| 1 | /* GNU Objective C Runtime class related functions
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| 2 | Copyright (C) 1993, 1995, 1996, 1997, 2001 Free Software Foundation, Inc.
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| 3 | Contributed by Kresten Krab Thorup and Dennis Glatting.
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| 4 |
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| 5 | Lock-free class table code designed and written from scratch by
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| 6 | Nicola Pero, 2001.
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| 7 |
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| 8 | This file is part of GNU CC.
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| 9 |
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| 10 | GNU CC is free software; you can redistribute it and/or modify it under the
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| 11 | terms of the GNU General Public License as published by the Free Software
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| 12 | Foundation; either version 2, or (at your option) any later version.
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| 13 |
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| 14 | GNU CC is distributed in the hope that it will be useful, but WITHOUT ANY
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| 15 | WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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| 16 | FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
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| 17 | details.
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| 18 |
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| 19 | You should have received a copy of the GNU General Public License along with
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| 20 | GNU CC; see the file COPYING. If not, write to the Free Software
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| 21 | Foundation, 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */
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| 22 |
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| 23 | /* As a special exception, if you link this library with files compiled with
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| 24 | GCC to produce an executable, this does not cause the resulting executable
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| 25 | to be covered by the GNU General Public License. This exception does not
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| 26 | however invalidate any other reasons why the executable file might be
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| 27 | covered by the GNU General Public License. */
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| 28 |
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| 29 | /*
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| 30 | The code in this file critically affects class method invocation
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| 31 | speed. This long preamble comment explains why, and the issues
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| 32 | involved.
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| 33 |
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| 34 |
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| 35 | One of the traditional weaknesses of the GNU Objective-C runtime is
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| 36 | that class method invocations are slow. The reason is that when you
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| 37 | write
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| 38 |
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| 39 | array = [NSArray new];
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| 40 |
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| 41 | this gets basically compiled into the equivalent of
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| 42 |
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| 43 | array = [(objc_get_class ("NSArray")) new];
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| 44 |
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| 45 | objc_get_class returns the class pointer corresponding to the string
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| 46 | `NSArray'; and because of the lookup, the operation is more
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| 47 | complicated and slow than a simple instance method invocation.
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| 48 |
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| 49 | Most high performance Objective-C code (using the GNU Objc runtime)
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| 50 | I had the opportunity to read (or write) work around this problem by
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| 51 | caching the class pointer:
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| 52 |
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| 53 | Class arrayClass = [NSArray class];
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| 54 |
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| 55 | ... later on ...
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| 56 |
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| 57 | array = [arrayClass new];
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| 58 | array = [arrayClass new];
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| 59 | array = [arrayClass new];
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| 60 |
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| 61 | In this case, you always perform a class lookup (the first one), but
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| 62 | then all the [arrayClass new] methods run exactly as fast as an
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| 63 | instance method invocation. It helps if you have many class method
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| 64 | invocations to the same class.
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| 65 |
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| 66 | The long-term solution to this problem would be to modify the
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| 67 | compiler to output tables of class pointers corresponding to all the
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| 68 | class method invocations, and to add code to the runtime to update
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| 69 | these tables - that should in the end allow class method invocations
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| 70 | to perform precisely as fast as instance method invocations, because
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| 71 | no class lookup would be involved. I think the Apple Objective-C
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| 72 | runtime uses this technique. Doing this involves synchronized
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| 73 | modifications in the runtime and in the compiler.
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| 74 |
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| 75 | As a first medicine to the problem, I [NP] have redesigned and
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| 76 | rewritten the way the runtime is performing class lookup. This
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| 77 | doesn't give as much speed as the other (definitive) approach, but
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