[LLVMbugs] [Bug 19901] New: The plugin can drop a symbol gold has asked for

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=19901

            Bug ID: 19901
           Summary: The plugin can drop a symbol gold has asked for
           Product: tools
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: gold-plugin
          Assignee: unassignedbugs at nondot.org
          Reporter: rafael.espindola at gmail.com
                CC: llvmbugs at cs.uiuc.edu, nlewycky at google.com
    Classification: Unclassified

$ cat test.ll
define linkonce_odr void @f() {
  ret void
}
$ cat test2.ll
define i32 @main() {
  call void @f()
  ret i32 0
}
define linkonce_odr void @f() {
  ret void
}
$ llc test2.ll -o test2.o -filetype=obj
$ llvm-as test.ll -o test.o
$ clang -flto test.o test2.o -o test
$ ./test
Segmentation fault

What is going on is:

gold decides to keep the 'f' defined in the IR file and tells the plugin that.
The plugin calls lto_codegen_add_must_preserve_symbol, but that does nothing
since the symbol is linkonce_odr and the IR linker will not copy it in if it is
not needed (from its narrow point of view).

The fix for the the gold plugin is clear if a bit laborious: apply the
restrictions to the unmerged module. If we are told to keep a symbol, change
the linkage to make sure we do. If we are told to drop a symbol, drop it before
even passing it down to the IR linker.

I will test this on OS X with ld64 in a sec.

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