How to use macros to replace/inject function calls in OpenCL - macros

Question

I am developing an algorithm using PyOpenCL. To avoid code duplication I am trying to use templating along with C macros to replace function calls, since OpenCL 1.2 does not support function pointers.
I currently have the following macro section in my OpenCL kernel code:
#define LINEAR_FIT_SEARCH_METHOD ${linear_fit_search_method}
#if LINEAR_FIT_SEARCH_METHOD == MIN_MAX_INTENSITY_SEARCH
#define LINEAR_FIT_SEARCH_METHOD_CALL() determineFitUsingMinMaxIntensitySearch(lineIntensities,imgSizeY,linFitParameter,linFitSearchRangeXvalues)
#elif LINEAR_FIT_SEARCH_METHOD == MAX_INCLINE_SEARCH
#define LINEAR_FIT_SEARCH_METHOD_CALL() determineFitUsingInclineSearch(lineIntensities,imgSizeY,linFitParameter,linFitSearchRangeXvalues,inclineRefinementRange)
#endif
In the kernel code I also define the corresponding functions determineFitUsingMinMaxIntensitySearch and determineFitUsingInclineSearch. I am now attempting to use the macro to exchange the function call like this:
__private struct linearFitResultStruct fitResult = LINEAR_FIT_SEARCH_METHOD_CALL();
so that I select the desired call (note: I always only need either one or the other and configuration is done before the program runs (no need for dynamically switching the two)).
Using PyOpenCL templating I now do something like this:
def applyTemplating(self):
tpl = Template(self.kernelString)
if self.positioningMethod == "maximumIntensityIncline":
linear_fit_search_method="MAX_INCLINE_SEARCH"
if self.positioningMethod == "meanIntensityIntercept":
linear_fit_search_method="MIN_MAX_INTENSITY_SEARCH"
rendered_tpl = tpl.render(linear_fit_search_method=linear_fit_search_method)
self.kernelString=str(rendered_tpl)
Where self.kernelString contains the macro above along with the code.
Unfortunately I am getting this error, which I do not understand:
1:455:53: error: implicit declaration of function 'determineFitUsingInclineSearch' is invalid in OpenCL
1:9:41: note: expanded from macro 'LINEAR_FIT_SEARCH_METHOD_CALL'
1:455:41: error: initializing 'struct linearFitResultStruct' with an expression of incompatible type 'int'
1:536:30: error: conflicting types for 'determineFitUsingInclineSearch'
1:455:53: note: previous implicit declaration is here
1:9:41: note: expanded from macro 'LINEAR_FIT_SEARCH_METHOD_CALL'
1:616:41: error: initializing 'struct linearFitResultStruct' with an expression of incompatible type 'int'
I have very little experience with macros so:
Is what I am attempting even possible in this way or do I need to go a different route?
UPDATE 1:
This code runs fine when I set self.positioningMethod = "meanIntensityIntercept" in my unit test, but fails when setting self.positioningMethod = "maximumIntensityIncline" with the error message above. I cannot spot the error at the yet.
UPDATE 2:
I was also inspired by this post, if that helps:
how to compare string in C conditional preprocessor-directives

Answer 1

As you say you have very little experience with macros then I would go for something simple. determineFitUsingMinMaxIntensitySearch and determineFitUsingInclineSearch accept different number of arguments, so this could done this way:
kernel_code = """
#ifdef USE_FUNCTION_A
void function_a(
int x,
int y,
int extra_param,
__global const int* restrict in,
__global int* restrict out
)
{
//...
}
#else
void function_b(
int x,
int y,
__global const int* restrict in,
__global int* restrict out
)
{
//...
}
#endif
__kernel void my_kernel(
int x,
int y,
__global const int* restrict in,
__global int* restrict out
)
{
// ...
#ifdef USE_FUNCTION_A
function_a(x,y,5,in,out);
#else
function_b(x,y,in,out);
#endif
// ...
}
"""
if use_function_a:
prg = cl.Program(ctx, kernel_code).build("-DUSE_FUNCTION_A")
else:
prg = cl.Program(ctx, kernel_code).build("")