[PATCH] D50480: [LV] Vectorizing loops of arbitrary trip count without remainder under opt for size

[Hideki Saito via Phabricator via llvm-commits]

hsaito added inline comments.


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Comment at: lib/Transforms/Vectorize/VPlan.h:609
   /// VPlan opcodes, extending LLVM IR with idiomatics instructions.
-  enum { Not = Instruction::OtherOpsEnd + 1 };
+  enum { Not = Instruction::OtherOpsEnd + 1, ICmpULE };
 
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Ayal wrote:
> dcaballe wrote:
> > I'm worried that this new opcode could be problematic since now we can have compare instructions represented as VPInstructions with Instruction::ICmp and Instruction::FCmp opcodes and VPInstructions with VPInstruction::ICmpULE. Internally, we have a VPCmpInst subclass to model I/FCmp opcodes and their predicates. Do you think it would be better to upstream that subclass first? 
> An alternative of leveraging `Instruction::ICmp` opcode and existing `ICmpInst` subclasses for keeping the Predicate, in a scalable way, could be (devised jointly w/ Gil):
> 
> ```
> +    // Introduce the early-exit compare IV <= BTC to form header block mask.
> +    // This is used instead of IV < TC because TC may wrap, unlike BTC.
> +    VPValue *IV = Plan->getVPValue(Legal->getPrimaryInduction());
> +    VPValue *BTC = Plan->getBackedgeTakenCount();
> +    Value *Undef = UndefValue::get(Legal->getPrimaryInduction()->getType());
> +    auto *ICmp = new ICmpInst(ICmpInst::ICMP_ULE, Undef, Undef);
> +    Plan->addDetachedValue(ICmp);
> +    BlockMask = Builder.createNaryOp(Instruction::ICmp, {IV, BTC}, ICmp);
>      return BlockMaskCache[BB] = BlockMask;
> ```
> 
> and then have `VPInstruction::generateInstruction()` do
> 
> ```
> +  case Instruction::ICmp: {
> +    Value *IV = State.get(getOperand(0), Part);
> +    Value *TC = State.get(getOperand(1), Part);
> +    auto *ICmp = cast<ICmpInst>(getUnderlyingValue());
> +    Value *V = Builder.CreateICmp(ICmp->getPredicate(), IV, TC);
> +    State.set(this, V, Part);
> +    break;
> +  }
> ```
> 
> where `VPlan::addDetachedValue()` is used for disposal purposes only. This has a minor (acceptable?) impact on the underlying IR: it creates/adds-users to `UndefValue`'s.
Pros/cons are easier to discuss with the code in hand. Diego, would you be able to upload the subclassing in Phabricator?

The alternative by Ayal/Gil works only because the VPlan modeling is done very late in the vectorization process. That'll make it very hard to move the modeling towards the beginning of vectorization. Please don't do that.

My preference is to be able to templatize VPInstruction and Instruction as much as feasible. Is that easier with subclassing? 


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Comment at: lib/Transforms/Vectorize/VPlan.h:1126
+  VPlan(VPBlockBase *Entry = nullptr) : Entry(Entry) {
+    BackedgeTakenCount = new VPValue();
+  }
----------------
Ayal wrote:
> dcaballe wrote:
> > Instead of using an "empty" VPValue to model the BTC, would it be possible to model the actual operations to compute the BTC? We would only need a sub, right?
> The BTC is computed by subtracting 1 from the Trip Count, which in turn is generated by SCEVExpander. To model this decrement would require using an "empty" VPValue to model its Trip Count operand. In any case, both involve scalar instructions that take place before the vectorized loop, currently outside the VPlan'd zone.
I'm not a big fan of allocating memory that goes unused in many situations. We can initialize this to nullptr, and create an instance once we know BTC is needed. That'll lose the convenience of being able to check NumUsers, but creating needsBackedgeTakenCount() member function shouldn't be that bad. It's just Legal->foldTailByMasking(), until something else needs BTC, right? 



Repository:
  rL LLVM

https://reviews.llvm.org/D50480