[llvm-dev] Handling misaligned array accesses

[Anna Thomas via llvm-dev]

Hi,

I have tried couple of c test cases with llvm to see if we handle misaligned accesses, but it seems we do not have transformations to align loop accesses. Misaligned accesses can worsen performance depending on the underlying target (severity of crossing cache line boundaries)

One example:
//unaligned load and store
int foo(short *a, int m){
  int i;
  for(i=1; i<m ; i++)
    a[i] *=2;
  return a[3];
}

IR generated though clang -O3 -mllvm -disable-llvm-optzns. Passed this through opt -O3 and the loop vectorizer adds vector code for this loop, but the GEP access starts at offset 1.

vector.body:                                      ; preds = %vector.body, %vector.body.preheader.new
  %index = phi i64 [ 0, %vector.body.preheader.new ], [ %index.next.3, %vector.body ]
  %niter = phi i64 [ %unroll_iter, %vector.body.preheader.new ], [ %niter.nsub.3, %vector.body ]
  %offset.idx = or i64 %index, 1
  %9 = getelementptr inbounds i16, i16* %a, i64 %offset.idx
  %10 = bitcast i16* %9 to <8 x i16>*
  %wide.load = load <8 x i16>, <8 x i16>* %10, align 2, !tbaa !2
  %11 = shl <8 x i16> %wide.load, <i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1>
  %12 = bitcast i16* %9 to <8 x i16>*
  store <8 x i16> %11, <8 x i16>* %12, align 2, !tbaa !2

Is there a reason we don’t support loop peeling for alignment handling?

Thanks,
Anna

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