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PR 23976 stores the currrent interpreter and the current Python thread state into a Thread Local Storage (TLS) using GCC/clang __thread keyword.
On x86-64 using LTO and GCC -O3, _PyThreadState_GET() and _PyInterpreterState_GET() become a single MOV.
Assembly code using LTO and gcc -O3.
_PyThreadState_GET() in _PySys_GetObjectId():
 0x00000000004adabe <+14>: mov rbx,QWORD PTR fs:0xfffffffffffffff8
_PyThreadState_GET() in PyThreadState_Get():
 0x000000000046b660 <+0>: mov rax,QWORD PTR fs:0xfffffffffffffff8
_PyInterpreterState_GET() in PyTuple_New():
 0x000000000048dfcc <+12>: mov rax,QWORD PTR fs:0xfffffffffffffff0
_PyInterpreterState_GET() in PyState_FindModule():
 0x000000000044bf20 <+16>: mov rax,QWORD PTR fs:0xfffffffffffffff0
---
Note: Without LTO, sometimes there is an indirection:
_PyThreadState_GET() in _PySys_GetObjectId(), 2 MOV (PIC indirection):
 mov rax,QWORD PTR [rip+0x1eb270] # 0x713fe0
 # rax = 0xfffffffffffffff0 (-16)
 mov r13,QWORD PTR fs:[rax]
_PyInterpreterState_GET() in PyTuple_New(), 2 MOV (PIC indirection):
 mov rax,QWORD PTR [rip+0x294d95] # 0x713ff8
 mov rax,QWORD PTR fs:[rax]
An optimized Python should always be built with LTO.