| 1 | mycpp
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| 2 | =====
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| 3 |
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| 4 | This is a Python-to-C++ translator based on MyPy. It only
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| 5 | handles the small subset of Python that we use in Oils.
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| 6 |
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| 7 | It's inspired by both mypyc and Shed Skin. These posts give background:
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| 8 |
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| 9 | - [Brief Descriptions of a Python to C++ Translator](https://www.oilshell.org/blog/2022/05/mycpp.html)
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| 10 | - [Oil Is Being Implemented "Middle Out"](https://www.oilshell.org/blog/2022/03/middle-out.html)
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| 11 |
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| 12 | As of March 2024, the translation to C++ is **done**. So it's no longer
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| 13 | experimental!
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| 14 |
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| 15 | However, it's still pretty **hacky**. This doc exists mainly to explain the
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| 16 | hacks. (We may want to rewrite mycpp as "yaks", although it's low priority
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| 17 | right now.)
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| 18 |
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| 19 | ---
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| 20 |
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| 21 | Source for this doc: [mycpp/README.md]($oils-src). The code is all in
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| 22 | [mycpp/]($oils-src).
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| 23 |
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| 24 |
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| 25 | <div id="toc">
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| 26 | </div>
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| 27 |
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| 28 | ## Instructions
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| 29 |
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| 30 | ### Translating and Compiling `oils-cpp`
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| 31 |
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| 32 | Running `mycpp` is best done on a Debian / Ubuntu-ish machine. Follow the
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| 33 | instructions at <https://github.com/oilshell/oil/wiki/Contributing> to create
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| 34 | the "dev build" first, which is DISTINCT from the C++ build. Make sure you can
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| 35 | run:
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| 36 |
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| 37 | oil$ build/py.sh all
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| 38 |
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| 39 | This will give you a working shell:
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| 40 |
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| 41 | oil$ bin/osh -c 'echo hi' # running interpreted Python
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| 42 | hi
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| 43 |
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| 44 | To run mycpp, we will build Python 3.10, clone MyPy, and install MyPy's
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| 45 | dependencies. First install packages:
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| 46 |
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| 47 | # We need libssl-dev, libffi-dev, zlib1g-dev to bootstrap Python
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| 48 | oil$ build/deps.sh install-ubuntu-packages
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| 49 |
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| 50 | You'll also need a C++17 compiler for code generated by Souffle datalog, used
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| 51 | by mycpp, although Oils itself only requires C++11.
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| 52 |
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| 53 | Then fetch data, like the Python 3.10 tarball and MyPy repo:
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| 54 |
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| 55 | oil$ build/deps.sh fetch
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| 56 |
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| 57 | Then build from source:
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| 58 |
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| 59 | oil$ build/deps.sh install-wedges
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| 60 |
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| 61 | To build oil-native, use:
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| 62 |
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| 63 | oil$ ./NINJA-config.sh
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| 64 | oil$ ninja # translate and compile, may take 30 seconds
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| 65 |
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| 66 | oil$ _bin/cxx-asan/osh -c 'echo hi' # running compiled C++ !
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| 67 | hi
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| 68 |
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| 69 | To run the tests and benchmarks:
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| 70 |
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| 71 | oil$ mycpp/TEST.sh test-translator
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| 72 | ... 200+ tasks run ...
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| 73 |
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| 74 | If you have problems, post a message on `#oil-dev` at
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| 75 | `https://oilshell.zulipchat.com`. Not many people have contributed to `mycpp`,
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| 76 | so I can use your feedback!
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| 77 |
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| 78 | Related:
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| 79 |
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| 80 | - [Oil Native Quick
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| 81 | Start](https://github.com/oilshell/oil/wiki/Oil-Native-Quick-Start) on the
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| 82 | wiki.
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| 83 | - [Oil Dev Cheat Sheet](https://github.com/oilshell/oil/wiki/Oil-Native-Quick-Start)
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| 84 |
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| 85 | ## Notes on the Algorithm / Architecture
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| 86 |
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| 87 | There are four passes over the MyPy AST.
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| 88 |
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| 89 | (1) `const_pass.py`: Collect string constants
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| 90 |
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| 91 | Turn turn the constant in `myfunc("foo")` into top-level `GLOBAL_STR(str1,
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| 92 | "foo")`.
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| 93 |
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| 94 | (2) Three passes in `cppgen_pass.py`.
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| 95 |
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| 96 | (a) Forward Declaration Pass.
