or like recursive calls? or like changing the order of the elements in the vector?

And another thing that confuses me is that `pad identity elements after the last element to widen the vector out to a power 2 `. According to the IR reference, is the neutral value just zero?

The last confusing point is `%start_value`, we just simply consider it is `0`, isn't it?

I would appreciate it if you can give me any hints, which I think is very helpful to my LLVM learning :-)


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D116736/new/

https://reviews.llvm.org/D116736