[LLVMdev] [lld] Wrong references for C++ COMDAT groups

Adhemerval Zanella
2015-05-06T14:43:40Z

Hi,

Checking the llvm test-suite SingleSource/Regression/C++/EH/class_hierarchy
testcase on aarch64 I noted something strange:

Dump of assembler code for function _Z4funcj:
   0x0000000000400650 <+0>:     stp     x22, x21, [sp,#-48]!
   0x0000000000400654 <+4>:     stp     x20, x19, [sp,#16]
   0x0000000000400658 <+8>:     stp     x29, x30, [sp,#32]
   0x000000000040065c <+12>:    add     x29, sp, #0x20
=> 0x0000000000400660 <+16>:    .inst   0x2bfffff7 ; undefined

Where it should be:

0000000000000000 <_Z4funcj>:
   0:   a9bd57f6        stp     x22, x21, [sp,#-48]!
   4:   a9014ff4        stp     x20, x19, [sp,#16]
   8:   a9027bfd        stp     x29, x30, [sp,#32]
   c:   910083fd        add     x29, sp, #0x20
  10:   2a0003f3        mov     w19, w0

And there is no relocation (static or dynamic) point to the faulty instruction.
It exist, however, a group section relocation with same offset, but for a
different text segments (the exception handler stub create by clang):

Relocation section '.rela.text' at offset 0xed8 contains 2 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000010  00480000011b R_AARCH64_CALL26  0000000000000000
_ZNSt9exceptionD2Ev + 0
000000000020  004d0000011a R_AARCH64_JUMP26  0000000000000000 _ZdlPv + 0

Relocation section '.rela.text' at offset 0xf08 contains 2 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000010  00480000011b R_AARCH64_CALL26  0000000000000000
_ZNSt9exceptionD2Ev + 0
000000000020  004d0000011a R_AARCH64_JUMP26  0000000000000000 _ZdlPv + 0

Relocation section '.rela.text' at offset 0xf38 contains 2 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000010  00480000011b R_AARCH64_CALL26  0000000000000000
_ZNSt9exceptionD2Ev + 0
000000000020  004d0000011a R_AARCH64_JUMP26  0000000000000000 _ZdlPv + 0

However seems that LLD is indeed condescending them, but create duplicated
references
for the wrong segments:

Writing atom: _Z4funcj | 1520
                Handle relocJump26 - S: 400460 A: 0 P: 400660 result: 3ffff98
                Handle relocJump26 - S: 400470 A: 0 P: 400670 result: 3ffff98

The first relocJump26 shouldn't be applied to .text segment 0x400660, but
solely on the
_ZN4BaseD0Ev section.  I am trying to debug how lld exactly is generating this
wrong
Reference, but I still can right figure out.  Any idea of what is happening
here?