Re: compiler bugs

anton@mips.complang.tuwien.ac.at (Anton Ertl)
Wed, 06 May 2009 11:37:27 GMT

          From comp.compilers
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[22 later articles]
| List of all articles for this month |
From: anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups: comp.compilers
Date: Wed, 06 May 2009 11:37:27 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
References: 09-04-072 09-04-086 09-05-010 09-05-022 09-05-028 09-05-038
Keywords: errors, optimize
Posted-Date: 07 May 2009 07:30:29 EDT

"Christopher Glaeser" <cdg@nullstone.com> writes:
[Anton Ertl:]
>> the optimizer should preserve the behaviour of the non-optimizing
>> compiler.
>
>In a language like C, the behavior of programs that reference
>uninitialized variables can be affected by the slightest change to
>register assignments.


Not if the unoptimizing compiler initializes all variables to some
known value.


>Given that optimizations like copy propagation and common
>subexpression elimination can potentially change register
>assignments, and thus change the behavior of these programs, why
>would you include these optimizations in your list of permissible
>optimizations?


As you can see, in this case this is easy to solve. It has the nice
side effect of making even the unoptimized behaviour more stable,
which programmers also like, because it makes, e.g., debugging easier.


- anton
--
M. Anton Ertl
anton@mips.complang.tuwien.ac.at
http://www.complang.tuwien.ac.at/anton/


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