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| 97 |
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| 98 | class Foo;
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| 99 | class Bar;
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| 100 |
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| 101 | This pass also determines which methods should be declared `virtual` in their
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| 102 | declarations. The `virtual` keyword is written in the next pass.
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| 103 |
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| 104 | (b) Declaration Pass.
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| 105 |
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| 106 | class Foo {
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| 107 | void method();
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| 108 | };
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| 109 | class Bar {
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| 110 | void method();
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| 111 | };
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| 112 |
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| 113 | More work in this pass:
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| 114 |
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| 115 | - Collect member variables and write them at the end of the definition
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| 116 | - Collect locals for "hoisting". Written in the next pass.
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| 117 |
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| 118 | (c) Definition Pass.
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| 119 |
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| 120 | void Foo:method() {
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| 121 | ...
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| 122 | }
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| 123 |
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| 124 | void Bar:method() {
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| 125 | ...
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| 126 | }
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| 127 |
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| 128 | Note: I really wish we were not using visitors, but that's inherited from MyPy.
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| 129 |
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| 130 | ## mycpp Idioms / "Creative Hacks"
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| 131 |
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| 132 | Oils is written in typed Python 2. It will run under a stock Python 2
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| 133 | interpreter, and it will typecheck with stock MyPy.
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| 134 |
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| 135 | However, there are a few language features that don't map cleanly from typed
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| 136 | Python to C++:
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| 137 |
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| 138 | - switch statements (unfortunately we don't have the Python 3 match statement)
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| 139 | - C++ destructors - the RAII ptatern
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| 140 | - casting - MyPy has one kind of cast; C++ has `static_cast` and
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| 141 | `reinterpret_cast`. (We don't use C-style casting.)
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| 142 |
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| 143 | So this describes the idioms we use. There are some hacks in
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| 144 | [mycpp/cppgen_pass.py]($oils-src) to handle these cases, and also Python
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| 145 | runtime equivalents in `mycpp/mylib.py`.
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| 146 |
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| 147 | ### `with {,tag,str_}switch` → Switch statement
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| 148 |
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| 149 | We have three constructs that translate to a C++ switch statement. They use a
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| 150 | Python context manager `with Xswitch(obj) ...` as a little hack.
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| 151 |
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| 152 | Here are examples like the ones in [mycpp/examples/test_switch.py]($oils-src).
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| 153 | (`ninja mycpp-logs-equal` translates, compiles, and tests all the examples.)
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| 154 |
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| 155 | Simple switch:
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| 156 |
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| 157 | myint = 99
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| 158 | with switch(myint) as case:
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| 159 | if case(42, 43):
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| 160 | print('forties')
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| 161 | else:
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| 162 | print('other')
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| 163 |
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| 164 | Switch on **object type**, which goes well with ASDL sum types:
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| 165 |
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| 166 | val = value.Str('foo) # type: value_t
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| 167 | with tagswitch(val) as case:
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| 168 | if case(value_e.Str, value_e.Int):
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| 169 | print('string or int')
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| 170 | else:
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| 171 | print('other')
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| 172 |
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| 173 | We usually need to apply the `UP_val` pattern here, described in the next
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| 174 | section.
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| 175 |
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| 176 | Switch on **string**, which generates a fast **two-level dispatch** -- first on
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| 177 | length, and then with `str_equals_c()`:
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| 178 |
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| 179 | s = 'foo'
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| 180 | with str_switch(s) as case:
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| 181 | if case("foo")
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| 182 | print('FOO')
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| 183 | else:
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| 184 | print('other')
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| 185 |
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| 186 | ### `val` → `UP_val` → `val` Downcasting pattern
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| 187 |
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| 188 | Summary: variable names like `UP_*` are **special** in our Python code.
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| 189 |
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| 190 | Consider the downcasts marked BAD:
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| 191 |
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| 192 | val = value.Str('foo) # type: value_t
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| 193 |
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| 194 | with tagswitch(obj) as case:
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| 195 | if case(value_e.Str):
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| 196 | val = cast(value.Str, val) # BAD: conflicts with first declaration
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| 197 | print('s = %s' % val.s)
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| 198 |
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| 199 | elif case(value_e.Int):
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| 200 | val = cast(value.Int, val) # BAD: conflicts with both
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| 201 | print('i = %d' % val.i)
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| 202 |
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| 203 | else:
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| 204 | print('other')
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| 205 |
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| 206 | MyPy allows this, but it translates to invalid C++ code. C++ can't have a
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| 207 | variable named `val`, with 2 related types `value_t` and `value::Str`.
