From: Daniel Jacobowitz <drow@false.org>
To: Eli Zaretskii <eliz@gnu.org>
Cc: gdb@sources.redhat.com, Reiner.Steib@gmx.de
Subject: Re: Variable "foo" is not available
Date: Mon, 04 Apr 2005 19:41:00 -0000 [thread overview]
Message-ID: <20050404194131.GA20570@nevyn.them.org> (raw)
In-Reply-To: <01c5394c$Blat.v2.4$e4580a80@zahav.net.il>
On Mon, Apr 04, 2005 at 10:30:23PM +0300, Eli Zaretskii wrote:
> But this kind of code is only possible if the compiler examines all
> the callers of `bar' and finds that none of them uses the value of
> `bar's argument after `bar' returns. So such code is probably only
> possible in practice with static functions, right? Or am I missing
> something?
That would be true if the ABI labeled the stack slots as preserved
across the function call. The consensus seems to be that it does not;
at the point of the function call they become part of the callee's
stack frame and it is free to modify them.
> Also, is such optimizations really worth it? I mean, the more
> traditional code will mov the argument into a register and do the math
> there; is adding to a memory location really faster than a mov and a
> register-based add?
Yes, especially on decoding-limited processors.
Also, sibling calling does the same thing. It reuses the stack slots
to pass arguments to a new function, and then jumps to the function.
> > GCC won't reuse the slot for an unrelated variable at present.
> > However, in the future, it would be a valid optimization.
>
> Again, only if the compiler has enough information about the callers,
> right?
No, as above.
--
Daniel Jacobowitz
CodeSourcery, LLC
next prev parent reply other threads:[~2005-04-04 19:41 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-04-01 16:40 Reiner Steib
2005-04-01 17:19 ` Daniel Jacobowitz
2005-04-02 9:49 ` Eli Zaretskii
2005-04-02 13:53 ` Reiner Steib
2005-04-02 14:27 ` Daniel Jacobowitz
2005-04-06 16:25 ` Reiner Steib
2005-04-02 14:26 ` Daniel Jacobowitz
2005-04-02 18:17 ` Eli Zaretskii
2005-04-02 18:40 ` Daniel Jacobowitz
2005-04-02 20:58 ` Eli Zaretskii
2005-04-02 21:05 ` Daniel Jacobowitz
2005-04-04 5:14 ` Eli Zaretskii
2005-04-04 6:00 ` Mark Kettenis
2005-04-04 7:58 ` Daniel THOMPSON
2005-04-04 19:28 ` Eli Zaretskii
2005-04-04 13:37 ` Daniel Jacobowitz
2005-04-04 19:35 ` Eli Zaretskii
2005-04-04 19:41 ` Daniel Jacobowitz [this message]
2005-04-03 18:16 ` Reiner Steib
2005-04-08 11:05 ` Eli Zaretskii
2005-04-04 9:26 ` Reiner Steib
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