llvm dev - May 2018 - Query on unswitching + vectorization

Hi,

I am going through analysis on unswitching + vectorization.
For the below test, llvm unswitches successfully but fails to vectorize the loop
after unswitching.
Llvm bails out saying "Found an outside user" apparently which is the
value of 'tmp'.

int i, w, x[1000], y[1000],tmp;
void fn()
{
  for (i = 0; i < 1000; i++) {
    if (w==1) {
        y[i] = 1; tmp = i*2;
    }
    else if (w==2) {
        y[i] = 2; tmp = i*4;
    }
    x[i] += y[i] + tmp;
  }
}

GCC vectorizes the loop after unswitching/if conversion. Request your help in
vectorizing this loop.

Regards
Ganesh

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On 5/10/2018 10:44 PM, Gopalasubramanian, Ganesh via llvm-dev
wrote:
>
> Hi,
>
> I am going through analysis on unswitching + vectorization.
>
> For the below test, llvm unswitches successfully but fails to 
> vectorize the loop after unswitching.
>
> Llvm bails out saying “Found an outside user” apparently which is the 
> value of ‘tmp’.
>
> int i, w, x[1000], y[1000],tmp;
>
> void fn()
>
> {
>
>   for (i = 0; i < 1000; i++) {
>
>     if (w==1) {
>
>         y[i] = 1; tmp = i*2;
>
>     }
>
>     else if (w==2) {
>
>         y[i] = 2; tmp = i*4;
>
>     }
>
>     x[i] += y[i] + tmp;
>
>   }
>
> }
>
> GCC vectorizes the loop after unswitching/if conversion. Request your 
> help in vectorizing this loop.
>
Looks like some sort of pass ordering issue; it will vectorize if 
indvars runs sometime between loop unswitch and the vectorizer.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
Foundation Collaborative Project

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*   Looks like some sort of pass ordering issue; it will vectorize if indvars
runs sometime between loop unswitch and the vectorizer.

That insight is helpful. I scheduled Canonicalization of induction variable
before loop vectorization and could get the loop vectorized.
The indvars are heavily dependent on SCEV. If there a scalar like tmp which is
of real type, we may not be able to get the indvars sorted out because of the
integer type limitation in SCEV!
Is there a way out when we have a scalar float computation that is dependent on
induction variable?
-Ganesh


From: Friedman, Eli [mailto:efriedma at codeaurora.org]
Sent: Saturday, May 12, 2018 12:36 AM
To: Gopalasubramanian, Ganesh <Ganesh.Gopalasubramanian at amd.com>;
llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Query on unswitching + vectorization

On 5/10/2018 10:44 PM, Gopalasubramanian, Ganesh via llvm-dev wrote:
Hi,

I am going through analysis on unswitching + vectorization.
For the below test, llvm unswitches successfully but fails to vectorize the loop
after unswitching.
Llvm bails out saying "Found an outside user" apparently which is the
value of 'tmp'.

int i, w, x[1000], y[1000],tmp;
void fn()
{
  for (i = 0; i < 1000; i++) {
    if (w==1) {
        y[i] = 1; tmp = i*2;
    }
    else if (w==2) {
        y[i] = 2; tmp = i*4;
    }
    x[i] += y[i] + tmp;
  }
}

GCC vectorizes the loop after unswitching/if conversion. Request your help in
vectorizing this loop.


Looks like some sort of pass ordering issue; it will vectorize if indvars runs
sometime between loop unswitch and the vectorizer.

-Eli



--

Employee of Qualcomm Innovation Center, Inc.

Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
Foundation Collaborative Project
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