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| 208 |
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| 209 | So we use this idiom instead, which takes advantage of **local vars in case
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| 210 | blocks** in C++:
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| 211 |
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| 212 | val = value.Str('foo') # type: value_t
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| 213 |
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| 214 | UP_val = val # temporary variable that will be casted
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| 215 |
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| 216 | with tagswitch(val) as case:
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| 217 | if case(value_e.Str):
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| 218 | val = cast(value.Str, UP_val) # this works
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| 219 | print('s = %s' % val.s)
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| 220 |
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| 221 | elif case(value_e.Int):
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| 222 | val = cast(value.Int, UP_val) # also works
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| 223 | print('i = %d' % val.i)
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| 224 |
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| 225 | else:
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| 226 | print('other')
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| 227 |
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| 228 | This translates to something like:
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| 229 |
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| 230 | value_t* val = Alloc<value::Str>(str42);
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| 231 | value_t* UP_val = val;
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| 232 |
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| 233 | switch (val->tag()) {
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| 234 | case value_e::Str: {
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| 235 | // DIFFERENT local var
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| 236 | value::Str* val = static_cast<value::Str>(UP_val);
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| 237 | print(StrFormat(str43, val->s))
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| 238 | }
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| 239 | break;
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| 240 | case value_e::Int: {
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| 241 | // ANOTHER DIFFERENT local var
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| 242 | value::Int* val = static_cast<value::Int>(UP_val);
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| 243 | print(StrFormat(str44, val->i))
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| 244 | }
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| 245 | break;
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| 246 | default:
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| 247 | print(str45);
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| 248 | }
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| 249 |
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| 250 | This works because there's no problem having **different** variables with the
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| 251 | same name within each `case { }` block.
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| 252 |
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| 253 | Again, the names `UP_*` are **special**. If the name doesn't start with `UP_`,
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| 254 | the inner blocks will look like:
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| 255 |
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| 256 | case value_e::Str: {
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| 257 | val = static_cast<value::Str>(val); // BAD: val reused
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| 258 | print(StrFormat(str43, val->s))
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| 259 | }
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| 260 |
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| 261 | And they will fail to compile. It's not valid C++ because the superclass
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| 262 | `value_t` doesn't have a field `val->s`. Only the subclass `value::Str` has
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| 263 | it.
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| 264 |
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| 265 | (Note that Python has a single flat scope per function, while C++ has nested
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| 266 | scopes.)
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| 267 |
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| 268 | ### Python context manager → C++ constructor and destructor (RAII)
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| 269 |
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| 270 | This Python code:
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| 271 |
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| 272 | with ctx_Foo(42):
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| 273 | f()
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| 274 |
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| 275 | translates to this C++ code:
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| 276 |
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| 277 | {
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| 278 | ctx_Foo tmp(42);
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| 279 | f()
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| 280 |
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| 281 | // destructor ~ctx_Foo implicitly called
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| 282 | }
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| 283 |
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| 284 | ## MyPy "Shimming" Technique
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| 285 |
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| 286 | We have an interesting way of "writing Python and C++ at the same time":
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| 287 |
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| 288 | 1. First, all Python code must pass the MyPy type checker, and run with a stock
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| 289 | Python 2 interpreter.
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| 290 | - This is the source of truth — the source of our semantics.
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| 291 | 1. We translate most `.py` files to C++, **except** some files, in particular
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| 292 | [mycpp/mylib.py]($oils-src) and files starting with `py` like
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| 293 | `core/{pyos.pyutil}.py`.
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| 294 | 1. In C++, we can substitute custom implementations with the properties we
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| 295 | want, like `Dict<K, V>` being ordered, `BigInt` being distinct from C `int`,
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| 296 | `BufWriter` being efficient, etc.
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| 297 |
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| 298 | The MyPy type system is very powerful! It lets us do all this.
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| 299 |
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| 300 | ### NewDict() for ordered dicts
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| 301 |
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| 302 | Dicts in Python 2 aren't ordered, but we make them ordered at **runtime** by
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| 303 | using `mylib.NewDict()`, which returns `collections_.OrderedDict`.
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| 304 |
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| 305 | The **static type** is still `Dict[K, V]`, but change the "spec" to be an
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| 306 | ordered dict.
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| 307 |
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| 308 | In C++, `Dict<K, V>` is implemented as an ordered dict. (Note: we don't
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| 309 | implement preserving order on deletion, which seems OK.)
