[llvm-dev] ShuffleKind SK_ExtractSubvector
Friedman, Eli via llvm-dev
llvm-dev at lists.llvm.org
Fri Feb 10 17:59:27 PST 2017
On 2/8/2017 11:00 PM, Jonas Paulsson wrote:
> On 2017-02-08 20:10, Friedman, Eli wrote:
>> On 2/8/2017 6:35 AM, Jonas Paulsson via llvm-dev wrote:
>>> I am a little unsure about the semantics of the ShuffleKind
>>> SK_ExtractSubvector. It seems a subvector is to be extracted,
>>> starting from a given index of a given subtype.
>>> First of all, if index 0 is passed, I suppose this would mean a noop?
>>> But what about calls like the one made of LoopVectorizer for
>>> Instruction::PHI in getInstructionCost():
>>> return TTI.getShuffleCost(TargetTransformInfo::SK_ExtractSubvector,
>>> VectorTy, VF - 1, VectorTy);
>>> Here the highest index is passed, which doesn't make sense to me.
>>> Nor does it make sense to pass the the same VectorTy in both
>>> In BBVectorize, start index 0 is passed in one place, but then in
>>> another place start index of 'VF' is passed, which should even be
>>> outside possible indexes, or?
>>> I guess this is confusing since there are those extra parameters and
>>> everything, but in the end it seems to me there is no code anywhere
>>> checking this particular ShuffleKind, or?
>> I think the intent is to match the rules for EXTRACT_SUBVECTOR in
>> isel (hence the name). But, as you've noted, no in-tree targets are
>> actually using it, so we can redefine it to use whatever definition
>> is convenient.
> I suppose it would make sense to follow the ISD::EXTRACT_SUBVECTOR
> rules, then.
> The indexes are passed currently in three places, and they are 0, VF -
> 1, and VF, where certainly the last wouldn't be legal, and the second
> would hardly make sense. I am not sure what the context is in the
> three cases this is used, but I could imagine that passing VF or VF -
> 1 should mean "the highest valid index" of the subvector type.
Some targets have other special-cases which are important; for example,
extracting the high half of a 128-bit vector on ARM is free.
> It seems the line in LoopVectorizer, which extracts a subvector of
> same type as the original type should be fixed, but I am not sure
> exactly how.
> On SystemZ, extracting from index 0 is a noop. If a vector type gets
> split is used as input, and the higher part is extracted, that should
> also be a noop.
> I don't know if this is true for all targets, but if it was this could
> be handled in common code.
It's not always a no-op because not all targets widen small vectors; we
promote to a wider integer type instead in some cases.
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