Kean Johnston - Re: Link-time optimzation (original) (raw)

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• From: Kean Johnston
• To: Mark Mitchell
• Cc: gcc mailing list
• Date: Wed, 16 Nov 2005 18:39:31 -0800
• Subject: Re: Link-time optimzation
• References: <437BB214.1070306@codesourcery.com>
• Reply-to: jkj at sco dot com

The document is on the web here: http://gcc.gnu.org/projects/lto/lto.pdfThe LaTeX sources are in htdocs/projects/lto/*.tex.Thoughts?

It may be worth mentioning that this type of optimization applies mainly to one given type of output: a non-symbolic a.out. When the output it a shared library or an a.out with exposed symbols (-Bexport, -E etc), then the semantics will need to be different.

In the simplest case where you are producing a non-symbolic a.out, you can detect that global functions or variables are completely unused, and therefore discard them. However, if the a.out has some of its symbols exposed by linker options, then those symbols, and any other symbols or functions which they reference, must be left intact. Similarly for shared libraries, although that case is slightly worse as any global symbol must be left intact, unless a specific export list is being used, in which case only those symbols, and symbols which they in turn reference, need survive.

At least thats my understanding.

Also, this whole issue of visible symbols and shared libraries may well be covered by requirement 3, and if so, it may be worth mentioning since there was no accompanying rationale for requirement 3.

Kean

• References:
  • Link-time optimzation
    * From: Mark Mitchell