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| 310 |
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| 311 | - TODO: `iteritems()` could go away
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| 312 |
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| 313 | ### StackArray[T]
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| 314 |
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| 315 | TODO: describe this when it works.
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| 316 |
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| 317 | ### BigInt
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| 318 |
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| 319 | - In Python, it's simply defined a a class with an integer, in
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| 320 | [mylib/mops.py]($oils-src).
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| 321 | - In C++, it's currently `typedef int64_t BigInt`, but we want to make it a big
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| 322 | integer.
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| 323 |
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| 324 | ### ByteAt(), ByteEquals(), ...
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| 325 |
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| 326 | Hand optimization to reduce 1-byte strings. For IFS algorithm,
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| 327 | `LooksLikeGlob()`, `GlobUnescape()`.
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| 328 |
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| 329 | ### File / LineReader / BufWriter
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| 330 |
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| 331 | TODO: describe how this works.
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| 332 |
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| 333 | Can it be more type safe? I think we can cast `File` to both `LineReader` and
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| 334 | `BufWriter`.
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| 335 |
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| 336 | Or can we invert the relationship, so `File` derives from **both** LineReader
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| 337 | and BufWriter?
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| 338 |
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| 339 | ### Fast JSON - avoid intermediate allocations
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| 340 |
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| 341 | - `pyj8.WriteString()` is shimmed so we don't create encoded J8 string objects,
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| 342 | only to throw them away and write to `mylib.BufWriter`. Instead, we append
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| 343 | an encoded strings **directly** to the `BufWriter`.
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| 344 | - Likewise, we have `BufWriter::write_spaces` to avoid temporary allocations
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| 345 | when writing indents.
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| 346 | - This could be generalized to `BufWriter::write_repeated(' ', 42)`.
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| 347 | - We may also want `BufWriter::write_slice()`
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| 348 |
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| 349 | ## Limitations Requiring Source Rewrites
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| 350 |
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| 351 | mycpp itself may cause limitations on expressiveness, or the C++ language may
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| 352 | be able express what we want.
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| 353 |
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| 354 | - C++ doesn't have `try / except / else`, or `finally`
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| 355 | - Use the `with ctx_Foo` pattern instead.
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| 356 | - `if mylist` tests if the pointer is non-NULL; use `if len(mylist)` for
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| 357 | non-empty test
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| 358 | - Functions can have at most one keyword / optional argument.
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| 359 | - We generate two methods: `f(x)` which calls `f(x, y)` with the default
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| 360 | value of `y`
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| 361 | - If there are two or more optional arguments:
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| 362 | - For classes, you can use the "builder pattern", i.e. add an
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| 363 | `Init_MyMember()` method
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| 364 | - If the arguments are booleans, translate it to a single bitfield argument
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| 365 | - C++ has nested scope and Python has flat function scope. This can cause name
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| 366 | collisions.
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| 367 | - Could enforce this if it becomes a problem
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| 368 |
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| 369 | Also see `mycpp/examples/invalid_*` for Python code that fails to translate.
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| 370 |
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| 371 | ## WARNING: Assumptions Not Checked
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| 372 |
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| 373 | ### Global Constants Can't Be Mutated
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| 374 |
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| 375 | We translate top level constants to statically initialized C data structures
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| 376 | (zero startup cost):
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| 377 |
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| 378 | gStr = 'foo'
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| 379 | gList = [1, 2] # type: List[int]
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| 380 | gDict = {'bar': 42} # type: Dict[str, int]
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| 381 |
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| 382 | Even though `List` and `Dict` are mutable in general, you should **NOT** mutate
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| 383 | these global instances! The C++ code will break at runtime.
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| 384 |
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| 385 | ### Gotcha about Returning Variants (Subclasses) of a Type
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| 386 |
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| 387 | MyPy will accept this code:
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| 388 |
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| 389 | ```
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| 390 | if cond:
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| 391 | sig = proc_sig.Open # type: proc_sig_t
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| 392 | # bad because mycpp HOISTS this
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| 393 | else:
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| 394 | sig = proc_sig.Closed.CreateNull()
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| 395 | sig.words = words # assignment fails
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| 396 | return sig
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| 397 | ```
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| 398 |
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| 399 | It will translate to C++, but fail to compile. Instead, rewrite it like this:
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| 400 |
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| 401 | ```
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| 402 | sig = None # type: proc_sig_t
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| 403 | if cond:
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| 404 | sig = proc_sig.Open # type: proc_sig_t
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| 405 | # bad because mycpp HOISTS this
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| 406 | else:
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| 407 | closed = proc_sig.Closed.CreateNull()
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| 408 | closed.words = words # assignment fails
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| 409 | sig = closed
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| 410 | return sig
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| 411 | ```
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| 412 |
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| 413 | ### Exceptions Can't Leave Destructors / Python `__exit__`
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| 414 |
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| 415 | Context managers like `with ctx_Foo():` translate to C++ constructors and
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| 416 | destructors.
