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I tested PR 10079 using gdb on Fedora 28 with GCC 8.1.1 to check if Py_INCREF/Py_DECREF functions are inlined.
I understand that "static inline void Py_INCREF()" is *not* inline by gcc -O0, but it *is* inlined using gcc -Og which is the *default* optimization level of ./configure --with-debug.
To develop on Python, I force -O0, because the compilation time matters more than runtime performance for me :-) Compilation on my laptop using MAKEFLAGS=-j9:
* -O0: 21s
* -0g: 36s (1.7x slower)
... But Py_INCREF/DECREF are always inlined, even with -O0, when using __attribute__((always_inline))! I will work on a change to use that.
== gcc -O0 ==
(gdb) disassemble Py_IncRef 
Dump of assembler code for function Py_IncRef:
 ...
 0x000000000047276a <+20>: 	cmpq 0ドルx0,-0x8(%rbp)
 0x000000000047276f <+25>:	je 0x47277d <Py_IncRef+39>
 0x0000000000472771 <+27>:	mov -0x8(%rbp),%rax
 0x0000000000472775 <+31>:	mov %rax,%rdi
 0x0000000000472778 <+34>:	callq 0x472523 <_Py_INCREF>
 0x000000000047277d <+39>:	nop
 0x000000000047277e <+40>:	leaveq 
 0x000000000047277f <+41>:	retq 
(gdb) disassemble Py_DecRef 
Dump of assembler code for function Py_DecRef:
 ...
 0x0000000000472794 <+20>:	cmpq 0ドルx0,-0x8(%rbp)
 0x0000000000472799 <+25>:	je 0x4727b1 <Py_DecRef+49>
 0x000000000047279b <+27>:	mov -0x8(%rbp),%rax
 0x000000000047279f <+31>:	mov %rax,%rdx
 0x00000000004727a2 <+34>:	mov 0ドルxe1,%esi
 0x00000000004727a7 <+39>:	mov 0ドルx65c550,%edi
 0x00000000004727ac <+44>:	callq 0x472554 <_Py_DECREF>
 0x00000000004727b1 <+49>:	nop
 0x00000000004727b2 <+50>:	leaveq 
 0x00000000004727b3 <+51>:	retq 
== gcc -Og ==
(gdb) disassemble Py_IncRef 
Dump of assembler code for function Py_IncRef:
 0x0000000000462de2 <+0>:	test %rdi,%rdi
 0x0000000000462de5 <+3>:	je 0x462dfb <Py_IncRef+25>
 0x0000000000462de7 <+5>:	addq 0ドルx1,0x4bfc09(%rip) # 0x9229f8 <_Py_RefTotal>
 0x0000000000462def <+13>:	mov 0x10(%rdi),%rax
 0x0000000000462df3 <+17>:	add 0ドルx1,%rax
 0x0000000000462df7 <+21>:	mov %rax,0x10(%rdi)
 0x0000000000462dfb <+25>:	retq 
(gdb) disassemble Py_DecRef 
Dump of assembler code for function Py_DecRef:
 0x0000000000463b2e <+0>:	test %rdi,%rdi
 0x0000000000463b31 <+3>:	je 0x463b6f <Py_DecRef+65>
 0x0000000000463b33 <+5>:	sub 0ドルx8,%rsp
 0x0000000000463b37 <+9>:	subq 0ドルx1,0x4beeb9(%rip) # 0x9229f8 <_Py_RefTotal>
 0x0000000000463b3f <+17>:	mov 0x10(%rdi),%rax
 0x0000000000463b43 <+21>:	sub 0ドルx1,%rax
 0x0000000000463b47 <+25>:	mov %rax,0x10(%rdi)
 0x0000000000463b4b <+29>:	je 0x463b68 <Py_DecRef+58>
 0x0000000000463b4d <+31>:	js 0x463b54 <Py_DecRef+38>
 0x0000000000463b4f <+33>:	add 0ドルx8,%rsp
 0x0000000000463b53 <+37>:	retq 
 0x0000000000463b54 <+38>:	mov %rdi,%rdx
 0x0000000000463b57 <+41>:	mov 0ドルxe1,%esi
 0x0000000000463b5c <+46>:	mov 0ドルx5e1120,%edi
 0x0000000000463b61 <+51>:	callq 0x462da4 <_Py_NegativeRefcount>
 0x0000000000463b66 <+56>:	jmp 0x463b4f <Py_DecRef+33>
 0x0000000000463b68 <+58>:	callq 0x463b0c <_Py_Dealloc>
 0x0000000000463b6d <+63>:	jmp 0x463b4f <Py_DecRef+33>
 0x0000000000463b6f <+65>:	retq