Re: [PATCH 01/18] flag parameters: helper function

[Ulrich Drepper]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Andrew Morton wrote:
> hm, that looks expensive.  The compiler will need to generate a deref of m
> and rf multiple times around the loop.  Copying them into locals does
> improve that a lot.

There really is no problem.  The value is in L1d when it is reused.
This is the generated code (%rdi is m):

   f:   85 17                   test   %edx,(%rdi)
  11:   74 0b                   je     1e <flags_remap+0x1e>
  13:   8b 47 04                mov    0x4(%rdi),%eax
  16:   09 01                   or     %eax,(%rcx)
  18:   8b 07                   mov    (%rdi),%eax
  1a:   f7 d0                   not    %eax
  1c:   21 c2                   and    %eax,%edx

At address 18 the load will be satisfied from L1d.  If you would want to
cache the value at address f you'd have to create one more instruction.

This really is the best code sequence.  The compiler could have chosen
to move the value into a register because the array is const.  But it
didn't.


> Also: sorry, but ugh-at-the-naming.  We don't *gain* anything from having
> idenitifers called f, of, m, n and rf.  And we lose quite a lot in
> readability and understandability.  It would be much nicer to invest a
> little bit more typing-time here, IMO.

That's Davide's code and I didn't change it because it doesn't really
matter.  This is a trivial function which doesn't need more than 10
seconds to be understood.  If you insist I'll rename the variables and
elements but I consider this just busy work.

- --
â Ulrich Drepper â Red Hat, Inc. â 444 Castro St â Mountain View, CA â
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.7 (GNU/Linux)

iD8DBQFIH8Tl2ijCOnn/RHQRAm+PAKDEArdgKWXLAMzzfYQ3Q8XzbJdWmgCfavq+
+THu/JEISm+IrX7oyhITYVU=
=F2Co
-----END PGP SIGNATURE-----
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/