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Murali, Sriram

2013-Sep-27 16:41 UTC

[LLVMdev] Trip count and Loop Vectorizer

Hi,
I am trying to get a small loop to *not vectorize* for cases where it
doesn't make sense. For instance, this loop:
void foo(int a[4][8], int n)
{
    int b[4][8];
    for(int i = 0; i < 4; i++) {
        for(int j = 0; j < n; j++) {
            a[i][j] = b[i][j];
        }
    }
}
* Has maximum of 8ints copy. LLVM tries to use Memcpy for the inner loop. It is
not helpful to perform memcpy for such small moves, especially when the outer
loop is unrolled since the trip count is constant (4). The 4 calls to memcpy is
not efficient.
* Therefore, I disabled the memcpy optimization for such cases, and found that
LLVM  LoopVectorizer successfully vectorizes and unrolls the inner loop.
However, in order to take the fast path (vmovups) it must copy at least 32 ints,
where as in this case we only do an 8int copy.
** Upon closer look, LoopVectorizer obtains the TripCount for the innerloop
using getSmallConstantTripCount(Loop,...). This value is 0 for the loop with
unknown trip count. Loop unrolling is disabled when TC > 0. Should this be
changed to TC >= 0 (which does the job for this testcase)? Or is there a
better way to disable loop unrolling for such trivial loops, at least the ones
with known array size?

Thanks for your feedback

Sriram

--
Sriram Murali
SSG/DPD/ECDL/DMP
+1 (519) 772 - 2579

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Nadav Rotem

2013-Sep-27 17:02 UTC

head link

[LLVMdev] Trip count and Loop Vectorizer

Hi Sriram, 

Thanks for performing this analysis. The problem here, both for memcpy and the
vectorizer, is that we can’t predict the size of “n”, even though the only use
of ’n’ is for the loop bound for the alloca [4 x [8 x i32]]. If you change the
unroll condition to TC >= 0 then you will disable loop unrolling for all
loops because getSmallConstantTripCount returns an unsigned number. You can
control the unroll factor using metadata or using the command line tools.

Thanks,
Nadav


On Sep 27, 2013, at 9:41 AM, Murali, Sriram <sriram.murali at intel.com>
wrote:
> Hi,
> I am trying to get a small loop to *not vectorize* for cases where it
doesn’t make sense. For instance, this loop:
> void foo(int a[4][8], int n)
> {   
>     int b[4][8];
>     for(int i = 0; i < 4; i++) {
>         for(int j = 0; j < n; j++) {
>             a[i][j] = b[i][j];           
>         }
>     }
> }
> * Has maximum of 8ints copy. LLVM tries to use Memcpy for the inner loop.
It is not helpful to perform memcpy for such small moves, especially when the
outer loop is unrolled since the trip count is constant (4). The 4 calls to
memcpy is not efficient.
> * Therefore, I disabled the memcpy optimization for such cases, and found
that LLVM  LoopVectorizer successfully vectorizes and unrolls the inner loop.
However, in order to take the fast path (vmovups) it must copy at least 32 ints,
where as in this case we only do an 8int copy.
> ** Upon closer look, LoopVectorizer obtains the TripCount for the innerloop
using getSmallConstantTripCount(Loop,…). This value is 0 for the loop with
unknown trip count. Loop unrolling is disabled when TC > 0. Should this be
changed to TC >= 0 (which does the job for this testcase)? Or is there a
better way to disable loop unrolling for such trivial loops, at least the ones
with known array size?
>  
> Thanks for your feedback
>  
> Sriram
>  
> --
> Sriram Murali
> SSG/DPD/ECDL/DMP
> +1 (519) 772 – 2579
>  
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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Murali, Sriram

2013-Sep-27 17:17 UTC

head link

[LLVMdev] Trip count and Loop Vectorizer

Hi Nadav,
Thanks for the response. I forgot to mention that there is an upper limit of 16
for the Trip Count check,
TinyTripCountVectorThreshold = 16;
if (TC > 0u && TC < TinyTripCountVectorThreshold). So right now,
any loop with Trip Count as 0, or with  value >=16, LV with unroll. With the
change to the lower bound, it will also include the loop with 0 trip count.
SCEV returns 0 trip count for this case, because it identifies that there is no
backedge taken.

ScalarEvolution::ComputeExitLimitFromCond () {
...
 if (L->contains(FBB) == !CI->getZExtValue())
   { }
else
   // The backedge is never taken.
   return getConstant(CI->getType(), 0);
}
From: Nadav Rotem [mailto:nrotem at apple.com]
Sent: Friday, September 27, 2013 1:03 PM
To: Murali, Sriram
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Trip count and Loop Vectorizer

Hi Sriram,

Thanks for performing this analysis. The problem here, both for memcpy and the
vectorizer, is that we can't predict the size of "n", even though
the only use of 'n' is for the loop bound for the alloca [4 x [8 x
i32]]. If you change the unroll condition to TC >= 0 then you will disable
loop unrolling for all loops because getSmallConstantTripCount returns an
unsigned number. You can control the unroll factor using metadata or using the
command line tools.

Thanks,
Nadav


On Sep 27, 2013, at 9:41 AM, Murali, Sriram <sriram.murali at
intel.com<mailto:sriram.murali at intel.com>> wrote:


Hi,
I am trying to get a small loop to *not vectorize* for cases where it
doesn't make sense. For instance, this loop:
void foo(int a[4][8], int n)
{
    int b[4][8];
    for(int i = 0; i < 4; i++) {
        for(int j = 0; j < n; j++) {
            a[i][j] = b[i][j];
        }
    }
}
* Has maximum of 8ints copy. LLVM tries to use Memcpy for the inner loop. It is
not helpful to perform memcpy for such small moves, especially when the outer
loop is unrolled since the trip count is constant (4). The 4 calls to memcpy is
not efficient.
* Therefore, I disabled the memcpy optimization for such cases, and found that
LLVM  LoopVectorizer successfully vectorizes and unrolls the inner loop.
However, in order to take the fast path (vmovups) it must copy at least 32 ints,
where as in this case we only do an 8int copy.
** Upon closer look, LoopVectorizer obtains the TripCount for the innerloop
using getSmallConstantTripCount(Loop,...). This value is 0 for the loop with
unknown trip count. Loop unrolling is disabled when TC > 0. Should this be
changed to TC >= 0 (which does the job for this testcase)? Or is there a
better way to disable loop unrolling for such trivial loops, at least the ones
with known array size?

Thanks for your feedback

Sriram

--
Sriram Murali
SSG/DPD/ECDL/DMP
+1 (519) 772 - 2579

_______________________________________________
LLVM Developers mailing list
LLVMdev at cs.uiuc.edu<mailto:LLVMdev at cs.uiuc.edu>        
http://llvm.cs.uiuc.edu<http://llvm.cs.uiuc.edu/>
http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

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