Re: [PATCH] lto-plugin: add support for feature detection

["Martin Liška" <mliska@suse.cz>]

CCing mold's author.

On 5/6/22 16:46, Alexander Monakov wrote:
> 
> 
> On Thu, 5 May 2022, Martin Liška wrote:
> 
>> On 5/5/22 12:52, Alexander Monakov wrote:
>>> Feels a bit weird to ask, but before entertaining such an API extension,
>>> can we step back and understand the v3 variant of get_symbols? It is not
>>> documented, and from what little I saw I did not get the "motivation" for
>>> its existence (what it is doing that couldn't be done with the v2 api).
>>
>> Please see here:
>> https://github.com/rui314/mold/issues/181#issuecomment-1037927757
> 
> Thanks. I've also re-read [1] and [2] which provided some relevant ideas.
> 
> [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86490
> [2] https://sourceware.org/bugzilla/show_bug.cgi?id=23411
> 
> 
> OK, so the crux of the issue is that sometimes the linker needs to feed the
> compiler plugin with LTO .o files extracted from static archives. This is
> not really obvious, because normally .a archives have an index that enumerates
> symbols defined/used by its .o files, and even during LTO the linker can simply
> consult the index to find out which members to extract.  In theory, at least.
> 
> The theory breaks in the following cases:
> 
>  - ld.bfd and common symbols (I wonder if weak/comdat code is also affected?):
>  archive index does not indicate which definitions are common, so ld.bfd
>  extracts the member and feeds it to the plugin to find out;
> 
>  - ld.gold and emulated archives via --start-lib a.o b.o ... --end-lib: here
>  there's no index to consult and ld.gold feeds each .o to the plugin.
> 
> In those cases it may happen that the linker extracts an .o file that would
> not be extracted during non-LTO link, and if that happens, the linker needs to
> inform the plugin. This is not the same as marking each symbol from spuriously
> extracted .o file as PREEMPTED when the .o file has constructors (the plugin
> will assume the constructors are kept while the linker needs to discard them).
> 
> So get_symbols_v3 allows the linker to discard an LTO .o file to solve this.
> 
> In absence of get_symbols_v3 mold tries to ensure correctness by restarting
> itself while appending a list of .o files to be discarded to its command line.
> 
> I wonder if mold can invoke plugin cleanup callback to solve this without
> restarting.
> 
> (also, hm, it seems to confirm my idea that LTO .o files should have had the
> correct symbol table so normal linker algorithms would work)
> 
> Hopefully this was useful.
> Alexander