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Vast majority of the time the system call returns 0. Letting the compiler know shortens the routine (119 -> 116) and the fast path. Disasm starting at the call to __fput_sync(): before: <+55>: call 0xffffffff816b0da0 <__fput_sync> <+60>: lea 0x201(%rbx),%eax <+66>: cmp $0x1,%eax <+69>: jbe 0xffffffff816ab707 <__x64_sys_close+103> <+71>: mov %ebx,%edx <+73>: movslq %ebx,%rax <+76>: and $0xfffffffd,%edx <+79>: cmp $0xfffffdfc,%edx <+85>: mov $0xfffffffffffffffc,%rdx <+92>: cmove %rdx,%rax <+96>: pop %rbx <+97>: pop %rbp <+98>: jmp 0xffffffff82242fa0 <__x86_return_thunk> <+103>: mov $0xfffffffffffffffc,%rax <+110>: jmp 0xffffffff816ab700 <__x64_sys_close+96> <+112>: mov $0xfffffffffffffff7,%rax <+119>: jmp 0xffffffff816ab700 <__x64_sys_close+96> after: <+56>: call 0xffffffff816b0da0 <__fput_sync> <+61>: xor %eax,%eax <+63>: test %ebp,%ebp <+65>: jne 0xffffffff816ab6ea <__x64_sys_close+74> <+67>: pop %rbx <+68>: pop %rbp <+69>: jmp 0xffffffff82242fa0 <__x86_return_thunk> # the jmp out <+74>: lea 0x201(%rbp),%edx <+80>: mov $0xfffffffffffffffc,%rax <+87>: cmp $0x1,%edx <+90>: jbe 0xffffffff816ab6e3 <__x64_sys_close+67> <+92>: mov %ebp,%edx <+94>: and $0xfffffffd,%edx <+97>: cmp $0xfffffdfc,%edx <+103>: cmovne %rbp,%rax <+107>: jmp 0xffffffff816ab6e3 <__x64_sys_close+67> <+109>: mov $0xfffffffffffffff7,%rax <+116>: jmp 0xffffffff816ab6e3 <__x64_sys_close+67> Signed-off-by: Mateusz Guzik <mjguzik@gmail.com> Link: https://lore.kernel.org/r/20250301104356.246031-1-mjguzik@gmail.com Signed-off-by: Christian Brauner <brauner@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.