[llvm-dev] RFC: Promoting experimental reduction intrinsics to first class intrinsics

Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 9 10:17:10 PDT 2020

That recent X86 bug isn't unique to the intrinsic. We generate the same
code from this which uses the shuffle sequence the vectorizers generated
before the reduction intrinsics existed.

declare i64 @llvm.experimental.vector.reduce.or.v2i64(<2 x i64>)·
declare void @TrapFunc(i64)

define void @parseHeaders(i64 * %ptr) {
  %vptr = bitcast i64 * %ptr to <2 x i64> *
  %vload = load <2 x i64>, <2 x i64> * %vptr, align 8

  %b = shufflevector <2 x i64> %vload, <2 x i64> undef, <2 x i32> <i32 1,
i32 undef>
  %c = or <2 x i64> %vload, %b
  %vreduce = extractelement <2 x i64> %c, i32 0

  %vcheck = icmp eq i64 %vreduce, 0
  br i1 %vcheck, label %ret, label %trap
  %v2 = extractelement <2 x i64> %vload, i32 1
  call void @TrapFunc(i64 %v2)
  ret void
  ret void


On Thu, Apr 9, 2020 at 10:04 AM Philip Reames via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> My experience with them so far is that the code generation for these
> intrinsics is still missing a lot of cases.  Some of them are X86
> specific (the target I look at mostly), but many of them have generic
> forms.
> As one recent example, consider
> https://bugs.llvm.org/show_bug.cgi?id=45378.  (There's nothing special
> about this one other than it was recent.)
> I'm not necessarily arguing they can't be promoted from experimental,
> but it would be a much easier case if the code gen was routinely as good
> or better than the scalar forms.  Or to say that a bit differently, if
> we could canonicalize to them in the IR without major regression.
> Having two ways to represent something in the IR without any agreed upon
> canonical form is always sub-optimal.
> Philip
> On 4/7/20 9:59 PM, Amara Emerson via llvm-dev wrote:
> > Hi,
> >
> > It’s been a few years now since I added some intrinsics for doing vector
> reductions. We’ve been using them exclusively on AArch64, and I’ve seen
> some traffic a while ago on list for other targets too. Sander did some
> work last year to refine the semantics after some discussion.
> >
> > Are we at the point where we can drop the “experimental” from the name?
> IMO all target should begin to transition to using these as the preferred
> representation for reductions. But for now, I’m only proposing the naming
> change.
> >
> > Cheers,
> > Amara
> > _______________________________________________
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> > llvm-dev at lists.llvm.org
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