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

[Amara Emerson via llvm-dev]

Proposed clarification here: https://reviews.llvm.org/D82034

> On Jun 17, 2020, at 5:52 AM, Simon Pilgrim via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> A minor point, but I think we need to more explicitly describe the order of floating point operations in the LangRef as well:
> 
> "If the intrinsic call has the ‘reassoc’ or ‘fast’ flags set, then the reduction will not preserve the associativity of an equivalent scalarized counterpart. Otherwise the reduction will be ordered, thus implying that the operation respects the associativity of a scalarized reduction."
> 
> Please could we add some pseudocode to show exactly how the intrinsic will be re-expanded for ordered cases?
> 
> On 16/06/2020 19:38, Sanjay Patel via llvm-dev wrote:
>> We switched over to producing the intrinsics for x86 with:
>> https://reviews.llvm.org/rGe50059f6b6b3 <https://reviews.llvm.org/rGe50059f6b6b3>
>> ...I'm not aware of any regressions yet.
>> 
>> https://bugs.llvm.org/show_bug.cgi?id=45378 <https://bugs.llvm.org/show_bug.cgi?id=45378> is also fixed as of today.
>> 
>> So that leaves the problem with fmin/fmax when no fast-math-flags are specified. We need to update the LangRef with whatever the expected behavior is for NaN and -0.0. 
>> x86 will probably be poor regardless of whether we choose "llvm.maxnum" or "llvm.maximum" semantics.
>> 
>> On Thu, Apr 9, 2020 at 1:28 PM Craig Topper via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> No we still use the shuffle expansion which is why the issue isn't unique to the intrinsic.
>> 
>> ~Craig
>> 
>> 
>> On Thu, Apr 9, 2020 at 10:21 AM Amara Emerson <aemerson at apple.com <mailto:aemerson at apple.com>> wrote:
>> Has x86 switched to the intrinsics now?
>> 
>>> On Apr 9, 2020, at 10:17 AM, Craig Topper <craig.topper at gmail.com <mailto:craig.topper at gmail.com>> wrote:
>>> 
>>> 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
>>> trap:
>>>   %v2 = extractelement <2 x i64> %vload, i32 1
>>>   call void @TrapFunc(i64 %v2)
>>>   ret void
>>> ret:
>>>   ret void
>>> }
>>> 
>>> ~Craig
>>> 
>>> 
>>> On Thu, Apr 9, 2020 at 10:04 AM Philip Reames via llvm-dev <llvm-dev at lists.llvm.org <mailto: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 <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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