[llvm-bugs] [Bug 47747] New: Clang produces an incomplete comdat

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47747

            Bug ID: 47747
           Summary: Clang produces an incomplete comdat
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: rafael at espindo.la
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Given

common.hh:
-------------------------
struct zed {
    zed() noexcept;
};
template <typename T>
thread_local zed var;
-------------------------

test1.cc:
-------------------------
#include "common.hh"
inline void* foobaz() {
    return &var<int>;
}
-------------------------

test2.cc:
-------------------------
#include "common.hh"
void* foobar() {
    return &var<int>;
}
-------------------------

Compile with just

$ clang++ -c test1.cc -o test1.o
$ clang++ -c test2.cc -o test2.o

$ readelf -gsW test2.o

....
COMDAT group section [    7] `.group' [_Z3varIiE] contains 4 sections:
   [Index]    Name
   [    8]   .text.startup
   [    9]   .rela.text.startup
   [   10]   .tbss._Z3varIiE
   [   11]   .tbss._ZGV3varIiE
...
    10: 0000000000000000    56 FUNC    WEAK   DEFAULT    8 _ZTH3varIiE


So the symbol _ZTH3varIiE is in a section that is in the _Z3varIiE
comdat.

But in the other file:

$ readelf -gsW test1.o

COMDAT group section [    3] `.group' [_Z3varIiE] contains 4 sections:
   [Index]    Name
   [    4]   .text.startup
   [    5]   .rela.text.startup
   [    6]   .tbss._Z3varIiE
   [    7]   .tbss._ZGV3varIiE

We have the same comdat, but the symbol _ZTH3varIiE is missing. This
means that if the two files are linked, _ZTH3varIiE can be resolved to
0.

GCC correctly outputs _ZTH3varIiE in both files.

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