Optimize flash storage of properties #6337
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Later, these can be changed in cunning ways to save flash storage.
This will enable setting data attributes, namely, the section of the symbol.
.. and enable it on atmel-samd and raspberrypi. On trinket_m0 this saves 96 net bytes of flash. There are 216 bytes actually saved by reducing the flash storage size of the property descriptors, but added code in several paths takes back over half of the 'raw savings'. By organizing the "get-only" and "get-set" (but no delete) properties each in a different section, we can represent then more efficiently. Testing performed: that a get-only property can still be gotten but can't be set or deleted; that a get-set property can sill be gotten or set but can't be deleted. Tested on pygamer. Because this requires linker file support, I only enabled it on two of the ports.
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This is clever! I have an experiment or two I would like to try before approving. |
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I did an experiment on the Trinket M0 build: I just removed the deleter slot from Removing the one slot saves 144 bytes. If all the Trinket M0 properties were read-only, which is not true, then the total savings would be 288 bytes. A quick manual count shows ~24 read-only properties. And this is all without any extra code to handle slot removal. While doing this, I discovered that some code in the |
In-flash properties with only a getter or only get+set are organized into two specific memory regions, so that it's possible to tell by their address whether they have 1, 2, or 3 proxy slots available. When accessing a property slot, code is added to check if the desired slot is available -- this affects setter & deleter paths, adding some additional conditional branching, so it probably has a minor negative performance effect as well (which I didn't attempt to measure)
A property with only a getter shrinks from 16 byte (type pointer + 3 proxy slots) to 8 bytes (type pointer + 1 proxy slot); a get+set property shrinks to 12 bytes (type pointer + 2 proxy slots). However, new tests are added in some fast paths, apparently costing ~120 bytes on samd21. Net, 96 bytes are saved on the trinket_m0 build.
Closes: #4899 (except for enabling it on all ports)