4eaebc1988
This commit adds many new sections to the existing "Developing and building MicroPython" chapter to make it all about the internals of MicroPython. This work was done as part of Google's Season of Docs 2020.
116 lines
5.3 KiB
ReStructuredText
116 lines
5.3 KiB
ReStructuredText
.. _qstr:
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MicroPython string interning
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============================
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MicroPython uses `string interning`_ to save both RAM and ROM. This avoids
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having to store duplicate copies of the same string. Primarily, this applies to
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identifiers in your code, as something like a function or variable name is very
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likely to appear in multiple places in the code. In MicroPython an interned
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string is called a QSTR (uniQue STRing).
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A QSTR value (with type ``qstr``) is a index into a linked list of QSTR pools.
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QSTRs store their length and a hash of their contents for fast comparison during
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the de-duplication process. All bytecode operations that work with strings use
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a QSTR argument.
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Compile-time QSTR generation
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----------------------------
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In the MicroPython C code, any strings that should be interned in the final
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firmware are written as ``MP_QSTR_Foo``. At compile time this will evaluate to
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a ``qstr`` value that points to the index of ``"Foo"`` in the QSTR pool.
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A multi-step process in the ``Makefile`` makes this work. In summary this
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process has three parts:
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1. Find all ``MP_QSTR_Foo`` tokens in the code.
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2. Generate a static QSTR pool containing all the string data (including lengths
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and hashes).
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3. Replace all ``MP_QSTR_Foo`` (via the preprocessor) with their corresponding
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index.
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``MP_QSTR_Foo`` tokens are searched for in two sources:
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1. All files referenced in ``$(SRC_QSTR)``. This is all C code (i.e. ``py``,
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``extmod``, ``ports/stm32``) but not including third-party code such as
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``lib``.
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2. Additional ``$(QSTR_GLOBAL_DEPENDENCIES)`` (which includes ``mpconfig*.h``).
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*Note:* ``frozen_mpy.c`` (generated by mpy-tool.py) has its own QSTR generation
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and pool.
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Some additional strings that can't be expressed using the ``MP_QSTR_Foo`` syntax
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(e.g. they contain non-alphanumeric characters) are explicitly provided in
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``qstrdefs.h`` and ``qstrdefsport.h`` via the ``$(QSTR_DEFS)`` variable.
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Processing happens in the following stages:
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1. ``qstr.i.last`` is the concatenation of putting every single input file
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through the C pre-processor. This means that any conditionally disabled code
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will be removed, and macros expanded. This means we don't add strings to the
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pool that won't be used in the final firmware. Because at this stage (thanks
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to the ``NO_QSTR`` macro added by ``QSTR_GEN_CFLAGS``) there is no
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definition for ``MP_QSTR_Foo`` it passes through this stage unaffected. This
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file also includes comments from the preprocessor that include line number
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information. Note that this step only uses files that have changed, which
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means that ``qstr.i.last`` will only contain data from files that have
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changed since the last compile.
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2. ``qstr.split`` is an empty file created after running ``makeqstrdefs.py split``
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on qstr.i.last. It's just used as a dependency to indicate that the step ran.
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This script outputs one file per input C file, ``genhdr/qstr/...file.c.qstr``,
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which contains only the matched QSTRs. Each QSTR is printed as ``Q(Foo)``.
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This step is necessary to combine the existing files with the new data
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generated from the incremental update in ``qstr.i.last``.
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3. ``qstrdefs.collected.h`` is the output of concatenating ``genhdr/qstr/*``
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using ``makeqstrdefs.py cat``. This is now the full set of ``MP_QSTR_Foo``'s
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found in the code, now formatted as ``Q(Foo)``, one-per-line, with duplicates.
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This file is only updated if the set of qstrs has changed. A hash of the QSTR
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data is written to another file (``qstrdefs.collected.h.hash``) which allows
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it to track changes across builds.
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4. Generate an enumeration, each entry of which maps a ``MP_QSTR_Foo`` to it's corresponding index.
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It concatenates ``qstrdefs.collected.h`` with ``qstrdefs*.h``, then it transforms
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each line from ``Q(Foo)`` to ``"Q(Foo)"`` so they pass through the preprocessor
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unchanged. Then the preprocessor is used to deal with any conditional
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compilation in ``qstrdefs*.h``. Then the transformation is undone back to
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``Q(Foo)``, and saved as ``qstrdefs.preprocessed.h``.
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5. ``qstrdefs.generated.h`` is the output of ``makeqstrdata.py``. For each
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``Q(Foo)`` in qstrdefs.preprocessed.h (plus some extra hard-coded ones), it outputs
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``QDEF(MP_QSTR_Foo, (const byte*)"hash" "Foo")``.
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Then in the main compile, two things happen with ``qstrdefs.generated.h``:
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1. In qstr.h, each QDEF becomes an entry in an enum, which makes ``MP_QSTR_Foo``
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available to code and equal to the index of that string in the QSTR table.
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2. In qstr.c, the actual QSTR data table is generated as elements of the
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``mp_qstr_const_pool->qstrs``.
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.. _`string interning`: https://en.wikipedia.org/wiki/String_interning
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Run-time QSTR generation
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------------------------
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Additional QSTR pools can be created at runtime so that strings can be added to
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them. For example, the code::
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foo[x] = 3
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Will need to create a QSTR for the value of ``x`` so it can be used by the
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"load attr" bytecode.
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Also, when compiling Python code, identifiers and literals need to have QSTRs
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created. Note: only literals shorter than 10 characters become QSTRs. This is
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because a regular string on the heap always takes up a minimum of 16 bytes (one
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GC block), whereas QSTRs allow them to be packed more efficiently into the pool.
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QSTR pools (and the underlying "chunks" that store the string data) are allocated
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on-demand on the heap with a minimum size.
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