Max (max@a.com) on 7/2/10 wrote:
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>Max (max@a.com) on 7/2/10 wrote:
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>>rwessel (robertwessel@yahoo.com) on 7/2/10 wrote:
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>>>In any event, LTCG is the solution to the optimization issue.
>>
>>If you don't care about compile speed, you can just compile everything at once.
>>That's simpler and more effective than duplicating parts of the compiler in the linker.
>
>On second thought, compile everything at once (starting from the intermediate language,
>not source) is probably exactly what you meant by LTCG. That is, LTCG is not enhanced
>traditional linking, it's a full blown compile with link semantics. Right?

Typically Link Time Code Generation works by having the compiler store the intermediate representation (usually something along the lines of the parse tree) in the "object" file, rather than passing it to the back end of the compiler for turning into object code. Usually the object file contains the name of the required back end (MSVC, for example, stores the full directory and file name of the compiler back end DLL).

The linker then collects all the intermediate "code" objects destined for a given back end, and passes then to that (actual) compiler back end, which then (typically) generates a "real" object file, which the linker then handles as usual. Other “real” object files are handled in the traditional way.

This usually has minimal impact on either the linker or compiler. The compiler back end has to handle names a bit differently (basically static names now need additional qualification), but not too much. The linker obviously has to recognize the new format, and deal with that, but usually then just has to deal with a normal object file.

I'm not sure what you mean by "compile all at once." Typically invoking the compiler with the command line specifying several source modules does *not* provide for any sort of inter-translation unit optimization, it just automatically compiles each source module as if you had invoked the compiler several times. It would, however, not be at all unreasonable for the compiler to invoke LTCG in such a case, but I don’t know of anyone who does that.