[LLVMbugs] [Bug 13581] New: integrated assembler gets .debug_frame wrong

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=13581

             Bug #: 13581
           Summary: integrated assembler gets .debug_frame wrong
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: nicholas at mxc.ca
                CC: llvmbugs at cs.uiuc.edu, rafael.espindola at gmail.com
    Classification: Unclassified


The integrated assembler produces a .debug_frame that doesn't work with gdb,
and is different from what GNU as produces. However, we produce the correct
.eh_frame, when we choose to do so.

$ cat 3.cc
int rs();
static int i = rs();
$ clang 3.cc -c -o 3.good.o -fPIC -g
$ clang 3.cc -c -o 3.bad.o -fPIC -g -fno-exceptions
$ clang 3.cc -c -o 3.gnuas.o -fPIC -g -fno-exceptions -no-integrated-as

If you check the assembly instead of going straight to .o files, the only
difference between -fno-exceptions and not is whether we emit a ".cfi_sections
.debug_frame" directive (we do in -fno-exceptions builds). Let's take a look at
what we just did:

$ objdump -Wf 3.good.o

3.good.o:     file format elf64-x86-64

Contents of the .eh_frame section:

00000000 00000014 00000000 CIE
  Version:               1
  Augmentation:          "zR"
  Code alignment factor: 1
  Data alignment factor: -8
  Return address column: 16
  Augmentation data:     1b

  DW_CFA_def_cfa: r7 (rsp) ofs 8
  DW_CFA_offset: r16 (rip) at cfa-8
  DW_CFA_nop
  DW_CFA_nop

00000018 00000018 0000001c FDE cie=00000000 pc=00000000..00000011
  DW_CFA_advance_loc: 1 to 00000001
  DW_CFA_def_cfa_offset: 16
  DW_CFA_offset: r6 (rbp) at cfa-16
  DW_CFA_advance_loc: 3 to 00000004
  DW_CFA_def_cfa_register: r6 (rbp)
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop

00000034 00000018 00000038 FDE cie=00000000 pc=00000020..0000002b
  DW_CFA_advance_loc: 1 to 00000021
  DW_CFA_def_cfa_offset: 16
  DW_CFA_offset: r6 (rbp) at cfa-16
  DW_CFA_advance_loc: 3 to 00000024
  DW_CFA_def_cfa_register: r6 (rbp)
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop

$ objdump -Wf 3.bad.o

3.bad.o:     file format elf64-x86-64

Contents of the .debug_frame section:

00000000 00000014 ffffffff CIE
  Version:               1
  Augmentation:          ""
  Code alignment factor: 1
  Data alignment factor: -8
  Return address column: 16

  DW_CFA_def_cfa: r7 (rsp) ofs 8
  DW_CFA_offset: r16 (rip) at cfa-8
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop

00000018 0000001c 00000000 FDE cie=00000000 pc=00000000..86100e4100000011
  DW_CFA_advance_loc1: 67 to 00000043
  DW_CFA_def_cfa_register: r6 (rbp)
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop

00000038 0000001c 00000000 FDE cie=00000000 pc=00000020..86100e410000002b
  DW_CFA_advance_loc1: 67 to 00000063
  DW_CFA_def_cfa_register: r6 (rbp)
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop

$ objdump -Wf 3.gnuas.o

3.gnuas.o:     file format elf64-x86-64

Contents of the .debug_frame section:

00000000 00000014 ffffffff CIE
  Version:               1
  Augmentation:          ""
  Code alignment factor: 1
  Data alignment factor: -8
  Return address column: 16

  DW_CFA_def_cfa: r7 (rsp) ofs 8
  DW_CFA_offset: r16 (rip) at cfa-8
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop

00000018 0000001c 00000000 FDE cie=00000000 pc=00000000..00000011
  DW_CFA_advance_loc: 1 to 00000001
  DW_CFA_def_cfa_offset: 16
  DW_CFA_offset: r6 (rbp) at cfa-16
  DW_CFA_advance_loc: 3 to 00000004
  DW_CFA_def_cfa_register: r6 (rbp)

00000038 0000001c 00000000 FDE cie=00000000 pc=00000020..0000002b
  DW_CFA_advance_loc: 1 to 00000021
  DW_CFA_def_cfa_offset: 16
  DW_CFA_offset: r6 (rbp) at cfa-16
  DW_CFA_advance_loc: 3 to 00000024
  DW_CFA_def_cfa_register: r6 (rbp)

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