[TUHS] /bin/true (was basic tools / Universal Unix)

Ralph Corderoy ralph at inputplus.co.uk
Wed Nov 29 09:25:24 AEST 2017


Hi Werner,

> >  * Exit with a zero value as quickly as possible.
> ...
> >         _exit(0);
...
> Then again _exit(0) is a useless optimization. It saves three closes
> for files that are bound to be closed at image tear down.

It also avoids checking the atexit(3) list, yet here on Linux x86_64
with glibc 2.26-6, `_exit(0)' is more instructions to execute than
`return 0', as measured with `perf stat -e instructions ./exit'.

`return 0' can just do

    xor    %eax, %eax
    retq   

whereas _exit makes room on the stack before the JSR, and that's through
the dynamic-linking table, `PLT'.

    sub    $0x8, %rsp
    xor    %edi, %edi
    callq  530 <_exit at plt>

Even with `-static' linking, `return 0' wins on instruction count.

-- 
Cheers, Ralph.
https://plus.google.com/+RalphCorderoy


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