[llvm-dev] Not able to vectorize struct members

[Ryan Taylor via llvm-dev]

I see Roman has commented on the ticket. The issue is alias analysis it
looks like, we have a partial fix in place in BasicAliasAnalysis.cpp:

    unsigned MinNumIndex = std::min(GEP1->getNumIndices(),
                                    GEP2->getNumIndices());
    if (MinNumIndex < 2)
      return MayAlias;
    SmallVector<Value *, 8> IntermediateIndices;
    IntermediateIndices.push_back(GEP1->getOperand(1));
    // Note,
    // 1. The index to struct field can only be literal constant.
    // 2. So far all the struct field indexes are the same respectively.
    // 3. The array indexes could be different, but they don't matter here.
    // 4. The types should be the same, say, different array indexes don't
    //    matter, and the struct indexes be the same respectively.
    // If current type is StructType, and the pair of corresponding indexes
    // are not equal, NoAlias
    for (unsigned i = 1; i != MinNumIndex - 1; ++i) {
      if (isa<StructType>(GetElementPtrInst::getIndexedType(
              GEP1->getSourceElementType(), IntermediateIndices))) {
        ConstantInt *C1 = dyn_cast<ConstantInt>(GEP1->getOperand(i + 1));
        ConstantInt *C2 = dyn_cast<ConstantInt>(GEP2->getOperand(i + 1));
        if (C1 && C2 && C1->getSExtValue() != C2->getSExtValue())
          return NoAlias;
      }
      IntermediateIndices.push_back(GEP1->getOperand(i + 1));
    }
  }
  // Note that we fall back the original logic here, there are still some
thing
  // not covered like p->a[2].x v.s. p->a[1].x.b[2]

but this seems to break some cases and isn't generalized.

-Ryan

On Thu, Jul 15, 2021 at 5:05 AM Simon Pilgrim via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I've reported this here: https://bugs.llvm.org/show_bug.cgi?id=51103
> On 15/07/2021 04:04, Ryan Taylor via llvm-dev wrote:
>
> Given a test case:
>
> #define N 10
> struct foo {
>   float a[N][N];
>   float b[N];
> };
>
> void compute(struct foo *p)
> {
>   for (int i = 0; i < N; i++) {
>     for (int j = 0; j < N; j++) {
>       p->a[i][j] += 0.1f * p->b[i];
>     }
>   }
> }
>
> LLVM isn't able to vectorize this code. I'm not sure what the issue is
> here as it seems to vectorize this code:
>
> #define N 10
> struct foo {
>   float a[N];
>   float b[N];
> };
>
> void compute(struct foo *p)
> {
>   for (int i = 0; i < N; i++) {
>       p->a[i] += 0.1f * p->b[i];
>   }
> }
>
> Is this a known issue?
>
> -Ryan
>
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