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GH-139922: add interpreter types to InternalDocs/interpreter.md (GH-148035)

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Chris Eibl
2026-04-05 17:25:14 +02:00
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@@ -507,6 +507,38 @@ After the last `DEOPT_IF` has passed, a hit should be recorded with
After an optimization has been deferred in the adaptive instruction,
that should be recorded with `STAT_INC(BASE_INSTRUCTION, deferred)`.
## Interpreter types
There are three different types of interpreters to choose from based on compiler support:
* traditional switch-case interpreter
Supported by all compilers covered in PEP 7.
* computed-gotos interpreter
Enabled using configure option `--with-computed-gotos` and used by default on supported compilers.
It uses [Labels as Values](https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html)
for more efficient dispatching.
* tail-calling interpreter
Enabled using configure option `--with-tail-call-interp` (or `--tail-call-interp` for build.bat on Windows).
It uses [tail calls](https://clang.llvm.org/docs/AttributeReference.html#musttail) and the
[preserve_none](https://clang.llvm.org/docs/AttributeReference.html#preserve-none)
calling convention between the small C functions that implement individual Python opcodes.
Not all compilers support these and if they do not all targets might be supported (for example,
MSVC currently only supports x64 and only in optimized builds).
In addition, compilers must do [escape analysis](https://gcc.gnu.org/onlinedocs/gcc/Common-Attributes.html#index-musttail)
of the lifetimes of automatic variables, function parameters, and temporaries to ensure proper tail-calls. They
emit a compile error in case of a violation or detection failure. The ability to detect this varies depending on the compiler and
also on the optimization level. Following techniques are particularly helpful to the MSVC compiler in this regard
* [Introducing additional scopes](https://github.com/python/cpython/blob/3908593039bde9d4b591ab09919003ee57418d64/Python/bytecodes.c#L2526)
* [extracting problematic code paths into a separate function](https://github.com/python/cpython/pull/143068/files#diff-729a985b0cb8b431cb291f1edb561bbbfea22e3f8c262451cd83328a0936a342R3724)
* [returning a pointer instead of taking it as an output parameter](https://github.com/python/cpython/blob/3908593039bde9d4b591ab09919003ee57418d64/Include/internal/pycore_ceval.h#L489-L492)
Using `restrict` is another (currently unused) remedy.
Additional resources
--------------------