According to the ia64 ABI standard, the @secrel(expr) directive is supposed to
compute a section-relative displacement as

 "the difference between expr and the address of the beginning of the (output)
  section that contains expr."

However, at the moment, GNU ld (via BFD) calculates @secrel(expr) as the
difference between expr and the address of the beginning of the (output) section
that contains the @secrel() directive.  The following example demostrates this:

On ia64 linux:

$ cat t.S
        .section "__my_text", "a"
        .section "__ex_table", "a"
        .section "__my_text"
        .global _start
_start:
{       nop 0
}
[1:]    nop 1
{
        nop 2
}
        .section "__ex_table"
        data4 @secrel(1b)
$ as t.S -o t.o
$ ld t.o
$ objdump --full --section __ex_table a.out 

a.out:     file format elf64-ia64-little

Contents of section __ex_table:
 40000000000000e0 00000000                             ....            
$ ld --version
GNU ld version 2.15
Copyright 2002 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License.  This program has absolutely no warranty.

If I do the same on HP-UX (using the GNU assembler but the HP-UX linker),
I get:

$ objdump --full --section __ex_table a.out

a.out:     file format elf32-ia64-hpux-big

Contents of section __ex_table:
 4000664 00000010                             ....

I believe the HP-UX linker is correct here: the difference between label 1b and
the beginning of the section containing label 1b is indeed 0x10.