[Beignet] a question about default optimize option when building

[Zhigang Gong]

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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