CMSIS-DSP fill flash with tables not in actual use about cmsis-dsp HOT 8 CLOSED

@ACleverDisguise You need to avoid using initialization functions like arm_cfft_init_f32 and instead use dedicated versions for the FFT lengths you need. For instance arm_cfft_init_512_f32

There are similar functions for other data types, and other transforms (rfft ...)

Like that, the linker will be able to deduce the table that are not used and they won't be included in the final build.

Those functions are documented in the doxygen generated documentation.

There is also a new section in the README : https://github.com/ARM-software/CMSIS-DSP?tab=readme-ov-file#code-size

(The required build and link options that are compiler dependent must also be used : like -ffunction-sections, -fdata-sections, -Wl,--gc-sections ...)

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I'm calling arm_cfft_init_256_q15().

It's still bringing in every size of every static variable.

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What's your compiler and command line options for compiling and linking ?

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IAR and I have no idea: the IDE does its level best to conceal that from me. I'll go digging to see what command line it comes up with.

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Ah, never mind. A third-party (as in another coworker's) file had a call for the general init and I wasn't expecting it to have any calls to the math lib at all, not to mention FFT.

Stripping that out and replacing it with the appropriately-sized init (64 in this case) solved our problem. Thanks, @christophe0606!

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Problem solved, but this really needs to be documented in the actual library docs, not buried in a README. Like perhaps arm_cfft_init_<type>()'s documentation should mention the sized variants?

Or optionally initialization could be a macro call that parametrizes by size and type like this: ARM_CFFT_INIT(&my_init_struct, q15, 256) and have that assemble the call to arm_cfft_init_256_q15(&my_init_struct)?

A macro I've (loosely) tested that does this is below:

#define ARM_CFFT_INIT_IMPL(S, T, N) arm_cfft_init ##N ##T(S)
#define ARM_CFFT_INIT_PREP(S, T, N) ARM_CFFT_INIT_IMPL(S, _ ##T, _ ##N)
#define ARM_CFFT_INIT(S, T, N) ARM_CFFT_INIT_PREP(S, T, N)

This permits me to write code like this:

#define DATA_TYPE q15
#define SAMPLE_SIZE 256
ARM_CFFT_INIT(&my_struct, DATA_TYPE, SAMPLE_SIZE)

And if I decide I want to expand or contract my sample size, I change it in one place and all uses are updated. Similarly if I decide I want to move to f32, I change it in one place and it gets updated everywhere.

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@ACleverDisguise We will upgrade the documentation so that it is more visible.
The macro is a good suggestion. Thank you.

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I've written now about a dozen such macros for the features I use. They've really helped in rapid changes while testing and prototyping. I'm sure if I spent actual time on it, I'd make macros that sucked slightly less. ;)

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