MicroPython: Avoid heap allocations in all C++ modules. #711
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This is a pre-requisite to bumping all of our MicroPython builds to the latest edge commit and - indeed - keeping ahead of MicroPython changes so we can incorporate Bluetooth when it arrives. Once merged, relevant commits from https://github.com/pimoroni/badger2040/tree/test/micropython-edge can be moved to pimoroni/badger2040#2 (and the GitHub actions .yml targeted back at the main branch here) |
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This was referenced Mar 13, 2023
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Pico RGB Keypad, Pico Scroll and Pico Unicorn have all been converted to MicroPython class-style modules. import picounicorn
picounicorn.init()Would become: from picounicorn import PicoUnicorn
picounicorn = PicoUnicorn() |
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The result of GET_STR_DATA_LEN should be null terminated, so converting to a std::string and then using .c_str() is both redundant and need s heap.
MicroPython's GET_STR_DATA_LEN macro returns a const byte array and len, which std::string would copy into heap. Using string_view lets us wrap the existing const values.
Because `mp_tracked_calloc` does not survive a soft reset but the memory region will, resulting in half-initialised frankenclasses that behave unpredictably. Using the class pattern fixes this since it's always guaranteed to be initialised when a user instantiates it, and __del__ can handle cleanup.
Borrow heavily from Galactic/Cosmic Unicorn for the PIO/chained-DMA setup.
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Remove builds for Badger 2040 and Badger 2040 W, these now live at: https://github.com/pimoroni/badger2040/
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As of micropython/micropython@c80e7c1 MicroPython's linker script includes a "greedy" heap, which attempts to use all free system RAM for MicroPython's heap.
Whereas previously we carefully avoided large heap allocations due to the very limited amount of heap available to C++ extensions, we must now avoid all allocations.
This changeset cleans up some uses of
std::stringwhich were implicitly allocating strings on heap. It does this by switching tostring_view, wrapping the existing byte array and length returned byGET_STR_DATA_LENinstead of copying them into astd::string.Additionally
set_font()performs Hershey font lookups via a newif/elsetable, since thestd::map<std::string, const *font_t>lookup table could not be used without constructing a string and risking a heap allocation.Finally I have removed the completely ridiculous and unnecessary conversion to/from a
std::stringin our jpegdec binding'sopen_filefunctionality.To make
string_viewavailable, all MicroPython builds needed to be forced to use C++17, this is achieved in their respective CMake files.In all cases I believe
std::stringwill implicitly cast tostd::string_viewso our examples will not break, and C++ code has always been set to C++17.