xref: /third_party/vk-gl-cts/doc/testspecs/GLES2/performance.shaders.operator.txt

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drawElements Quality Program Test Specification
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    Shader operator performance tests

Tests:
 + dEQP-GLES2.performance.shaders.operator.*

Includes:
 + Arithmetic operators in vertex and fragment shaders
   - Scalar and vector types
 + Computation-only built-in functions

Excludes:
 + Texture lookup built-in functions
   - Covered in performance.texture.

Description:

Each test case draws multiple times with different workload sizes. A workload
size means the iteration count of a uniform loop in the shader. Time for each
frame is measured, and the slope of the workload size vs frame time data is
estimated. This slope tells us the estimated increase in frame time caused by
a workload increase of 1 loop iteration.

Generally, the shaders contain not just the operation we're interested in (e.g.
addition) but also some other things (e.g. loop overhead). To eliminate this
cost, we actually do the measurements described above paragraph with two
programs, which contain different amounts of computation in the loop. Then we
can compute the cost of just one operation by appropriately subtracting the
estimated slopes, and dividing by the operation count difference between the
two programs.

At this point, the result tells us the increase in frame time caused by the
addition of one operation. Dividing this by the amount of draw calls in a frame,
and further by the amount of vertices or fragments in a draw call, we get the
time cost of one operation.

In reality, there sometimes isn't just a trivial linear dependence between
workload size and frame time. Instead, there tends to be some amount of initial
"free" operations. That is, it may be that all workload sizes below some number
C yield the same frame time, and only workload sizes beyond C increase the frame
time in a supposedly linear manner. Graphically, this means that there graph
consists of two parts: a horizontal left part, and a linearly increasing right
part; the right part starts where the left parts ends. The principal task of
these tests is to look at the slope of the increasing right part. Additionally
an estimate for the amount of initial free operations is calculated. Note that
it is also normal to get graphs where the horizontal left part is of zero width,
i.e. there are no free operations.

Note that the tests use several fixed constants, such as the extent to which the
loops in the shaders are unrolled. These may not be the most suitable for all
platforms, and can be modified near the top of es2pShaderOperatorTests.cpp .

The unit of the test result is millions of operations per second.

See performance.txt for more details on shader performance testing.