[PATCH] D31488: [SimplifyIndvar] Replace the sdiv used by IV if we can prove both of its operands are non-negative

[Tobias Grosser via Phabricator via llvm-commits]

grosser added a comment.

Hi Hongbin,

I am not sure what you are trying to achieve here. This kernel is already optimized by -instcombine to

  define void @test(i32* %a) {
  entry:
    br label %for.body
  
  for.body:                                         ; preds = %for.body, %entry
    %i.01 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
    %div = shl nsw i32 %i.01, 5
    %idxprom = sext i32 %div to i64
    %arrayidx = getelementptr inbounds i32, i32* %a, i64 %idxprom
    store i32 %i.01, i32* %arrayidx, align 4
    %inc = add nsw i32 %i.01, 1
    %cmp = icmp slt i32 %i.01, 63
    br i1 %cmp, label %for.body, label %for.end
  
  for.end:                                          ; preds = %for.body
    ret void
  }

So the sdiv is replaced by a lsh, taking into account the information that sdiv is working on unsigned inputs only.

If you divide by a non-power-of-two, instcombine does not change the code and leaves the sdiv in. However, if you would like to move to an sdiv even in this case this should probably be added to instcombine.

I would also be interested to learn why an 'udiv' is a better canonical form than an 'sdiv'. Is it faster to execute (non-power-of-two) udivs on FPGAs than sdivs?


Repository:
  rL LLVM

https://reviews.llvm.org/D31488