llvm dev - Dec 2015 - Question about __builtin_assume()

void test_copy_vec(const short* restrict src, short* restrict res, int N) {
 __builtin_assume( (N > 1) && (N%2 == 0) );
  #pragma clang loop vectorize(enable) vectorize_width(2) interleave_count(1)
  for (int j=0; j<N; ++j)
    *res++ = *src++;
}

If I use __builtin_assume(N>1) then llvm knows the loop will execute and not
check for (j <= 0), but I can't seem to get it to accept N is even. Is
there a way to get llvm to vectorize the loop and not generate the additional
scalar loop conditions?

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It looks like we've never added V % C == C2 in 
computeKnownBitsFromAssume.  This would be a simple patch to add if 
you're interested in fixing the compiler to handle this case.

You might also get this to work by using N & 0x1 == 0.  It looks like we 
do handle that case.  If that doesn't work, it probably means the 
vectorizer isn't asking the right questions here.

Philip

On 12/22/2015 02:34 PM, Mark Schimmel via llvm-dev
wrote:
>
> void test_copy_vec(const short* restrict src, short* restrict res, int 
> N) {
>
>  __builtin_assume( (N > 1) && (N%2 == 0) );
>
>   #pragma clang loop vectorize(enable) vectorize_width(2) 
> interleave_count(1)
>
>   for (int j=0; j<N; ++j)
>
>     *res++ = *src++;
>
> }
>
> If I use __builtin_assume(N>1) then llvm knows the loop will execute 
> and not check for (j <= 0), but I can’t seem to get it to accept N is 
> even. Is there a way to get llvm to vectorize the loop and not 
> generate the additional scalar loop conditions?
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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Hi Philip, Mark,

To add to this two comments:

 1. Since the vectorizer is using SCEV here to do the expression simplification,
it is possible that SCEV is what needs to be enhanced in this case (along with,
or in addition to, computeKnownBitsFromAssume) because SCEV also independently
searches for dominating assumes.

 2. You can tell the vectorizer that the loop is safe to vectorize, without any
runtime checks, by using 'vectorize(assume_safety)' (instead of just
'vectorize(enable)').

 -Hal

----- Original Message -----
> From: "Philip Reames via llvm-dev" <llvm-dev at
lists.llvm.org>
> To: "Mark Schimmel" <Mark.Schimmel at synopsys.com>,
llvm-dev at lists.llvm.org
> Sent: Tuesday, December 22, 2015 5:52:23 PM
> Subject: Re: [llvm-dev] Question about __builtin_assume()
> 
> 
> It looks like we've never added V % C == C2 in
> computeKnownBitsFromAssume. This would be a simple patch to add if
> you're interested in fixing the compiler to handle this case.
> 
> You might also get this to work by using N & 0x1 == 0. It looks like
> we do handle that case. If that doesn't work, it probably means the
> vectorizer isn't asking the right questions here.
> 
> Philip
> 
> 
> On 12/22/2015 02:34 PM, Mark Schimmel via llvm-dev wrote:
> 
> 
> void test_copy_vec(const short* restrict src, short* restrict res,
> int N) {
> 
> __builtin_assume( (N > 1) && (N%2 == 0) );
> 
> #pragma clang loop vectorize(enable) vectorize_width(2)
> interleave_count(1)
> 
> for (int j=0; j<N; ++j)
> 
> *res++ = *src++;
> 
> }
> 
> 
> 
> If I use __builtin_assume(N>1) then llvm knows the loop will execute
> and not check for (j <= 0), but I can’t seem to get it to accept N
> is even. Is there a way to get llvm to vectorize the loop and not
> generate the additional scalar loop conditions?
> 
> 
> 
> _______________________________________________
> LLVM Developers mailing list llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory