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VGPRs as intermediate storage

Question asked by aazmp on Dec 21, 2013
Latest reply on Feb 14, 2014 by aazmp

how to prevent usage of VGPRs for storing intermediate data, that compiler find to be useful in future?




In my program each kernels stores 8x8 pixel(8bit) data to Local memory; pixels are packed into uchar4 by 4.

position of quadropixel is   array_in_LDS[get_local_size(0)*qpixel_num + get_local_id(0)]


At some time i place them in local memory and after some time load from local memory.

between these two times compiler stores each index in VGPR => 16 vgprs are used in this example


There is a workaround:

if at store time calculate index differently, for example mad24(get_local_size(0), qpixel_num, get_local_id(0)),

then no additional VGPRs are used


Calculating index is ~5 ticks or even 1 with mad24, while VGPRs are always expensive.

Are there any solutions to prevent storing such easy calculated data without fooling compiler? Maybe some directive or some option to clBuildProgram?