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57 lines
2.6 KiB
Plaintext
Date: Tue, 18 Sep 2001 00:38:37 -0500 (CDT)
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From: Chris Lattner <sabre@nondot.org>
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To: Vikram S. Adve <vadve@cs.uiuc.edu>
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Subject: Idea for a simple, useful link time optimization
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In C++ programs, exceptions suck, and here's why:
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1. In virtually all function calls, you must assume that the function
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throws an exception, unless it is defined as 'nothrow'. This means
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that every function call has to have code to invoke dtors on objects
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locally if one is thrown by the function. Most functions don't throw
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exceptions, so this code is dead [with all the bad effects of dead
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code, including icache pollution].
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2. Declaring a function nothrow causes catch blocks to be added to every
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call that isnot provably nothrow. This makes them very slow.
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3. Extra extraneous exception edges reduce the opportunity for code
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motion.
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4. EH is typically implemented with large lookup tables. Ours is going to
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be much smaller (than the "standard" way of doing it) to start with,
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but eliminating it entirely would be nice. :)
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5. It is physically impossible to correctly put (accurate, correct)
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exception specifications on generic, templated code. But it is trivial
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to analyze instantiations of said code.
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6. Most large C++ programs throw few exceptions. Most well designed
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programs only throw exceptions in specific planned portions of the
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code.
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Given our _planned_ model of handling exceptions, all of this would be
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pretty trivial to eliminate through some pretty simplistic interprocedural
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analysis. The DCE factor alone could probably be pretty significant. The
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extra code motion opportunities could also be exploited though...
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Additionally, this optimization can be implemented in a straight forward
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conservative manner, allowing libraries to be optimized or individual
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files even (if there are leaf functions visible in the translation unit
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that are called).
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I think it's a reasonable optimization that hasn't really been addressed
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(because assembly is way too low level for this), and could have decent
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payoffs... without being a overly complex optimization.
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After I wrote all of that, I found this page that is talking about
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basically the same thing I just wrote, except that it is translation unit
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at a time, tree based approach:
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http://www.ocston.org/~jls/ehopt.html
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but is very useful from "expected gain" and references perspective. Note
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that their compiler is apparently unable to inline functions that use
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exceptions, so there numbers are pretty worthless... also our results
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would (hopefully) be better because it's interprocedural...
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What do you think?
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-Chris
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