Re: [PATCH] elf: Add the soname to the libname_list eagerly on loading a library

[Florian Weimer <fweimer@redhat.com>]

* Fangrui Song via Libc-alpha:

> Original author is Ambrose Feinstein while working at Google.
>
> _dl_map_object iterates over loaded objects and calls _dl_name_match_p.
> If l->l_soname_added is 0, we incur two costs.
>
> First, loading l->l_info[DT_SONAME]->d_un.d_val has TLB pressure as
> every library's string table is in a different page.  The cost will be
> avoided if the string is on the heap.
>
> Second, add_name_to_object repeats the l_libname comparison already done
> by the _dl_name_match_p call.
>
> To remove these costs, we eagerly add the SONAME to the libname_list.
> l_soname_added is typically 1, so laziness doesn't provide savings.
> ---
>  elf/dl-load.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/elf/dl-load.c b/elf/dl-load.c
> index 9a0e40c0e9..1b17410ce0 100644
> --- a/elf/dl-load.c
> +++ b/elf/dl-load.c
> @@ -1451,10 +1451,11 @@ cannot enable executable stack as shared object requires");
>  
>    /* When we profile the SONAME might be needed for something else but
>       loading.  Add it right away.  */
> -  if (__glibc_unlikely (GLRO(dl_profile) != NULL)
> -      && l->l_info[DT_SONAME] != NULL)
> +  if (l->l_info[DT_SONAME] != NULL) {
>      add_name_to_object (l, ((const char *) D_PTR (l, l_info[DT_STRTAB])
>  			    + l->l_info[DT_SONAME]->d_un.d_val));
> +    l->l_soname_added = 1;
> +  }
>  
>    /* If we have newly loaded libc.so, update the namespace
>       description.  */

The comment is now outdated.

Not sure if this kind of micro-optimization makes sense.  Maybe until we
add a hash table here …

Thanks,
Florian