How to calculate power index in CMake file - foreach

Question

I am working on CMake unit test cases that is using ctest.
I am having one question here.
Some part of my CMake is as below:
set(size_w 32 )
set(powerof2_w 5 )
foreach(size ${size_w})
foreach(pwr_of_2 ${powerof2_w})
...
FUNCTION_EXE(${size} ${pwr_of_2})
endforeach(pwr_of_2)
endforeach(size)
set(size_w 64 )
set(powerof2_w 6 )
foreach(size ${size_w})
foreach(pwr_of_2 ${powerof2_w})
...
FUNCTION_EXE(${size} ${pwr_of_2})
endforeach(pwr_of_2)
endforeach(size)
set(size_w 128 )
set(powerof2_w 7 )
foreach(size ${size_w})
foreach(pwr_of_2 ${powerof2_w})
...
FUNCTION_EXE(${size} ${pwr_of_2})
endforeach(pwr_of_2)
endforeach(size)
set(size_w 256 )
set(powerof2_w 8 )
foreach(size ${size_w})
foreach(pwr_of_2 ${powerof2_w})
...
FUNCTION_EXE(${size} ${pwr_of_2})
endforeach(pwr_of_2)
endforeach(size)
Expectation:
I want to reduce that one loop which is with powerof2_w parameter:
foreach(pwr_of_2 ${powerof2_w})
Is it possible to calculate the pwr_of_2 parameter from the size_w parameter inside the foreach(size ${size_w}) for-loop itself?
Note: Also, I want to combine all four of these for-loops into one for-loop using an array index.
Is this possible in CMake?

Answer 1

If I understand correctly, you are looking to calculate the exponential component for the powers of two for the given sizes:
32, 64, 128, 256, 512
These are powers of two with corresponding exponents of:
5, 6, 7, 8, 9
which we can calculate.
Unfortunately, CMake's math() function does not support exponential arithmetic. But luckily, powers of two are easy to manipulate using bit-shifting, which is supported in CMake. We can create a simple CMake function to calculate the (power of 2) exponents used to derive the sizes 32, 64, 128, etc.
function(calc_power_of_two_exponent num exponent)
set(counter 0)
# Shift until our number equals 1.
while(num GREATER 1)
# Right shift by 1
math(EXPR num "${num} >> 1")
# Count the number of times we shift.
math(EXPR counter "${counter} + 1")
endwhile()
# Return the number of times shifted, which is the exponent.
set(exponent ${counter} PARENT_SCOPE)
endfunction()
It looks like you want to iterate through these size and exponent values in pairs. We can set a list of sizes to iterate over, and calculate the corresponding exponent as we go.
set(sizes 32 64 128 256 512)
# Iterate through each size.
foreach(size ${sizes})
# Call the function to calculate its base-2 power (or index).
calc_power_of_two_exponent(${size} exponent)
message(STATUS "${size} ${exponent}")
FUNCTION_EXE(${size} ${exponent})
endforeach(size)
The status message can be used to confirm we pass the correct values to the FUNCTION_EXE function. This code prints:
32 5
64 6
128 7
256 8
512 9