docs/develop/qstr.rst
.. _qstr:
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.
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
a 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_Foo tokens 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. py,
extmod, ports/stm32) but not including third-party code such as
lib.
Additional $(QSTR_GLOBAL_DEPENDENCIES) (which includes mpconfig*.h).
Note: frozen_mpy.c (generated by mpy-tool.py) has its own QSTR generation
and pool.
Some additional strings that can't be expressed using the MP_QSTR_Foo syntax
(e.g. they contain non-alphanumeric characters) are explicitly provided in
qstrdefs.h and qstrdefsport.h via the $(QSTR_DEFS) variable.
Processing happens in the following stages:
qstr.i.last is 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_QSTR macro added by QSTR_GEN_CFLAGS) there is no
definition for MP_QSTR_Foo it 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.last will only contain data from files that have
changed since the last compile.
qstr.split is an empty file created after running makeqstrdefs.py split
on 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 qstr.i.last.
qstrdefs.collected.h is the output of concatenating genhdr/qstr/*
using 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_Foo to it's corresponding index.
It concatenates qstrdefs.collected.h with qstrdefs*.h, then it transforms
each line from Q(Foo) to "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.preprocessed.h.
qstrdefs.generated.h is 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 qstrdefs.generated.h:
In qstr.h, each QDEF becomes an entry in an enum, which makes MP_QSTR_Foo
available 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
mp_qstr_const_pool->qstrs.
.. _string interning: https://en.wikipedia.org/wiki/String_interning
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.