[llvm] r290537 - Test the different scenarios of GlobalDCE and comdats more

[Rafael Avila de Espindola via llvm-commits]

Chandler Carruth via llvm-commits <llvm-commits at lists.llvm.org> writes:
> For example, it seemed like a bug that GlobalDCE didn't consider comdat
> participation of ifuncs. Specifically it seemed like an accident because
> testing didn't really cover that case. But in fact, ifuncs specifically
> cannot participate in a comdat despite having that API. The new test
> case covers this and explicitly documents that DCE gets to fire here
> even though there are comdats involved.

I think your first impression was correct and you found a bug.

Given just

$test5_c = comdat any
@test5_if = linkonce_odr ifunc void (), void ()* @test5_f
define linkonce_odr void @test5_f() comdat($test5_c) {
  ret void
}

llc produces

        .weak   test5_if
        .type   test5_if, at function
        .type   test5_if, at gnu_indirect_function
test5_if = test5_f

which both gas and MC map to test5_f and test5_if having the same
section and that section being in a comdat:

COMDAT group section [    3] `.group' [test5_c] contains 1 sections:
   [Index]    Name
   [    4]   .text.test5_f

Symbol table '.symtab' contains 6 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS test.ll
     2: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    3 test5_c
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    4
     4: 0000000000000000     1 FUNC    WEAK   DEFAULT    4 test5_f
     5: 0000000000000000     1 IFUNC   WEAK   DEFAULT    4 test5_if

So if this comdat is not selected, the linker will treat test5_f and
test5_if as undefined.

Cheers,
Rafael