David Sherwood 44031ae79f [LV] Fix issue in VPFirstOrderRecurrencePHIRecipe::usesFirstLaneOnly (#179977) ...

In some cases we decide to vectorise loops with first-order recurrences
using VF=1, IC>1. We then attempt to unroll a vplan in replicateByVF,
however when trying to erase the list of values from the parent we
trigger the following assert:

```
virtual llvm::VPRecipeValue::~VPRecipeValue(): Assertion `Users.empty()
  && "trying to delete a VPRecipeValue with remaining users"' failed.
```

The problem seems to stem from this code:

```
  DefR->replaceUsesWithIf(LaneDefs[0], [DefR](VPUser &U, unsigned) {
    return U.usesFirstLaneOnly(DefR);
  });
```

since usesFirstLaneOnly returns false and we fail to replace uses of
DefR with LaneDefs[0]. Upon inspection the only VPUser objects that
return false are VPInstruction::FirstOrderRecurrenceSplice and
VPFirstOrderRecurrencePHIRecipe. Since the values are all scalar it's
simply not possible for us to be using anything other than the first
lane. I've fixed this by bailing out of replicateByVF early for plans with
only a scalar VF.

Fixes https://github.com/llvm/llvm-project/issues/179671

2026-02-09 13:42:26 +00:00

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