[PATCH] D121899: [LoopVectorize] Optimise away the icmp when tail-folding for some low trip counts

[Sander de Smalen via Phabricator via llvm-commits]

sdesmalen added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/VPlan.cpp:750-760
+    uint64_t TCVal = 0;
+    if (auto *TC2 = dyn_cast<ConstantInt>(TC))
+      TCVal = TC2->getZExtValue();
+
+    Value *Cond;
+    // When we know there will only be one vector iteration there is no need to
+    // create the comparison, since we already know the answer.
----------------
david-arm wrote:
> sdesmalen wrote:
> > nit: How about
> > 
> >   Value *ConstCmp = nullptr;
> >   if (auto *C = dyn_cast<ConstantInt>(TC))
> >     if (C->getZExtValue() <= State.UF * State.VF.getKnownMinValue())
> >       ConstCmp = Builder.getInt1(true);
> >   Value *Cond = ConstCmp ? ConstCmp : Builder.CreateICmpEQ(IV, VTC);
> It would have to be something like this I think:
> 
> ```Value *ConstCmp = nullptr;
> if (auto *C = dyn_cast<ConstantInt>(TC)) {
>   uint64_t TCVal = C->getZExtValue();
>   if (TCVal && TCVal  <= State.UF * State.VF.getKnownMinValue())
>     ConstCmp = Builder.getInt1(true);
> }
> Value *Cond = ConstCmp ? ConstCmp : Builder.CreateICmpEQ(IV, VTC);```
> 
> because I'm pretty sure I found a test where the trip count was actually defined as a zero constant. I'm happy to change it to the code above, but not sure it looks much better to be honest. :)
If the trip-count is zero, wouldn't `true` be the correct value for the condition?


Repository:
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  https://reviews.llvm.org/D121899/new/

https://reviews.llvm.org/D121899