bpo-40280: Add Tools/wasm with helpers for cross building (GH-29984)

[tiran]

https://bugs.python.org/issue40280

[tiran]

$ CONFIG_SITE=../../Tools/wasm/config.site-wasm32-emscripten \
emconfigure ../../configure -C \
--host=wasm32-unknown-emscripten \
--build=$(../../config.guess) \
--with-build-python=$(pwd)/../build/python
$ emmake make -j$(nproc) python.html
$ ls -s --si python.*
3.0M python.data
107k python.html
361k python.js
6.6M python.wasm

[emmatyping]

This will definitely make experimenting with Emscripten based CPython a lot easier!

I have a few suggestions/comments but overall this looks good.

[emmatyping]

The binary size looks really great! Though with the number of things disabled, it will probably grow over time...

[tiran]

With --disabled-test-modules, without decimal unittest xml and without some C extension

# Modules/Setup.local
*disabled*
_decimal
pyexpat
_elementtree
_lsprof
audioop

I'm able to shrink the core to about 6 MB

# du --si -s -c python.*
2.7M python.data
107k python.html
275k python.js
3.3M python.wasm
6.4M total

The python.wasm file can be compressed well. If we assume that all browsers support transparent compression, then we are down to about 4 MB with room for more removals.

# du --si -s -c python.*
2.7M python.data
107k python.html
275k python.js
1.1M python.wasm.gz
4.2M total

The largest 25 files are:

# unzip -l usr/local/lib/python311.zip | sort -n | tail -n 26
57953 2021-12-13 19:58 threading.pyc
59930 2021-12-13 19:59 optparse.pyc
61650 2021-12-13 19:58 subprocess.pyc
61916 2021-12-13 19:56 collections/__init__.pyc
63723 2021-12-13 19:56 enum.pyc
63980 2021-12-13 19:59 pickle.pyc
66135 2021-12-13 19:59 pdb.pyc
67936 2021-12-13 19:58 locale.pyc
68530 2021-12-13 19:58 asyncio/base_events.pyc
70300 2021-12-14 09:56 _sysconfigdata__emscripten_.pyc
71134 2021-12-13 19:59 difflib.pyc
77197 2021-12-13 19:58 datetime.pyc
77293 2021-12-13 19:59 ipaddress.pyc
78354 2021-12-13 19:56 ast.pyc
79675 2021-12-13 19:59 pickletools.pyc
82993 2021-12-13 19:58 logging/__init__.pyc
85730 2021-12-13 19:56 zipfile.pyc
86382 2021-12-13 19:59 mailbox.pyc
86537 2021-12-13 19:56 argparse.pyc
88029 2021-12-13 19:59 tarfile.pyc
92877 2021-12-13 19:59 doctest.pyc
108300 2021-12-13 19:56 inspect.pyc
109640 2021-12-13 19:58 typing.pyc
118759 2021-12-13 19:59 pydoc.pyc
253750 2021-12-13 19:59 html/entities.pyc
5711264 347 files

@brettcannon now is a good time to discuss what modules belong in a minimum stdlib.

[emmatyping]

Wow that's a great reduction in size! I played with thin lto and for some reason it greatly increases the binary size (to 21MB!). I guess inlining makes the build a lot bigger. One other idea I had which gives small gains in binary size is compiling with -Os, which seems to not make much of a difference vs -Oz.

[tiran]

Actually it is less. I think I introduced a regression in GH-30001. makesetup is dropping too many modules from the final binary.

[brettcannon]

Suggested change

	tool chain and a build Python interpreter.
	tool chain and a built Python interpreter.

[emmatyping]

build here is I believe a reference to the autotools/configure --build argument. (Same for line 11 below).

[tiran]

That is correct. In lack of a better term I introduced the term "build Python"/

[brettcannon]

My brain just keeps reading "build" as a verb instead of an adjective. 😄