Developer Guide


Strategies for Inferring the Accumulator

To leverage the single cycle floating-point accumulator feature, you can modify the accumulator description in your kernel code to improve efficiency or work around programming restrictions.

Describe an Accumulator Using Multiple Loops

Consider a case where you want to describe an accumulator using multiple loops, with some of the loops being unrolled:
float acc = 0.0f; for (i = 0; i < k; i++) { #pragma unroll for (j = 0; j < 16; j++) acc += (x[i+j]*y[i+j]); }
With fast math enabled by default, the
Intel® oneAPI
automatically rearranges operations in a way that exposes the accumulation.

Modify a Multi-Loop Accumulator Description

In cases where you cannot compile an accumulator description using the 
 compiler command option, rewrite the code to expose the accumulation.
For the code example above, rewrite it in the following manner:
float acc = 0.0f; for (i = 0; i < k; i++) { float my_dot = 0.0f; #pragma unroll for (j = 0; j < 16; j++) my_dot += (x[i+j]*y[i+j]); acc += my_dot; }

Modify an Accumulator Description Containing a Variable or Non-Zero Initial Value

Consider a situation where you might want to apply an offset to a description of an accumulator that begins with a non-zero value:
float acc = array[0]; for (i = 0; i < k; i++) { acc += x[i]; }
Because the accumulator hardware does not support variable or non-zero initial values in a description, you must rewrite the description.
float acc = 0.0f; for (i = 0; i < k; i++) { acc += x[i]; } acc += array[0];
Rewriting the description in the above manner enables the kernel to use an accumulator in a loop. The loop structure is then followed by an increment of 

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