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

From: Ulrich Drepper <[email-masked]>
Date: Mon, 05 May 2008 19:39:34 -0700

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.

- --
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