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Analysis of CPython binary assembly

Usually, you should not care how the C compiler optimizes Python. Analyzing the assembly code helps to check if the C compiler is able to optimize Python as you might expect.

See also :ref:`Python builds <python-builds>` and Assembly Intel x86.

Inline libpython function calls and LTO

Link Time Optimization (LTO) helps a lot to inline function calls.

If Python is configured with --enable-shared (Python executable is linked to libpythonX.Y.so), the -fno-semantic-interposition compiler flag is needed by GCC to inline libpython function calls. This flag is now enabled by --enable-optimizations since Python 3.10. Clang disables semantic interposition by default and so doesn't need this flag.

See Red Hat Enterprise Linux 8.2 brings faster Python 3.8 run speeds for a concrete analysis of Python 3.8 performance on RHEL 8 with --enable-shared and -fno-semantic-interposition.

macOS doesn't use LTO

The official Python macOS binaries are not built with LTO to keep support of old clang versions of macOS 10.6: see bpo-41181. See also bpo-42235: [macOS] Use --enable-optimizations in build-installer.py.

Concrete example of performance issue with the lack of LTO on macOS: bpo-39542. Converting PyTuple_Check() macro to a function call introduced a performance slowndown on macOS beause clang was unable to inline the PyTuple_Check() function call. The change was reverted to restore performance on macOS.

In Python 3.10, LTO is used on macOS but on macOS 10.15 and newer (bpo-42235).

Security compiler flags

Position Independent Code (-fPIC)

On Fedora, Python is built with -fPIC for security. See Wikipedia: Position-independent code.

Control flow Enforcement Technology (CET) hardening

GCC has a -fcf-protection=branch flag which emits ENDBR64 ("End Branch 64 bit") instructions at functions entry point. It is used on Fedora.

Compiler and linker flags

Get compiler (CFLAGS) and linker (LDFLAGS) flags:

$ python3
Python 3.9.1 (default, Jan 20 2021, 00:00:00)
>>> import sysconfig
>>> cflags = sysconfig.get_config_var('PY_CFLAGS') + sysconfig.get_config_var('PY_CFLAGS_NODIST')
>>> ldflags = sysconfig.get_config_var('PY_LDFLAGS') + sysconfig.get_config_var('PY_LDFLAGS_NODIST')
>>> '-fPIC' in cflags
True
>>> '-fno-semantic-interposition' in cflags
True
>>> '-flto' in ldflags
True

Python thread state (tstate)

Since Python 3.8, there is an on-going effect to pass explicitly the current Python thread state ("tstate") to internal functions:

  • It avoids having to read an atomic variable: _PyThreadState_GET() reads _PyRuntime.gilstate.tstate_current atomic variable with _Py_atomic_load_relaxed().
  • It should help the C compiler to inline more code.

See Pass the Python thread state explicitly.

PyErr_Occurred()

Simplified C code of PyErr_Occurred():

PyObject* PyErr_Occurred(void)
{
    _PyRuntimeState *runtime = &_PyRuntime;
    _Py_atomic_address *ptstate = &runtime->gilstate.tstate_current;
    PyThreadState *tstate = (PyThreadState*)_Py_atomic_load_relaxed(ptstate)
    return tstate->curexc_type;
}

PyErr_Occurred() of Fedora Python 3.9 (built with -fPIC):

endbr64

# rax = &_PyRuntime = *(void **)0x7ffff7f45d38
mov rax, QWORD PTR [rip+0x1fef9d]  # 0x7ffff7f45d38

# offsetof(_PyRuntimeState, gilstate.tstate_current) = 0x238
# rdx = tstate = *(_PyRuntime.gilstate.tstate_current) = *(void **)($rax + 0x238)
mov rdx, QWORD PTR [rax+0x238]

# offsetof(PyThreadState, curexc_type) = 0x58
# rax = tstate->curexc_type = *(void **)($rdx + 0x58)
mov rax, QWORD PTR [rdx+0x58]

ret

Getting tstate requires two pointer deferences (two MOV):

  • runtime = *($rip + 0x1fef9d) (&_PyRuntime)
  • tstate = runtime->gilstate.tstate

PyErr_Occurred() requires 3 pointer deferences.

Note: the $rip indirection is needed by -fPIC flag and endbr64 instruction is related to CET hardening flag.

_PyErr_Occurred()

C code:

static inline PyObject* _PyErr_Occurred(PyThreadState *tstate)
{
    assert(tstate != NULL);
    return tstate->curexc_type;
}

_PyErr_Occurred() of Fedora Python 3.9 (built with -fPIC), inlined in _Py_CheckFunctionResult+12()::

# $rdi = tstate argument
# offsetof(PyThreadState, curexc_type) = 0x58
mov rax, QWORD PTR [rdi+0x58]                                                                           │

The function calls becomes a single pointer deference (one MOV):

  • result = (*tstate).curexc_type

On Fedora, calling PyErr_Occurred() requires 6 instructions (CALL, ENDBR64, 3 MOV, RET), whereas inlined _PyErr_Occurred is a single MOV instruction.