[llvm-commits] [PATCH] generate ABI-compliant sections for uninitialized static data members in template instantiations

[Kristof Beyls]

The attached patch fixes a problem where LLVM doesn't generate .bss sections
in a C++ ABI-compliant way.
Would someone be able to review and apply the patch if fine?

Thanks,

Kristof


Here is some background information:

The following section of the generic C++ ABI
(http://sourcery.mentor.com/public/cxx-abi/abi.html#vague-itemplate) states:
"""
5.2.6 Instantiated Templates

An instantiation of a class template requires:
....
* Any static member data object is emitted in a COMDAT identified by its
mangled name, in any object file with a reference to its name symbol.
....
"""

However, for uninitialized static member data objects in instantiated
templates, LLVM does not produce a COMDAT.
For the following test case:

template<class T> struct C {
    static int i;
    static int j;
};
template<class T> int C<T>::i;
template<class T> int C<T>::j = 12;
void f(C<int> c) { c.i++; c.j++; }

LLVM produces the following assembler output for the template instantiations
of C<T>::i and C<T>::j:

        .type   _ZN1CIiE1iE,%object     @ @_ZN1CIiE1iE
        .bss
        .weak   _ZN1CIiE1iE
        .align  2
_ZN1CIiE1iE:
        .long   0                       @ 0x0
        .size   _ZN1CIiE1iE, 4

        .type   _ZN1CIiE1jE,%object     @ @_ZN1CIiE1jE
        .section        .data._ZN1CIiE1jE,"aGw",%progbits,_ZN1CIiE1jE,comdat
        .weak   _ZN1CIiE1jE
        .align  2
_ZN1CIiE1jE:
        .long   12                      @ 0xc
        .size   _ZN1CIiE1jE, 4

Note that for C<T>::i (mangled: _ZN1CIiE1iE), no comdat section is produced.
OTOH, for C<T>::j, a comdat section is produced.
The correct behaviour would be to produce the following section for C<T>::i

        .type   _ZN1CIiE1iE,%object     @ @_ZN1CIiE1iE
        .section        .bss._ZN1CIiE1jE,"aGw",%nobits,_ZN1CIiE1jE,comdat
        .weak   _ZN1CIiE1iE
        .align  2
_ZN1CIiE1iE:
        .long   0                       @ 0x0
        .size   _ZN1CIiE1iE, 4


This is also what gcc produces.
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