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[ThinLTO/gold] Perform index-based weak/linkonce resolution

Authored by tejohnson on Jul 13 2016, 6:29 AM.



Invoke the weak/linkonce symbol resolution support (already used by
libLTO) that operates via the summary index.

This ensures prevailing linkonce are kept, by making them weak, and
marks preempted copies as available_externally when possible.

With this change, the older support for keeping the prevailing linkonce
(by changing their symbol resolution) is removed.

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tejohnson updated this revision to Diff 63800.Jul 13 2016, 6:29 AM
tejohnson retitled this revision from to [ThinLTO/gold] Perform index-based weak/linkonce resolution.
tejohnson updated this object.
tejohnson added a reviewer: mehdi_amini.
tejohnson added a subscriber: llvm-commits.
tejohnson updated this revision to Diff 63813.Jul 13 2016, 9:10 AM

Add back a missing piece that I accidentally removed during patch
cleanup: add save-temps dumping of module just after index-based
optimizations and importing, but before the rest of the optimizations.

mehdi_amini accepted this revision.Jul 14 2016, 12:28 PM
mehdi_amini edited edge metadata.

LGTM, with some comments.

1023 ↗(On Diff #63813)

Is this related to the rest of this patch? How was it handled before?

1364 ↗(On Diff #63813)

I'm not sure what is this doing, the comment said "identify symbols referenced by more than a single IR module".

After checking ; it is Preserving any symbols that is not *exclusively referenced from an IR file* (i.e. the comment above the loop is incomplete: it will "identify symbols that can needs to be preserved outside of LTO, i.e. symbols that LTO can't eliminate under any condition".

1370 ↗(On Diff #63813)

GlobalValue::getGUID isn't free, you may want to CSE on these two lines.

This revision is now accepted and ready to land.Jul 14 2016, 12:28 PM
Prazek added a subscriber: Prazek.Jul 14 2016, 12:58 PM
Prazek added inline comments.
1018–1019 ↗(On Diff #63813)

I think it would be good to put this comment in an assert.

Thanks for the comments, will address as noted below before submitting.

1018–1019 ↗(On Diff #63813)

Ok, will do when I add this later in a separate patch.

1023 ↗(On Diff #63813)

It is related in the sense that the new test uses the .import.bc file to do some checking (before the optimization pipeline kicks in and does inlining, e.g.). But actually, I just changed the test to check the .opt.bc saved temps files and it still works - looks like the linkonce_odr removal doesn't happen until code gen. I will remove this new block (which is also useful for debugging, but not then related to this patch, I can submit separately) and change the test.

1364 ↗(On Diff #63813)

Right, I was trying to include both scenarios where we need to preserve this in one comment. I.e. if is is not simply referenced by a single IR modue (referenced from other IR modules or from a non-IR file) then it will not have LDPR_PREVAILING_DEF_IRONLY somewhere and will end up in the Preserve set. I will clarify this in the comment.

1370 ↗(On Diff #63813)

Will do.

mehdi_amini added inline comments.Jul 14 2016, 1:36 PM
1023 ↗(On Diff #63813)

I'm not sure what you mean with linkonce_odr removal does not happen until codegen?
If you turn it into an available_externally, we should always remove them after the inliner is complete.
If you don't turn them into available_externally but they are inlined, the inline is supposed to remove them:

/// Remove now-dead linkonce functions at the end of
/// processing to avoid breaking the SCC traversal.
bool Inliner::doFinalization(CallGraph &CG) {
  return removeDeadFunctions(CG);
tejohnson updated this revision to Diff 64039.Jul 14 2016, 1:47 PM
tejohnson edited edge metadata.
  • Address review comments
This revision was automatically updated to reflect the committed changes.