[LLVMdev] loop vectorizer issue

[Sara Elshobaky]

Actually what I meant in my original loop, that there is a dependency
between every two consecutive iterations. So, how the loop vectorizer says
'we can vectorize this loop'?

for(int k=20;k<50;k++)

     dataY[k] = dataY[k-1];

 

 

From: Henrique Santos [mailto:henrique.nazare.santos at gmail.com] 
Sent: Sunday, November 03, 2013 4:28 PM
To: Sara Elshobaky
Cc: <llvmdev at cs.uiuc.edu>
Subject: Re: [LLVMdev] loop vectorizer issue

 

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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