[LLVMdev] loop vectorizer issue

[Henrique Santos]

Notice that the code you provided, for globals and stack allocations, at
least,
is semantically equivalent to:

  int a = d[19];
  for(int k = 20; k < 50; k++)
    dataY[k] = a;

Like so, the load you see missing was redundant, probably hoisted by
GVN/PRE
and replaced with "%.pre".

H.



On Sun, Nov 3, 2013 at 11:26 AM, Sara Elshobaky <sara.elshobaky at gmail.com>wrote:

>  Hello,
>
> I was trying to trace the Loop vectorizer of the LLVM, I wrote a simple
> loop with a clear dependency.
>
> But found that the debug shows that ‘we can vectorize this loop’
>
>
>
> Here you are my  loop with dependency:
>
> for(int k=20;k<50;k++)
>
>      dataY[k] = dataY[k-1];
>
>
>
> And the debug prints:
>
> LV: Checking a loop in "main"
>
> LV: Found a loop: for.body4
>
> LV: Found an induction variable.
>
> LV: Found a write-only loop!
>
> LV: We can vectorize this loop!
>
> ...
>
> LV: Vectorization is possible but not beneficial.
>
>
>
> From the LLVM IR, it contains only one ‘store’ instruction with ‘%.pre’.
> Seems that no ‘load’ instruction prevented the Vectorizer to detect
> dependency.
>
> Is that a bug, or I’m missing something? Please advice
>
>
>
> for.body4:                                        ; preds = %for.body4,
> %for.cond2.preheader
>
>   %k.030 = phi i32 [ 20, %for.cond2.preheader ], [ %inc8, %for.body4 ]
>
>   %arrayidx6 = getelementptr inbounds i32* %0, i32 %k.030
>
>   store i32 %.pre, i32* %arrayidx6, align 4, !tbaa !0
>
>   %inc8 = add nsw i32 %k.030, 1
>
>   %exitcond32 = icmp eq i32 %inc8, 50
>
>   br i1 %exitcond32, label %for.cond10.preheader, label %for.body4
>
>
>
>
>
> Thanks in advance,
>
> Sara Elshobaky
>
>
>
>
>
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