[PATCH] D72486: Add ThinLtoJIT example

[Teresa Johnson via Phabricator via llvm-commits]

tejohnson added inline comments.


================
Comment at: llvm/examples/ThinLtoJIT/ThinLtoModuleIndex.cpp:54
+  if (VI.getSummaryList().size() > 1) {
+    LLVM_DEBUG(dbgs() << "SummaryList with multiple entries!\n");
+  }
----------------
sgraenitz wrote:
> @tejohnson In which cases does the summary list for a global value have more than 1 entry? Is it reasonable to always refer to the first one (while this is a work in progress)? I am currently limited to sources in C and never see it happen.
In C you can presumably still get duplicates on symbols declared with __attribute__((weak)), a GNU extension supported by gcc and clang.

Additionally, you could also get duplicates when there are same-named local symbols in same-named source files in different subdirectories and the user has not compiled with enough distinguishing source path. E.g. assume you have local symbol foo defined in path1/bar.c and path2/bar.c, and the user has done the following:

```
$ cd path1
$ clang -c -flto=thin bar.c
$ cd ../path2
$ clang -c -flto=thin bar.c
```

When you link those two bar.o together it will have two matching GUID since the globalized name for locals used to compute the GUID has the relative path (encoded in the module) prepended, and in this case that is both "." essentially. It happens, and we support this typically by looking for one in the same module as the caller:
http://llvm-cs.pcc.me.uk/lib/Transforms/IPO/FunctionImport.cpp#216

or in the current module when doing some handling in the ThinLTO backend, e.g.:
http://llvm-cs.pcc.me.uk/lib/Transforms/Utils/FunctionImportUtils.cpp#62




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