[PATCH] D16440: [ThinLTO] Link in only necessary DICompileUnit operands

[Mehdi Amini via llvm-commits]

> On Mar 23, 2016, at 6:39 AM, Teresa Johnson <tejohnson at google.com> wrote:
> 
> 
> 
> On Tue, Mar 22, 2016 at 4:36 PM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
> Hey Teresa,
> 
> How will the changes we were discussing with Adrian yesterday on IRC will affect this patch?
> 
> Yes, not from the moving of metadata into the function block where possible, but with the reversing of the DICompileUnit -> DISubprogram links. 
> 
> With the link reversal, the DICompileUnit will automatically be fully mapped in when the function bodies are mapped and the associated metadata including the DISubprogram are mapped in, thus pulling in all the retained types and other fields on the CU we are trying to drop here for imported modules. So we'd either need to figure out how to stop the metadata mapping that happens in MapMetadata when we hit a DICompileUnit, so that we could map it manually as we do here, or figure out what to map via some callbacks from the ValueMapper (similar to how we prevent unneeded DISubprogram from getting mapped into the DISubprogram list hanging of the DICompileUnit currently).
> 
> If the retained types list is also dropped from the DICompileUnit, which I see is also on Duncan's list, then at least the above mapping would not automatically pull in all types. But if we don't do that before reversing the CU->SP link then it needs to be handled.
> 
> So what we want to do with this patch will depend on how fast some of these changes under discussion will go in. It results in a greatly reduced amount of type data being pulled in with -flto=thin -g, so I'm not sure we want to wait until all the debug info changes under discussion are in.

In case we were not clear: the changes we're working on on our side are intended to be on a couple of weeks timeframe from now. The only I don't know the exact plan is the retained types.

-- 
Mehdi


> 
> One way to handle this with the current patch if the link reversal goes in before the retained types etc are removed from the CU is to put back the findReachedCompositeTypes helper that I just removed with my latest update and use it to find the needed types and manually create/map the new pruned DICompileUnit *before* we do any function body linking. Then when the DISubprogram are mapped during function body linking the mapper will find the new DICompileUnit in the ValueMap and use it. I *think* I can do this now with some restructuring of this patch, or we could do that just after the link reversal goes in (we would temporarily lose out on the benefits of this patch in between when the link reversal and the change to this handling goes in).
> 
> Teresa
> 
> 
> --
> Mehdi
> 
> > On Mar 22, 2016, at 2:35 PM, Teresa Johnson <tejohnson at google.com <mailto:tejohnson at google.com>> wrote:
> >
> > tejohnson updated this revision to Diff 51343.
> > tejohnson added a comment.
> >
> > - Address review comments from dblaikie.
> >
> >
> > http://reviews.llvm.org/D16440 <http://reviews.llvm.org/D16440>
> >
> > Files:
> >  include/llvm/Linker/IRMover.h
> >  lib/Linker/IRMover.cpp
> >  lib/Linker/LinkModules.cpp
> >  lib/Transforms/Utils/ValueMapper.cpp
> >  test/Linker/thinlto_funcimport_debug.ll
> >  test/Transforms/FunctionImport/Inputs/funcimport_debug.ll
> >  test/Transforms/FunctionImport/funcimport_debug.ll
> >
> > <D16440.51343.patch>
> 
> 
> 
> 
> -- 
> Teresa Johnson |	 Software Engineer |	 tejohnson at google.com <mailto:tejohnson at google.com> |	 408-460-2413 <tel:408-460-2413>
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