[lld] r207141 - [PECOFF] Define implicit symbols for exported ones.

Nick Kledzik kledzik at apple.com
Thu Apr 24 14:38:27 PDT 2014

On Apr 24, 2014, at 1:57 PM, Rui Ueyama wrote:

> On Thu, Apr 24, 2014 at 1:29 PM, Chandler Carruth <chandlerc at google.com> wrote:
> On Thu, Apr 24, 2014 at 1:12 PM, Rui Ueyama <ruiu at google.com> wrote:
> Using __imp_ symbols locally is I think not a good coding style. One
> should just take an address using "&" operator rather than appending
> __imp_ prefix. However, there are programs in the wild that depends
> on this link.exe's behavior, so we need this feature.
> I wonder if we should have a warning about this?
> Ideally, yes, but there's no good way to print a warning message when some symbol is actually linked. In this patch I create __imp_ symbols unconditionally for all dllexported symbols. Creating them is not visible to user -- using them should be warned.
It is possible to create the __imp_ atoms lazily.  You have to add a linker internal File object which acts like a static archive. It only returns atoms when asked for an __imp_ symbol.  The File object could then warn when an ImpPointerAtom is instantiated.

I'm not sure if the warning is worth the extra machinery this would require.  Is there a (size) cost to always generating all the ImpPointerAtom atoms?  The lazy way only instantiates ones that are used. 


> I also wonder whether the cause in the wild is source code being shared between the DLL and the users of the DLL and being written "generically".
> The program I spotted in the wild is ffmpeg. That has /include:__imp_sym where sym is a defined function in ffmpeg (/include is equivalent to --undefined). Not sure why it has such option as it is basically no-op to link.exe.
> I don't think writing code generically cannot really justify to use __imp_ symbols, as you can just use "&", so I'm still wondering what this feature is really for...
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