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| 417 |
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| 418 | In C++, a destructor can't "leave" an exception. It results in a runtime error.
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| 419 |
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| 420 | You can throw and CATCH an exception WITHIN a destructor, but you can't let it
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| 421 | propagate outside.
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| 422 |
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| 423 | This means you must be careful when coding the `__exit__` method. For example,
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| 424 | in `vm::ctx_Redirect`, we had this bug due to `IOError` being thrown and not
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| 425 | caught when restoring/popping redirects.
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| 426 |
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| 427 | To fix the bug, we rewrote the code to use an out param
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| 428 | `List[IOError_OSError]`.
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| 429 |
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| 430 | Related:
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| 431 |
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| 432 | - <https://akrzemi1.wordpress.com/2011/09/21/destructors-that-throw/>
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| 433 |
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| 434 | ## More Translation Notes
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| 435 |
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| 436 | ### Hacky Heuristics
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| 437 |
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| 438 | - `callable(arg)` to either:
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| 439 | - function call `f(arg)`
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| 440 | - instantiation `Alloc<T>(arg)`
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| 441 | - `name.attr` to either:
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| 442 | - `obj->member`
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| 443 | - `module::Func`
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| 444 | - `cast(MyType, obj)` to either
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| 445 | - `static_cast<MyType*>(obj)`
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| 446 | - `reinterpret_cast<MyType*>(obj)`
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| 447 |
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| 448 | ### Hacky Hard-Coded Names
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| 449 |
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| 450 | These are signs of coupling between mycpp and Oils, which ideally shouldn't
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| 451 | exist.
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| 452 |
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| 453 | - `mycpp_main.py`
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| 454 | - `ModulesToCompile()` -- some files have to be ordered first, like the ASDL
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| 455 | runtime.
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| 456 | - TODO: Pea can respect parameter order? So we do that outside the project?
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| 457 | - Another ordering constraint comes from **inheritance**. The forward
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| 458 | declaration is NOT sufficient in that case.
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| 459 | - `cppgen_pass.py`
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| 460 | - `_GetCastKind()` has some hard-coded names
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| 461 | - `AsdlType::Create()` is special cased to `::`, not `->`
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| 462 | - Default arguments e.g. `scope_e::Local` need a repeated `using`.
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| 463 |
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| 464 | Issue on mycpp improvements: <https://github.com/oilshell/oil/issues/568>
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| 465 |
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| 466 | ### Major Features
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| 467 |
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| 468 | - Python `int` and `bool` → C++ `int` and `bool`
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| 469 | - `None` → `nullptr`
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| 470 | - Statically Typed Python Collections
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| 471 | - `str` → `Str*`
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| 472 | - `List[T]` → `List<T>*`
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| 473 | - `Dict[K, V]` → `Dict<K, V>*`
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| 474 | - tuples → `Tuple2<A, B>`, `Tuple3<A, B, C>`, etc.
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| 475 | - Collection literals turn into initializer lists
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| 476 | - And there is a C++ type inference issue which requires an explicit
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| 477 | `std::initializer_list<int>{1, 2, 3}`, not just `{1, 2, 3}`
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| 478 | - Python's polymorphic iteration → `StrIter`, `ListIter<T>`, `DictIter<K,
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| 479 | V`
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| 480 | - `d.iteritems()` is rewritten `mylib.iteritems()` → `DictIter`
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| 481 | - TODO: can we be smarter about this?
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| 482 | - `reversed(mylist)` → `ReverseListIter`
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| 483 | - Python's `in` operator:
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| 484 | - `s in mystr` → `str_contains(mystr, s)`
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| 485 | - `x in mylist` → `list_contains(mylist, x)`
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| 486 | - Classes and inheritance
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| 487 | - `__init__` method becomes a constructor. Note: initializer lists aren't
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| 488 | used.
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| 489 | - Detect `virtual` methods
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| 490 | - TODO: could we detect `abstract` methods? (`NotImplementedError`)
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| 491 | - Python generators `Iterator[T]` → eager `List<T>` accumulators
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| 492 | - Python Exceptions → C++ exceptions
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| 493 | - Python Modules → C++ namespace (we assume a 2-level hierarchy)
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| 494 | - TODO: mycpp need real modules, because our `oils_for_unix.mycpp.cc`
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| 495 | translation unit is getting big.
