From: Paul Eggert <eggert@cs.ucla.edu>
To: Andreas Schwab <schwab@linux-m68k.org>
Cc: emacs-devel@gnu.org
Subject: Re: VIRT_ADDR_VARIES
Date: Sun, 06 Nov 2011 17:50:59 -0800 [thread overview]
Message-ID: <4EB73983.1060000@cs.ucla.edu> (raw)
In-Reply-To: <m2lirtnp8g.fsf@igel.home>
On 11/06/11 09:18, Andreas Schwab wrote:
> Is there any reason not to define VIRT_ADDR_VARIES?
I expect the main reason is performance. For example,
on x86-64 with GCC 4.6.2 -O2, the function:
int pure (Lisp_Object obj) { return PURE_P (obj); }
has 8 instructions (not counting the 'ret') if VIRT_ADDR_VARIES
is defined, and 4 instructions if it is not defined. The key difference
is that VIRT_ADDRESS_VARIES generates this:
cmpq $pure+1000000, %rdi
jge .L2
xorl %eax, %eax
cmpq $pure, %rdi
setge %al
.L2: rep
(where the "1000000" is a function of PURESIZE),
whereas omitting VIRT_ADDRESS_VARIES generates this:
cmpq $my_edata, %rdi
setl %al
> The only difference
> it makes is that the bounds of the pure array are checked accurately
True, but for Emacs it shouldn't matter whether PURE_P checks
accurately or loosely -- either way Emacs should operate correctly.
next prev parent reply other threads:[~2011-11-07 1:50 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-11-06 17:18 VIRT_ADDR_VARIES Andreas Schwab
2011-11-07 1:50 ` Paul Eggert [this message]
2011-11-07 8:38 ` VIRT_ADDR_VARIES Andreas Schwab
2011-11-07 23:23 ` VIRT_ADDR_VARIES Paul Eggert
2011-11-08 8:56 ` VIRT_ADDR_VARIES Andreas Schwab
2011-11-08 17:34 ` VIRT_ADDR_VARIES Paul Eggert
2011-11-08 18:38 ` VIRT_ADDR_VARIES Andreas Schwab
2011-11-08 21:17 ` VIRT_ADDR_VARIES Paul Eggert
2011-11-08 22:06 ` VIRT_ADDR_VARIES Andreas Schwab
2011-11-09 17:44 ` VIRT_ADDR_VARIES Paul Eggert
2011-11-09 21:32 ` VIRT_ADDR_VARIES Andreas Schwab
2011-11-10 8:17 ` VIRT_ADDR_VARIES Paul Eggert
2011-11-10 9:29 ` VIRT_ADDR_VARIES Andreas Schwab
2011-11-10 16:19 ` VIRT_ADDR_VARIES Paul Eggert
2011-11-10 11:06 ` VIRT_ADDR_VARIES Eli Zaretskii
2011-11-10 11:20 ` VIRT_ADDR_VARIES Andreas Schwab
2011-11-10 16:29 ` VIRT_ADDR_VARIES Paul Eggert
2011-11-10 16:43 ` VIRT_ADDR_VARIES Eli Zaretskii
2011-11-10 16:50 ` VIRT_ADDR_VARIES Paul Eggert
2011-11-10 18:06 ` VIRT_ADDR_VARIES Eli Zaretskii
2011-11-14 4:57 ` VIRT_ADDR_VARIES Paul Eggert
2011-11-10 4:05 ` VIRT_ADDR_VARIES Stephen J. Turnbull
2011-11-10 5:40 ` VIRT_ADDR_VARIES Paul Eggert
2011-11-10 17:05 ` VIRT_ADDR_VARIES Andreas Schwab
2011-11-10 17:15 ` VIRT_ADDR_VARIES Paul Eggert
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