MicroPython string interning¶
MicroPython uses string interning to save both RAM and ROM. This avoids having to store duplicate copies of the same string. Primarily, this applies to identifiers in your code, as something like a function or variable name is very likely to appear in multiple places in the code. In MicroPython an interned string is called a QSTR (uniQue STRing).
A QSTR value (with type
qstr) is a index into a linked list of QSTR pools.
QSTRs store their length and a hash of their contents for fast comparison during
the de-duplication process. All bytecode operations that work with strings use
a QSTR argument.
Compile-time QSTR generation¶
In the MicroPython C code, any strings that should be interned in the final
firmware are written as
MP_QSTR_Foo. At compile time this will evaluate to
qstr value that points to the index of
"Foo" in the QSTR pool.
A multi-step process in the
Makefile makes this work. In summary this
process has three parts:
- Find all
MP_QSTR_Footokens in the code.
- Generate a static QSTR pool containing all the string data (including lengths and hashes).
- Replace all
MP_QSTR_Foo(via the preprocessor) with their corresponding index.
MP_QSTR_Foo tokens are searched for in two sources:
- All files referenced in
$(SRC_QSTR). This is all C code (i.e.
ports/stm32) but not including third-party code such as
frozen_mpy.c (generated by mpy-tool.py) has its own QSTR generation
Some additional strings that can’t be expressed using the
(e.g. they contain non-alphanumeric characters) are explicitly provided in
qstrdefsport.h via the
Processing happens in the following stages:
qstr.i.lastis the concatenation of putting every single input file through the C pre-processor. This means that any conditionally disabled code will be removed, and macros expanded. This means we don’t add strings to the pool that won’t be used in the final firmware. Because at this stage (thanks to the
NO_QSTRmacro added by
QSTR_GEN_CFLAGS) there is no definition for
MP_QSTR_Fooit passes through this stage unaffected. This file also includes comments from the preprocessor that include line number information. Note that this step only uses files that have changed, which means that
qstr.i.lastwill only contain data from files that have changed since the last compile.
qstr.splitis an empty file created after running
makeqstrdefs.py spliton qstr.i.last. It’s just used as a dependency to indicate that the step ran. This script outputs one file per input C file,
genhdr/qstr/...file.c.qstr, which contains only the matched QSTRs. Each QSTR is printed as
Q(Foo). This step is necessary to combine the existing files with the new data generated from the incremental update in
qstrdefs.collected.his the output of concatenating
makeqstrdefs.py cat. This is now the full set of
MP_QSTR_Foo’s found in the code, now formatted as
Q(Foo), one-per-line, with duplicates. This file is only updated if the set of qstrs has changed. A hash of the QSTR data is written to another file (
qstrdefs.collected.h.hash) which allows it to track changes across builds.
- Generate an enumeration, each entry of which maps a
MP_QSTR_Footo it’s corresponding index. It concatenates
qstrdefs*.h, then it transforms each line from
"Q(Foo)"so they pass through the preprocessor unchanged. Then the preprocessor is used to deal with any conditional compilation in
qstrdefs*.h. Then the transformation is undone back to
Q(Foo), and saved as
qstrdefs.generated.his the output of
makeqstrdata.py. For each
Q(Foo)in qstrdefs.preprocessed.h (plus some extra hard-coded ones), it outputs
QDEF(MP_QSTR_Foo, (const byte*)"hash" "Foo").
Then in the main compile, two things happen with
- In qstr.h, each QDEF becomes an entry in an enum, which makes
MP_QSTR_Fooavailable to code and equal to the index of that string in the QSTR table.
- In qstr.c, the actual QSTR data table is generated as elements of the
Run-time QSTR generation¶
Additional QSTR pools can be created at runtime so that strings can be added to them. For example, the code:
foo[x] = 3
Will need to create a QSTR for the value of
x so it can be used by the
“load attr” bytecode.
Also, when compiling Python code, identifiers and literals need to have QSTRs created. Note: only literals shorter than 10 characters become QSTRs. This is because a regular string on the heap always takes up a minimum of 16 bytes (one GC block), whereas QSTRs allow them to be packed more efficiently into the pool.
QSTR pools (and the underlying “chunks” that store the string data) are allocated on-demand on the heap with a minimum size.