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| 496 | - And `cpp/preamble.h` is a hack to work around the lack of modules.
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| 497 |
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| 498 | ### Minor Translations
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| 499 |
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| 500 | - `s1 == s2` → `str_equals(s1, s2)`
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| 501 | - `'x' * 3` → `str_repeat(globalStr, 3)`
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| 502 | - `[None] * 3` → `list_repeat(nullptr, 3)`
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| 503 | - Omitted:
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| 504 | - If the LHS of an assignment is `_`, then the statement is omitted
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| 505 | - This is for `_ = log`, which shuts up Python lint warnings for 'unused
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| 506 | import'
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| 507 | - Code under `if __name__ == '__main__'`
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| 508 |
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| 509 | ### Optimizations
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| 510 |
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| 511 | - Returning Tuples by value. To reduce GC pressure, we we return
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| 512 | `Tuple2<A, B>` instead of `Tuple2<A, B>*`, and likewise for `Tuple3` and `Tuple4`.
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| 513 |
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| 514 | ### Rooting Policy
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| 515 |
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| 516 | The translated code roots local variables in every function
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| 517 |
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| 518 | StackRoots _r({&var1, &var2});
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| 519 |
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| 520 | We have two kinds of hand-written code:
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| 521 |
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| 522 | 1. Methods like `Str::strip()` in `mycpp/`
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| 523 | 2. OS bindings like `stat()` in `cpp/`
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| 524 |
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| 525 | Neither of them needs any rooting! This is because we use **manual collection
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| 526 | points** in the interpreter, and these functions don't call any functions that
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| 527 | can collect. They are "leaves" in the call tree.
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| 528 |
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| 529 | ## The mycpp Runtime
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| 530 |
|
| 531 | The mycpp translator targets a runtime that's written from scratch. It
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| 532 | implements garbage-collected data structures like:
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| 533 |
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| 534 | - Typed records
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| 535 | - Python classes
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| 536 | - ASDL product and sum types
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| 537 | - `Str` (immutable, as in Python)
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| 538 | - `List<T>`
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| 539 | - `Dict<K, V>`
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| 540 | - `Tuple2<A, B>`, `Tuple3<A, B, C>`, ...
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| 541 |
|
| 542 | It also has functions based on CPython's:
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| 543 |
|
| 544 | - `mycpp/gc_builtins.{h,cc}` corresponds roughly to Python's `__builtin__`
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| 545 | module, e.g. `int()` and `str()`
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| 546 | - `mycpp/gc_mylib.{h,cc}` corresponds `mylib.py`
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| 547 | - `mylib.BufWriter` is a bit like `cStringIO.StringIO`
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| 548 |
|
| 549 | ### Differences from CPython
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| 550 |
|
| 551 | - Integers either C `int` or `mylib.BigInt`, not Python's arbitrary size
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| 552 | integers
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| 553 | - `NUL` bytes are allowed in arguments to syscalls like `open()`, unlike in
|
| 554 | CPython
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| 555 | - `s.strip()` is defined in terms of ASCII whitespace, which does not include
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| 556 | say `\v`.
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| 557 | - This is done to be consistent with JSON and J8 Notation.
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| 558 |
|
| 559 | ## C++ Notes
|
| 560 |
|
| 561 | ### Gotchas
|
| 562 |
|
| 563 | - C++ classes can have 2 member variables of the same name! From the base
|
| 564 | class and derived class.
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| 565 | - Failing to declare methods `virtual` can involve the wrong one being called
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| 566 | at runtime
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| 567 |
|
| 568 | ### Minor Features Used
|
| 569 |
|
| 570 | In addition to classes, templates, exceptions, etc. mentioned above, we use:
|
| 571 |
|
| 572 | - `static_cast` and `reinterpret_cast`
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| 573 | - `enum class` for ASDL
|
| 574 | - Function overloading
|
| 575 | - For equality and hashing?
|
| 576 | - `offsetof` for introspection of field positions for garbage collection
|
| 577 | - `std::initializer_list` for `StackRoots()`
|
| 578 | - Should we get rid of this?
|
| 579 |
|
| 580 | ### Not Used
|
| 581 |
|
| 582 | - I/O Streams, RTTI, etc.
|
| 583 | - `const`
|
| 584 | - Smart pointers
|
| 585 |
|