[Beignet] a question about default optimize option when building

[Zhigang Gong]

I just found a non-standard performance hint keyword __restrict__ is supported by CLANG.
You could try it this way

__global unsigned char __restrict__ *p;
 int a, b, c, d;

res1 = *p * (a*b + c*d);
<some code here >
res2 = *p * (a*b + c*d + 1);


Then the compiler will treat pointer p pointing to an unique chunk of memory which could
only be accessed by p. Then if there is no storing to pointer p between the res1 and res2
assignment, the res2 will not generate an extra load.

Please have a try at your kernel and check the difference by enable the LLVM output as below:

export OCL_OUTPUT_LLVM_AFTER_GEN=1

Please be warned, this is not a portable way even it works for beignet, may not work with other OpenCL library.

> -----Original Message-----
> From: Beignet [mailto:beignet-bounces at lists.freedesktop.org] On Behalf Of
> Zhigang Gong
> Sent: Thursday, February 12, 2015 4:57 PM
> To: 彭席汉
> Cc: 褰腑姹?peng_xihan at dahuatec; beignet at lists.freedesktop.org; Song,
> Ruiling
> Subject: Re: [Beignet] a question about default optimize option when building
> 
> Athough theoretically this is doable for OpenCL, if user could guarantee all the
> buffers are not overlapped, then the compiler could do more aggresive
> optimization to merge duplicate loads.
> 
> But you can check the OpenCL C spec or C99 spec, there are no such type of
> attribute qualifiers defined, so there is no safe/portable way to do this type of
> optimization from compiler side currently.
> 
> On Thu, Feb 12, 2015 at 05:32:08PM +0800, 彭席汉 wrote:
> > Any way to tell compiler I have no side effect for memory pointer p and I don't
> want to load it again?
> >
> >
> >
> >
> > 彭席汉
> > 2015-02-12
> >
> >
> >
> > 发件人： Zhigang Gong
> > 发送时间： 2015-02-12 17:14:36
> > 收件人： Song, Ruiling
> > 抄送： beignet at lists.freedesktop.org; 彭席汿
> peng_xihan at dahuatech.com>
> > 主题： Re: [Beignet] a question about default optimize option when
> > building
> >
> > Some additional analysis based on ruiling's comment.
> >
> > The second load from p(to calculate res2) may or may not be issued.
> > It depends on whether there are some side effect instructions between
> > res1 and res2's assignment. For example, if there is a store
> > instruction or there is a barrier, the second load will be issued and
> > you will see two loads for the same pointer in the final instruction stream.
> >
> > As to the a * b + c*d, it will always be optimized and be reused when
> > calculate for res2 which means at the res2 assignment it will only
> > generate one add instruction to add the 1 to the previous calculated
> > value.
> >
> > On Thu, Feb 12, 2015 at 08:56:57AM +0000, Song, Ruiling wrote:
> > > It should not read global memory again. We already enable such kind of
> optimization pass in LLVM.
> > > And (a*b+c*d) should not calculate again. This is common-subexpression.
> Clang should do it easily. But I am not quite sure whether clang is affected by
> -O2 or -O0. Anyone know details?
> > >
> > > To check specific kernel. You may need to ‘export
> OCL_OUTPUT_LLVM_AFTER_GEN=1’ and build your program again to get the
> LLVM IR.
> > >
> > > From: Beignet [mailto:beignet-bounces at lists.freedesktop.org] On
> > > Behalf Of 彭席汉
> > > Sent: Thursday, February 12, 2015 4:40 PM
> > > To: beignet at lists.freedesktop.org
> > > Subject: [Beignet] a question about default optimize option when
> > > building
> > >
> > > Hi:
> > >
> > > My CL kernel program looks like as follow:
> > >
> > > __global unsigned char *p;
> > > int a, b, c, d;
> > >
> > > res1 = *p * (a*b + c*d);
> > >
> > >  <some code here >
> > >
> > > res2 = *p * (a*b + c*d + 1);
> > >
> > >
> > > If I use default build option, for res2, what will EU do? read global memory
> for pointer p again and do computing of "a*b + c*d" again?
> >
> > > _______________________________________________
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> >
> >
> >
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