Subject: Re: Bogus variable in ld.elf_so
To: None <>
From: Charles Hannum <>
List: tech-toolchain
Date: 09/05/2002 18:43:08
> I don't understand this in ld.elf_so:
>                 tmp = (Elf_Addr)(defobj->relocbase + def->st_value);
>                 if (*where != tmp)
>                         *where = tmp;
> If *where is not same as tmp, value of tmp is assigned to *where and
> they have same value. If *where is same as tmp, they are still same.
> I can see there lots of unnecessary operations like above. Wouldn't
> it be made simpler like
>                 *where = (Elf_Addr)(defobj->relocbase + def->st_value);

You'll find that executables, at least, already have *where set to
point to the relevant function, if it's in the executable.  Thus, the
conditional store potentially avoids copy-on-write on some number of
pages in each executable.

In practice it doesn't actually help *much*, but it's not entirely
gratuitous.  It might help more if the linker actually sorted the PLT
and GOT by whether or not the symbol was defined...