Re: sort dynamic linking overhead

[Bruno Haible]

Paul Eggert wrote:
> Thanks for the patch. I was hoping that we didn't need to worry about 
> older platforms needing -ldl. Oh well.

Distros with glibc < 2.34 include:
  Debian 11
  CentOS 8 stream
  Fedora 34
  openSUSE 15.5
  Slackware 15
These are not obsolete, so far.

> The patch causes 'configure' to search for dlopen even when there's no 
> crypto library. 'configure' could instead use AC_SEARCH_LIBS only if the 
> AC_LINK_IFELSE fails (or simply put AC_LINK_ELSE in an 'for LIB_DL in "" 
> -ldl' loop). But perhaps it's better to leave things be, in case we ever 
> need dlopen for something else.

If we were to start optimizing configure.ac scripts like this, they would
quickly become hard to maintain, because here and there we would be
accessing a shell variable that has not been initialized (because the
initialization was in an 'if' branch). (There's a reason why AC_REQUIRE
has been invented...)

The configure tests which nag me the most are those which take one second
of time or more: the fcntl test, the sleep test, the strstr test, and a few
others. If someone is starting to optimize, here would be the starting point :)

> Also, if I understand things correctly, with this patch it's 
> theoretically possible to pass -ldl to gcc even when 'sort' doesn't do 
> dynamic linking, and we could complicate configure.ac further to omit 
> -ldl in that case. But I doubt whether it's worth worrying about this.

Correct, but this applies only to glibc and Android systems, and on these
systems configure finds

  checking for C compiler flag to ignore unused libraries... -Wl,--as-needed

So, if $(LIB_DL) is -ldl and this linker option gets used for 'sort', but
'sort' does not use dlopen(), the linker will eliminate it.

Bruno