[llvm-dev] Handling misaligned array accesses

[Hal Finkel via llvm-dev]

----- Original Message -----
> From: "Anna Thomas" <anna at azul.com>
> To: llvm-dev at lists.llvm.org
> Cc: hfinkel at anl.gov, anemet at apple.com
> Sent: Thursday, May 12, 2016 4:20:24 PM
> Subject: Handling misaligned array accesses
> 
> 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?
> 

No. AFAIK, just no one has done the work to implement it yet. I'd certainly be quite interested in it, however. Several targets I care about would benefit from alignment-based peeling.

 -Hal

> 
> Thanks,
> 
> Anna
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory