[llvm-dev] Is it feasible for LV to vectorize a loop accessing A[i] using VF>2 when A has only 2 elements?

[Björn Pettersson A via llvm-dev]

Hi!

Consider an example like this

int G[1000];

void gazonk(int N) {
    int A[2] = {0};
    for (int i = 0; i < N; ++i)
      G[i] = A[i] + i;
}


When compiling with "-O3 -emit-llvm" ( see https://godbolt.org/z/f6jWfM )
the loop is vectorized with VF=4 and we get

  %2 = alloca i64, align 8
  %3 = bitcast i64* %2 to [2 x i32]*
  ...
 vector.body:
  ...
  %24 = getelementptr inbounds [2 x i32], [2 x i32]* %3, i64 0, i64 %21, !dbg !35
  %25 = bitcast i32* %24 to <4 x i32>*, !dbg !35
  %26 = load <4 x i32>, <4 x i32>* %25, align 8, !dbg !35, !tbaa !36
  ...


Loading <4 x i32> from something pointing into [2 x i32] seems like a bad thing (UB?).
And I believe that for example BasicAliasAnalysis will assume that the load won't alias with anything else since the size of the access is greater than the underlying object, so the code in the vector body is just crap afaict.

There are some loop guards that perhaps (hopefully) protects from running the vector body here,
but isn't it a bit weird thing to introduce such code anyway?

BR,
Björn