[LLVMdev] Interesting post increment situation in DAG combiner

Sergei Larin slarin at codeaurora.org
Fri Mar 1 08:24:39 PST 2013

Hal, (and everyone who might care about post increment generation)...

I have an interesting question/observation. Consider this vector loop.

void vec_add_const(unsigned N, short __attribute__ ((aligned (16))) *A,
                                  short __attribute__ ((aligned (16))) val)
 unsigned i,j;
 for (i=0; i<N; i++) {
  for (j=0; j<N; j++) {
   A[i*N+j] += val;

The innermost loop looks like this right before the DAG selection begins.

  p.loop_body.us65:                             ; preds =
%p.loop_body.lr.ph.us78, %p.loop_body.us65
  %p_arrayidx.us69.phi = phi i16* [ %p_arrayidx.us69.gep,
%p.loop_body.lr.ph.us78 ], [ %p_arrayidx.us69.inc, %p.loop_body.us65 ]
  %p.loopiv48.us66 = phi i32 [ 0, %p.loop_body.lr.ph.us78 ], [
%p.next_loopiv.us67, %p.loop_body.us65 ]
  %vector_ptr.us70 = bitcast i16* %p_arrayidx.us69.phi to <4 x i16>*
  %p.next_loopiv.us67 = add nsw i32 %p.loopiv48.us66, 4  <<<<<<<<<<<<<<<<<<
  %_p_vec_full.us71 = load <4 x i16>* %vector_ptr.us70, align 16
  %add5p_vec.us72 = add <4 x i16> %_p_vec_full.us71, %5
  store <4 x i16> %add5p_vec.us72, <4 x i16>* %vector_ptr.us70, align 16
  %p_arrayidx.us69.inc = getelementptr i16* %p_arrayidx.us69.phi, i32 4
<<<<<<<<<<<<<<< Common Ptr
  %11 = icmp slt i32 %p.next_loopiv.us67, %leftover_lb
  br i1 %11, label %p.loop_body.us65, label %p.loop_header38.preheader.us84

When it gets to the DAG Combiner, in CombineToPostIndexedLoadStore() two
opportunities for post inc are recognized - the load and the store.
Now, you can easily see that in this case you would want the store to get
the post inc, not the load, but since the DAG combiner simply scans
top-down, the opposite happens.

  So here is the question - do you recognize this as a deficiency, and can
you see the same in PPC? The fix is code trivial, but it would introduce a
general concept of a primitive cost function to the PostInc candidacy
selection in DAG combine. If you recognize the issue, I will post a patch
with more details, but if I am missing the big picture here, please advise.


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