Add Small String Optimization #5690
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added 2 commits
January 28, 2019 14:03
Reduce String memory overhead from 24 bytes to 16 bytes by limiting the maximum string length to <64Kbytes (which is larger than heap so no effective problem). Add Small String Optimization, SSO, which instead of allocating pointers to small strings on the heap will store the string in place of the pointer in the class. This should reduce memory fragmentation as Strings of 0-3 characters will not need extra malloc()s. No user code changes should be required to work with this optimization.
We only have 15, not 16, bits of length, so adjust the check accordingly.
Memory fragmentation is worse than not saving any RAM per String instance, so extend the SSO size to 8 bytes (7 chars + '\0'), and making the total String size the same size as it was before. Remove any hardcoded SSO size and allow configuration via an enum, and replace the magic max-capacity with an enum tor clarity. Add a host test that verifies that no memory is allocated until a full 8 characters are assigned to a string, as well as checking all intermediate values.
Save up to 12 chars (11 + \0) in String itself by using the terminating \0 in the inline string as a flag to identify if this is a SSO or a heap string. Fix DOS endlines present in StreamString, for some reason.
earlephilhower
changed the title
Because pointers are 8 bytes (and 8-bytes aligned) on x64, the structure used to store SSO needs to make sure the terminal \0 won't overwrite some of that buffer. Adjust the SSOSIZE dynamically depending on the size of the struct ptr. Refactor to add a setBuffer method, like the setConfig, setSSO, etc. Fix some issues found when using SSO strings and functions which modify len() dynamically.
devyte
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Feb 8, 2019
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TD-er
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Mar 18, 2019
A fix for some issue introduced in PR esp8266#5690 See discussion in esp8266#5883
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Add Small String Optimization, SSO, which instead of allocating pointers
to small strings on the heap will store the string in place of the pointer
in the class. This should reduce memory fragmentation as Strings
of up to 12 chars (11 + \0) will not need any space on the heap at all.
No user code changes should be required to work with this optimization.